fix wix installer bug (#8)

Reviewed-on: #8
Co-authored-by: Aleksandr Neychev <alexnejchev73@gmail.com>

.editorconfig

@@ -0,0 +1,335 @@
+# Unified code style for CursorLang.
+# The rules are understood by Visual Studio, Rider, VS Code and `dotnet format`.
+# https://learn.microsoft.com/dotnet/fundamentals/code-analysis/code-style-rule-options
+
+root = true
+
+# ==========================================================================
+# Common to all files
+# ==========================================================================
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+indent_style = space
+indent_size = 4
+
+# Build scripts are read by Windows PowerShell 5.1: without a BOM it takes the
+# file for ANSI and mangles the Russian text in the messages
+[*.{ps1,psm1,psd1}]
+charset = utf-8-bom
+
+# In Markdown two trailing spaces at the end of a line mean a line break
+[*.{md,markdown}]
+trim_trailing_whitespace = false
+
+[*.{json,jsonc,yml,yaml}]
+indent_size = 2
+
+# Project files, XAML and other XML keep the same four spaces as the code
+[*.{csproj,vbproj,props,targets,sln,slnx,xaml,xml,resx,manifest,config,appxmanifest,nuspec}]
+indent_size = 4
+
+# Files written by the SDK rather than by a human
+[{obj,bin,artifacts}/**]
+generated_code = true
+
+# ==========================================================================
+# C#
+# ==========================================================================
+[*.cs]
+
+# A guideline for wrapping lines, not a hard limit
+max_line_length = 120
+
+#### using directives ####
+
+# System comes first and in a group separate from the rest
+dotnet_sort_system_directives_first = true
+dotnet_separate_import_directive_groups = false
+
+# using outside the namespace — more familiar and compatible with file-scoped namespace
+csharp_using_directive_placement = outside_namespace:warning
+dotnet_diagnostic.IDE0005.severity = warning
+
+#### File layout ####
+
+csharp_style_namespace_declarations = file_scoped:warning
+
+# The namespace mirrors the file path from the project root
+dotnet_diagnostic.IDE0130.severity = warning
+
+#### this. and Me. ####
+
+dotnet_style_qualification_for_field = false:warning
+dotnet_style_qualification_for_property = false:warning
+dotnet_style_qualification_for_method = false:warning
+dotnet_style_qualification_for_event = false:warning
+
+#### Language keywords instead of BCL type names ####
+
+dotnet_style_predefined_type_for_locals_parameters_members = true:warning
+dotnet_style_predefined_type_for_member_access = true:warning
+
+#### Modifiers ####
+
+dotnet_style_require_accessibility_modifiers = for_non_interface_members:warning
+csharp_preferred_modifier_order = public,private,protected,internal,file,static,extern,new,virtual,abstract,sealed,override,readonly,unsafe,required,volatile,async:suggestion
+dotnet_style_readonly_field = true:warning
+csharp_prefer_static_local_function = true:suggestion
+csharp_prefer_static_anonymous_function = true:suggestion
+
+#### var and explicit types ####
+# The type is written explicitly, except where it is already visible on the right of the equals sign
+
+csharp_style_var_for_built_in_types = false:suggestion
+csharp_style_var_when_type_is_apparent = true:suggestion
+csharp_style_var_elsewhere = false:suggestion
+
+#### Expression-bodied members ####
+# Allowed but not enforced: for long expressions a block reads better
+
+csharp_style_expression_bodied_methods = when_on_single_line:silent
+csharp_style_expression_bodied_constructors = when_on_single_line:silent
+csharp_style_expression_bodied_operators = when_on_single_line:silent
+csharp_style_expression_bodied_properties = true:silent
+csharp_style_expression_bodied_indexers = true:silent
+csharp_style_expression_bodied_accessors = true:silent
+csharp_style_expression_bodied_lambdas = true:silent
+csharp_style_expression_bodied_local_functions = when_on_single_line:silent
+
+#### Pattern matching and null checks ####
+
+csharp_style_pattern_matching_over_is_with_cast_check = true:warning
+csharp_style_pattern_matching_over_as_with_null_check = true:warning
+csharp_style_prefer_switch_expression = true:suggestion
+csharp_style_prefer_pattern_matching = true:suggestion
+csharp_style_prefer_not_pattern = true:warning
+csharp_style_prefer_extended_property_pattern = true:suggestion
+csharp_style_conditional_delegate_call = true:warning
+csharp_style_throw_expression = true:suggestion
+
+dotnet_style_coalesce_expression = true:warning
+dotnet_style_null_propagation = true:warning
+dotnet_style_prefer_is_null_check_over_reference_equality_method = true:warning
+
+#### Expressions ####
+
+dotnet_style_object_initializer = true:suggestion
+dotnet_style_collection_initializer = true:suggestion
+dotnet_style_prefer_collection_expression = when_types_loosely_match:suggestion
+dotnet_style_explicit_tuple_names = true:warning
+dotnet_style_prefer_inferred_tuple_names = true:suggestion
+dotnet_style_prefer_inferred_anonymous_type_member_names = true:suggestion
+dotnet_style_prefer_auto_properties = true:silent
+dotnet_style_prefer_conditional_expression_over_assignment = true:silent
+dotnet_style_prefer_conditional_expression_over_return = true:silent
+dotnet_style_prefer_compound_assignment = true:suggestion
+dotnet_style_prefer_simplified_boolean_expressions = true:suggestion
+dotnet_style_prefer_simplified_interpolation = true:suggestion
+dotnet_style_operator_placement_when_wrapping = beginning_of_line
+csharp_style_prefer_index_operator = true:suggestion
+csharp_style_prefer_range_operator = true:suggestion
+csharp_style_implicit_object_creation_when_type_is_apparent = true:suggestion
+csharp_prefer_simple_default_expression = true:suggestion
+csharp_style_deconstructed_variable_declaration = true:suggestion
+csharp_style_inlined_variable_declaration = true:suggestion
+csharp_style_unused_value_assignment_preference = discard_variable:suggestion
+csharp_style_unused_value_expression_statement_preference = discard_variable:silent
+csharp_prefer_simple_using_statement = true:suggestion
+csharp_style_prefer_utf8_string_literals = true:suggestion
+csharp_style_prefer_readonly_struct = true:suggestion
+csharp_style_prefer_readonly_struct_member = true:suggestion
+csharp_style_prefer_primary_constructors = false:silent
+
+#### Braces and blocks ####
+
+# Braces are always used, even around a single line
+csharp_prefer_braces = true:warning
+csharp_style_prefer_top_level_statements = false:silent
+
+# Parentheses that clarify the order of operations are welcome. The hint is
+# silent: parentheses around operations of the same precedence also count as
+# redundant, and in the coordinate calculations they are there on purpose
+dotnet_style_parentheses_in_arithmetic_binary_operators = always_for_clarity:silent
+dotnet_style_parentheses_in_relational_binary_operators = always_for_clarity:silent
+dotnet_style_parentheses_in_other_binary_operators = always_for_clarity:silent
+dotnet_style_parentheses_in_other_operators = never_if_unnecessary:silent
+
+#### Unused code ####
+
+dotnet_code_quality_unused_parameters = non_public:suggestion
+dotnet_remove_unnecessary_suppression_exclusions = none
+csharp_style_prefer_method_group_conversion = true:silent
+
+# ==========================================================================
+# C#: formatting
+# ==========================================================================
+
+#### Line breaks before a brace — Allman style ####
+
+csharp_new_line_before_open_brace = all
+csharp_new_line_before_else = true
+csharp_new_line_before_catch = true
+csharp_new_line_before_finally = true
+csharp_new_line_before_members_in_object_initializers = true
+csharp_new_line_before_members_in_anonymous_types = true
+csharp_new_line_between_query_expression_clauses = true
+
+#### Indentation ####
+
+csharp_indent_case_contents = true
+csharp_indent_case_contents_when_block = false
+csharp_indent_switch_labels = true
+csharp_indent_labels = one_less_than_current
+csharp_indent_block_contents = true
+csharp_indent_braces = false
+
+#### Spacing ####
+
+csharp_space_after_cast = false
+csharp_space_after_keywords_in_control_flow_statements = true
+csharp_space_before_colon_in_inheritance_clause = true
+csharp_space_after_colon_in_inheritance_clause = true
+csharp_space_around_binary_operators = before_and_after
+csharp_space_between_method_declaration_parameter_list_parentheses = false
+csharp_space_between_method_declaration_empty_parameter_list_parentheses = false
+csharp_space_between_method_declaration_name_and_open_parenthesis = false
+csharp_space_between_method_call_parameter_list_parentheses = false
+csharp_space_between_method_call_empty_parameter_list_parentheses = false
+csharp_space_between_method_call_name_and_opening_parenthesis = false
+csharp_space_after_comma = true
+csharp_space_before_comma = false
+csharp_space_after_dot = false
+csharp_space_before_dot = false
+csharp_space_after_semicolon_in_for_statement = true
+csharp_space_before_semicolon_in_for_statement = false
+csharp_space_around_declaration_statements = false
+csharp_space_before_open_square_brackets = false
+csharp_space_between_empty_square_brackets = false
+csharp_space_between_square_brackets = false
+
+#### Line wrapping ####
+
+# Every statement and every member on its own line
+csharp_preserve_single_line_statements = false
+csharp_preserve_single_line_blocks = true
+
+# ==========================================================================
+# C#: naming
+# The rules are checked top to bottom, the first matching one wins
+# ==========================================================================
+
+#### Naming styles ####
+
+dotnet_naming_style.pascal_case.capitalization = pascal_case
+
+dotnet_naming_style.camel_case.capitalization = camel_case
+
+dotnet_naming_style.underscore_camel_case.capitalization = camel_case
+dotnet_naming_style.underscore_camel_case.required_prefix = _
+
+dotnet_naming_style.i_pascal_case.capitalization = pascal_case
+dotnet_naming_style.i_pascal_case.required_prefix = I
+
+dotnet_naming_style.t_pascal_case.capitalization = pascal_case
+dotnet_naming_style.t_pascal_case.required_prefix = T
+
+#### Interfaces: IStartupService ####
+
+dotnet_naming_symbols.interfaces.applicable_kinds = interface
+dotnet_naming_symbols.interfaces.applicable_accessibilities = *
+
+dotnet_naming_rule.interfaces_are_i_pascal_case.symbols = interfaces
+dotnet_naming_rule.interfaces_are_i_pascal_case.style = i_pascal_case
+dotnet_naming_rule.interfaces_are_i_pascal_case.severity = warning
+
+#### Type parameters: TValue ####
+
+dotnet_naming_symbols.type_parameters.applicable_kinds = type_parameter
+dotnet_naming_symbols.type_parameters.applicable_accessibilities = *
+
+dotnet_naming_rule.type_parameters_are_t_pascal_case.symbols = type_parameters
+dotnet_naming_rule.type_parameters_are_t_pascal_case.style = t_pascal_case
+dotnet_naming_rule.type_parameters_are_t_pascal_case.severity = warning
+
+#### Types and members: PascalCase ####
+
+dotnet_naming_symbols.types.applicable_kinds = class,struct,enum,delegate
+dotnet_naming_symbols.types.applicable_accessibilities = *
+
+dotnet_naming_rule.types_are_pascal_case.symbols = types
+dotnet_naming_rule.types_are_pascal_case.style = pascal_case
+dotnet_naming_rule.types_are_pascal_case.severity = warning
+
+dotnet_naming_symbols.non_field_members.applicable_kinds = method,property,event,local_function
+dotnet_naming_symbols.non_field_members.applicable_accessibilities = *
+
+dotnet_naming_rule.non_field_members_are_pascal_case.symbols = non_field_members
+dotnet_naming_rule.non_field_members_are_pascal_case.style = pascal_case
+dotnet_naming_rule.non_field_members_are_pascal_case.severity = warning
+
+#### Constants and static fields: PascalCase ####
+# Comes before the rule about private fields, otherwise they would fall under _camelCase
+
+dotnet_naming_symbols.constants.applicable_kinds = field
+dotnet_naming_symbols.constants.applicable_accessibilities = *
+dotnet_naming_symbols.constants.required_modifiers = const
+
+dotnet_naming_rule.constants_are_pascal_case.symbols = constants
+dotnet_naming_rule.constants_are_pascal_case.style = pascal_case
+dotnet_naming_rule.constants_are_pascal_case.severity = suggestion
+
+dotnet_naming_symbols.static_readonly_fields.applicable_kinds = field
+dotnet_naming_symbols.static_readonly_fields.applicable_accessibilities = *
+dotnet_naming_symbols.static_readonly_fields.required_modifiers = static,readonly
+
+dotnet_naming_rule.static_readonly_fields_are_pascal_case.symbols = static_readonly_fields
+dotnet_naming_rule.static_readonly_fields_are_pascal_case.style = pascal_case
+dotnet_naming_rule.static_readonly_fields_are_pascal_case.severity = suggestion
+
+#### Public fields: PascalCase ####
+
+dotnet_naming_symbols.public_fields.applicable_kinds = field
+dotnet_naming_symbols.public_fields.applicable_accessibilities = public,internal,protected,protected_internal
+
+dotnet_naming_rule.public_fields_are_pascal_case.symbols = public_fields
+dotnet_naming_rule.public_fields_are_pascal_case.style = pascal_case
+dotnet_naming_rule.public_fields_are_pascal_case.severity = warning
+
+#### Private fields: _camelCase ####
+
+dotnet_naming_symbols.private_fields.applicable_kinds = field
+dotnet_naming_symbols.private_fields.applicable_accessibilities = private,private_protected
+
+dotnet_naming_rule.private_fields_are_underscore_camel_case.symbols = private_fields
+dotnet_naming_rule.private_fields_are_underscore_camel_case.style = underscore_camel_case
+dotnet_naming_rule.private_fields_are_underscore_camel_case.severity = suggestion
+
+#### Parameters and local variables: camelCase ####
+
+dotnet_naming_symbols.parameters_and_locals.applicable_kinds = parameter,local
+dotnet_naming_symbols.parameters_and_locals.applicable_accessibilities = *
+
+dotnet_naming_rule.parameters_and_locals_are_camel_case.symbols = parameters_and_locals
+dotnet_naming_rule.parameters_and_locals_are_camel_case.style = camel_case
+dotnet_naming_rule.parameters_and_locals_are_camel_case.severity = suggestion
+
+# ==========================================================================
+# Tests
+# ==========================================================================
+[CursorLang.Tests/**/*.cs]
+
+# A test name is a sentence with underscores instead of spaces:
+# in the run report it is read by a human, not called by other code
+dotnet_diagnostic.IDE1006.severity = none
+
+# ==========================================================================
+# Win32 and WinRT wrappers
+# ==========================================================================
+[CursorLang/Interop/*.cs]
+
+# Function, constant and field names repeat the Windows documentation: they are
+# easier to search for in the code when written the same way as in the API description
+dotnet_diagnostic.IDE1006.severity = none

.gitattributes

@@ -0,0 +1,65 @@
+# Text files are stored and checked out with LF line endings — the same as
+# .editorconfig describes them. Otherwise some files arrive with CRLF and edits
+# in them look like the whole file was rewritten
+* text=auto eol=lf
+
+# ==========================================================================
+# Git LFS
+# ==========================================================================
+# Git stores binary files in full for every version: a redrawn icon settles in
+# the history together with all its previous shapes. LFS keeps only a pointer
+# in the history and fetches the files themselves on checkout.
+#
+# Working with the repository requires git-lfs installed and a one-time
+# `git lfs install`. Without it the working copy will contain text pointer
+# files instead of the images.
+#
+# The -text flag is mandatory here: it forbids Git from touching the line
+# endings that are allowed for everything else above.
+
+# Application icon and MSIX package logos. They are drawn by
+# Packaging\New-Assets.ps1, and every run of the script produces whole new files
+*.png filter=lfs diff=lfs merge=lfs -text
+*.ico filter=lfs diff=lfs merge=lfs -text
+*.jpg filter=lfs diff=lfs merge=lfs -text
+*.jpeg filter=lfs diff=lfs merge=lfs -text
+*.gif filter=lfs diff=lfs merge=lfs -text
+*.bmp filter=lfs diff=lfs merge=lfs -text
+*.tif filter=lfs diff=lfs merge=lfs -text
+*.tiff filter=lfs diff=lfs merge=lfs -text
+*.webp filter=lfs diff=lfs merge=lfs -text
+
+# Windows packages and installers. The build puts them into artifacts\, where
+# Git does not pick them up, but a released version sometimes has to be attached
+# to a tag — they run into tens of megabytes, and their only place is LFS
+*.msix filter=lfs diff=lfs merge=lfs -text
+*.msixbundle filter=lfs diff=lfs merge=lfs -text
+*.appx filter=lfs diff=lfs merge=lfs -text
+*.appxbundle filter=lfs diff=lfs merge=lfs -text
+*.appxupload filter=lfs diff=lfs merge=lfs -text
+*.msi filter=lfs diff=lfs merge=lfs -text
+*.msp filter=lfs diff=lfs merge=lfs -text
+*.cab filter=lfs diff=lfs merge=lfs -text
+
+# Prebuilt binaries, in case one has to be attached to the repository
+*.dll filter=lfs diff=lfs merge=lfs -text
+*.exe filter=lfs diff=lfs merge=lfs -text
+
+# Fonts and archives
+*.ttf filter=lfs diff=lfs merge=lfs -text
+*.otf filter=lfs diff=lfs merge=lfs -text
+*.woff filter=lfs diff=lfs merge=lfs -text
+*.woff2 filter=lfs diff=lfs merge=lfs -text
+*.zip filter=lfs diff=lfs merge=lfs -text
+*.7z filter=lfs diff=lfs merge=lfs -text
+
+# ==========================================================================
+# Binary, but not in LFS
+# ==========================================================================
+# Signing keys never reach the repository — .gitignore keeps them out. The mark
+# is left in case such a file still ends up in the working copy: there is no
+# point in showing its contents in a diff
+*.pfx binary
+*.snk binary
+*.cer binary
+*.p12 binary

.gitea/workflows/pull-request.yml

@@ -0,0 +1,82 @@
+# The check every pull request goes through: the solution builds and the tests pass.
+#
+# The runner has to be a Windows one with the .NET 10 SDK: the application is
+# WPF, so neither the build nor the tests happen anywhere else. The tests raise
+# real windows and ask Windows for the foreground one, so the runner has to work
+# in an interactive desktop session — as a service in session 0 the end-to-end
+# checks have no window to wait for.
+name: Pull request
+
+on:
+  pull_request:
+    branches:
+      - master
+
+# A new push into the branch makes the previous run pointless
+concurrency:
+  group: pull-request-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+defaults:
+  run:
+    shell: pwsh
+
+jobs:
+  build:
+    runs-on: windows-x64
+
+    steps:
+      - name: Check out the sources
+        uses: actions/checkout@v4
+
+      # The exe icon lives in Git LFS, and without it the checkout leaves a text
+      # pointer that the build cannot read as an icon.
+      #
+      # The objects are fetched here rather than by `lfs: true` on the checkout:
+      # that way they arrive over a request the LFS endpoint accepts. See the
+      # comment on the header below
+      - name: Fetch the LFS objects
+        run: |
+          $ErrorActionPreference = 'Stop'
+
+          # actions/checkout leaves its own token in the config as an
+          # http.<server>/.extraheader, and git-lfs sends that header on to the
+          # LFS endpoint, which turns down the token of a workflow: every object
+          # comes back 401 and the fetch gives up. The repository is public and
+          # its LFS objects are readable without a token at all, so the header
+          # simply goes. A private repository would need credentials of its own
+          # in lfs.url instead
+          $keys = git config --local --list --name-only | Where-Object { $_ -like '*.extraheader' }
+          foreach ($key in $keys) { git config --local --unset-all $key }
+
+          git lfs pull
+          if ($LASTEXITCODE -ne 0) { throw "git lfs pull ended with exit code $LASTEXITCODE." }
+
+          # A pointer left in place of a file shows itself much later and in a
+          # way that is hard to read back: the build breaks on the icon
+          $pointers = git lfs ls-files --name-only |
+              Where-Object { (Get-Content $_ -TotalCount 1) -like 'version https://git-lfs*' }
+
+          if ($pointers) {
+              throw "Git LFS left pointers instead of files: $($pointers -join ', ')."
+          }
+
+      - name: Show the toolchain
+        run: dotnet --info
+
+      - name: Restore
+        run: dotnet restore CursorLang.sln --nologo
+
+      - name: Build
+        run: dotnet build CursorLang.sln --configuration Release --no-restore --nologo
+
+      # The tests take the application from bin\Release, which is why the whole
+      # run is a Release one: in Debug they would find no executable and skip
+      # themselves — a green run that checked nothing
+      - name: Test
+        run: >
+          dotnet test CursorLang.sln
+          --configuration Release
+          --no-build
+          --nologo
+          --settings coverage.runsettings

.gitea/workflows/release.yml

@@ -0,0 +1,196 @@
+# The release: a tag of the form v1.2.3 builds the solution, runs the tests and
+# packs the MSIX with the version taken from the tag — three numbers of the tag
+# and a zero the Store keeps for itself.
+#
+# The package goes to the Store and nowhere else, so it leaves the run as an
+# artifact: someone picks it up and uploads it to Partner Center, which puts its
+# own signature on it. Nothing is signed here and nothing is attached to the
+# release — a publicly trusted code signing certificate is not to be had, and an
+# unsigned package would look like something to install and install nowhere.
+# Gitea makes the release for the tag itself, and it carries the tag alone.
+#
+# The same requirements to the runner as in pull-request.yml apply: Windows, the
+# .NET 10 SDK and an interactive desktop session for the tests. makeappx comes
+# with a NuGet package (Packaging\Tools\SdkTools.csproj), so the Windows SDK does
+# not have to be installed.
+name: Release
+
+on:
+  push:
+    tags:
+      - 'v*'
+
+defaults:
+  run:
+    shell: pwsh
+
+jobs:
+  release:
+    runs-on: windows-x64
+
+    steps:
+      - name: Check out the sources
+        uses: actions/checkout@v4
+
+      # The exe icon and the MSIX logos live in Git LFS, and without them the
+      # checkout leaves text pointers in their place — the build fails on the
+      # icon and the package would carry broken logos.
+      #
+      # They are fetched here rather than by `lfs: true` on the checkout: that
+      # way the objects arrive over a request the LFS endpoint accepts. See the
+      # comment on the header below
+      - name: Fetch the LFS objects
+        run: |
+          $ErrorActionPreference = 'Stop'
+
+          # actions/checkout leaves its own token in the config as an
+          # http.<server>/.extraheader, and git-lfs sends that header on to the
+          # LFS endpoint, which turns down the token of a workflow: every object
+          # comes back 401 and the fetch gives up. The repository is public and
+          # its LFS objects are readable without a token at all, so the header
+          # simply goes. A private repository would need credentials of its own
+          # in lfs.url instead
+          $keys = git config --local --list --name-only | Where-Object { $_ -like '*.extraheader' }
+          foreach ($key in $keys) { git config --local --unset-all $key }
+
+          git lfs pull
+          if ($LASTEXITCODE -ne 0) { throw "git lfs pull ended with exit code $LASTEXITCODE." }
+
+          # A pointer left in place of a file shows itself much later and in a
+          # way that is hard to read back: the icon breaks the build, and a logo
+          # quietly ends up broken inside the package
+          $pointers = git lfs ls-files --name-only |
+              Where-Object { (Get-Content $_ -TotalCount 1) -like 'version https://git-lfs*' }
+
+          if ($pointers) {
+              throw "Git LFS left pointers instead of files: $($pointers -join ', ')."
+          }
+
+      # The tag is the only place the version comes from, and it is a plain
+      # version of three numbers — the same shape the application itself looks
+      # for in the releases when it checks for an update. A tag of any other
+      # shape is stopped here rather than halfway through the packaging
+      - name: Read the version from the tag
+        id: version
+        run: |
+          $ErrorActionPreference = 'Stop'
+          $tag = '${{ github.ref_name }}'
+
+          if ($tag -notmatch '^v\d+\.\d+\.\d+$') {
+              throw "The tag '$tag' does not fit: a release is tagged as v1.2.3 — three numbers. A fourth one does not belong in the tag: the Store keeps the revision for itself, and the package always gets a zero there."
+          }
+
+          # The package takes four numbers with a zero at the end: the Store
+          # reserves the last one, so it carries nothing the tag could tell
+          "version=$($tag.Substring(1)).0" | Out-File $env:GITHUB_OUTPUT -Append -Encoding utf8
+
+          # The installer answers to nobody about a fourth number and takes the
+          # tag as it is
+          "plain=$($tag.Substring(1))" | Out-File $env:GITHUB_OUTPUT -Append -Encoding utf8
+
+      - name: Show the toolchain
+        run: dotnet --info
+
+      - name: Restore
+        run: dotnet restore CursorLang.sln --nologo
+
+      - name: Build
+        run: dotnet build CursorLang.sln --configuration Release --no-restore --nologo
+
+      - name: Test
+        run: >
+          dotnet test CursorLang.sln
+          --configuration Release
+          --no-build
+          --nologo
+          --settings coverage.runsettings
+
+      # The package comes out as Partner Center wants it — the Store puts its own
+      # signature on it. The identity comes from repository variables and falls
+      # back to the defaults of the script when a variable is not set.
+      - name: Pack the MSIX
+        env:
+          IDENTITY_NAME: ${{ vars.MSIX_IDENTITY_NAME }}
+          PUBLISHER: ${{ vars.MSIX_PUBLISHER }}
+          PUBLISHER_DISPLAY_NAME: ${{ vars.MSIX_PUBLISHER_DISPLAY_NAME }}
+        run: |
+          $ErrorActionPreference = 'Stop'
+
+          $arguments = @{ Version = '${{ steps.version.outputs.version }}' }
+
+          # An empty variable is left out rather than passed on: the script has
+          # defaults of its own, and an empty string would wipe them
+          $variables = @{
+              IdentityName         = $env:IDENTITY_NAME
+              Publisher            = $env:PUBLISHER
+              PublisherDisplayName = $env:PUBLISHER_DISPLAY_NAME
+          }
+
+          foreach ($name in $variables.Keys) {
+              if ($variables[$name]) { $arguments[$name] = $variables[$name] }
+          }
+
+          ./Packaging/build-msix.ps1 @arguments
+
+      # The other half of the release: the same application as an ordinary
+      # installer, for handing round outside the Store. Nobody signs it, so
+      # SmartScreen warns about it — see Packaging\installer.iss
+      - name: Build the installer
+        run: ./Packaging/build-installer.ps1 -Version ${{ steps.version.outputs.plain }}
+
+      # The artifact is where the package waits to be uploaded to Partner Center
+      - name: Keep the package
+        uses: actions/upload-artifact@v4
+        with:
+          name: msix-${{ steps.version.outputs.version }}
+          path: artifacts/packages/
+          if-no-files-found: error
+
+      # The .wixpdb next to each installer is left out on purpose: it is of use
+      # only when something has to be traced back to the WiX source
+      - name: Keep the installer
+        uses: actions/upload-artifact@v4
+        with:
+          name: installer-${{ steps.version.outputs.plain }}
+          path: artifacts/installers/*.msi
+          if-no-files-found: error
+
+      # Only the installers go into the release. The MSIX stays in the artifacts
+      # of the run: unsigned, it installs nowhere, and its one destination is
+      # Partner Center
+      - name: Publish the release
+        env:
+          GITEA_TOKEN: ${{ secrets.GITEA_TOKEN }}
+          TAG: ${{ github.ref_name }}
+        run: |
+          $ErrorActionPreference = 'Stop'
+
+          # The GITHUB_ names are what Gitea itself hands to the workflow — its
+          # actions repeat those of GitHub, and the addresses in them point at
+          # this Gitea instance. GITHUB_API_URL used not to reach the steps at
+          # all, so the address is put together from the server one when empty
+          $root = if ($env:GITHUB_API_URL) { $env:GITHUB_API_URL } else { "$env:GITHUB_SERVER_URL/api/v1" }
+          $api = "$root/repos/$env:GITHUB_REPOSITORY/releases"
+          $headers = @{ Authorization = "token $env:GITEA_TOKEN" }
+
+          # Gitea makes a release of its own for a pushed tag, so the release is
+          # looked up first and only made when it is not there
+          $release = $null
+          try { $release = Invoke-RestMethod "$api/tags/$env:TAG" -Headers $headers } catch { }
+
+          if (-not $release) {
+              $body = @{ tag_name = $env:TAG; name = $env:TAG; draft = $false; prerelease = $false } | ConvertTo-Json
+              $release = Invoke-RestMethod $api -Method Post -Headers $headers -ContentType 'application/json' -Body $body
+          }
+
+          foreach ($file in Get-ChildItem artifacts/installers -File -Filter *.msi) {
+              # A tag can be pushed again after it was deleted; the old file of
+              # the same name is dropped, otherwise the upload is refused
+              $existing = $release.assets | Where-Object { $_.name -eq $file.Name }
+              foreach ($asset in $existing) {
+                  Invoke-RestMethod "$api/$($release.id)/assets/$($asset.id)" -Method Delete -Headers $headers | Out-Null
+              }
+
+              Write-Host "Uploading $($file.Name)"
+              Invoke-RestMethod "$api/$($release.id)/assets?name=$($file.Name)" -Method Post -Headers $headers -Form @{ attachment = $file } | Out-Null
+          }

.gitignore

@@ -0,0 +1,484 @@
+## Ignore Visual Studio temporary files, build results, and
+## files generated by popular Visual Studio add-ons.
+##
+## Get latest from `dotnet new gitignore`
+
+# dotenv files
+.env
+
+# User-specific files
+*.rsuser
+*.suo
+*.user
+*.userosscache
+*.sln.docstates
+
+# User-specific files (MonoDevelop/Xamarin Studio)
+*.userprefs
+
+# Mono auto generated files
+mono_crash.*
+
+# Build results
+[Dd]ebug/
+[Dd]ebugPublic/
+[Rr]elease/
+[Rr]eleases/
+x64/
+x86/
+[Ww][Ii][Nn]32/
+[Aa][Rr][Mm]/
+[Aa][Rr][Mm]64/
+bld/
+[Bb]in/
+[Oo]bj/
+[Ll]og/
+[Ll]ogs/
+
+# Visual Studio 2015/2017 cache/options directory
+.vs/
+# Uncomment if you have tasks that create the project's static files in wwwroot
+#wwwroot/
+
+# Visual Studio 2017 auto generated files
+Generated\ Files/
+
+# MSTest test Results
+[Tt]est[Rr]esult*/
+[Bb]uild[Ll]og.*
+
+# NUnit
+*.VisualState.xml
+TestResult.xml
+nunit-*.xml
+
+# Build Results of an ATL Project
+[Dd]ebugPS/
+[Rr]eleasePS/
+dlldata.c
+
+# Benchmark Results
+BenchmarkDotNet.Artifacts/
+
+# .NET
+project.lock.json
+project.fragment.lock.json
+artifacts/
+
+# Tye
+.tye/
+
+# ASP.NET Scaffolding
+ScaffoldingReadMe.txt
+
+# StyleCop
+StyleCopReport.xml
+
+# Files built by Visual Studio
+*_i.c
+*_p.c
+*_h.h
+*.ilk
+*.meta
+*.obj
+*.iobj
+*.pch
+*.pdb
+*.ipdb
+*.pgc
+*.pgd
+*.rsp
+*.sbr
+*.tlb
+*.tli
+*.tlh
+*.tmp
+*.tmp_proj
+*_wpftmp.csproj
+*.log
+*.tlog
+*.vspscc
+*.vssscc
+.builds
+*.pidb
+*.svclog
+*.scc
+
+# Chutzpah Test files
+_Chutzpah*
+
+# Visual C++ cache files
+ipch/
+*.aps
+*.ncb
+*.opendb
+*.opensdf
+*.sdf
+*.cachefile
+*.VC.db
+*.VC.VC.opendb
+
+# Visual Studio profiler
+*.psess
+*.vsp
+*.vspx
+*.sap
+
+# Visual Studio Trace Files
+*.e2e
+
+# TFS 2012 Local Workspace
+$tf/
+
+# Guidance Automation Toolkit
+*.gpState
+
+# ReSharper is a .NET coding add-in
+_ReSharper*/
+*.[Rr]e[Ss]harper
+*.DotSettings.user
+
+# TeamCity is a build add-in
+_TeamCity*
+
+# DotCover is a Code Coverage Tool
+*.dotCover
+
+# AxoCover is a Code Coverage Tool
+.axoCover/*
+!.axoCover/settings.json
+
+# Coverlet is a free, cross platform Code Coverage Tool
+coverage*.json
+coverage*.xml
+coverage*.info
+
+# Visual Studio code coverage results
+*.coverage
+*.coveragexml
+
+# NCrunch
+_NCrunch_*
+.*crunch*.local.xml
+nCrunchTemp_*
+
+# MightyMoose
+*.mm.*
+AutoTest.Net/
+
+# Web workbench (sass)
+.sass-cache/
+
+# Installshield output folder
+[Ee]xpress/
+
+# DocProject is a documentation generator add-in
+DocProject/buildhelp/
+DocProject/Help/*.HxT
+DocProject/Help/*.HxC
+DocProject/Help/*.hhc
+DocProject/Help/*.hhk
+DocProject/Help/*.hhp
+DocProject/Help/Html2
+DocProject/Help/html
+
+# Click-Once directory
+publish/
+
+# Publish Web Output
+*.[Pp]ublish.xml
+*.azurePubxml
+# Note: Comment the next line if you want to checkin your web deploy settings,
+# but database connection strings (with potential passwords) will be unencrypted
+*.pubxml
+*.publishproj
+
+# Microsoft Azure Web App publish settings. Comment the next line if you want to
+# checkin your Azure Web App publish settings, but sensitive information contained
+# in these scripts will be unencrypted
+PublishScripts/
+
+# NuGet Packages
+*.nupkg
+# NuGet Symbol Packages
+*.snupkg
+# The packages folder can be ignored because of Package Restore
+**/[Pp]ackages/*
+# except build/, which is used as an MSBuild target.
+!**/[Pp]ackages/build/
+# Uncomment if necessary however generally it will be regenerated when needed
+#!**/[Pp]ackages/repositories.config
+# NuGet v3's project.json files produces more ignorable files
+*.nuget.props
+*.nuget.targets
+
+# Microsoft Azure Build Output
+csx/
+*.build.csdef
+
+# Microsoft Azure Emulator
+ecf/
+rcf/
+
+# Windows Store app package directories and files
+AppPackages/
+BundleArtifacts/
+Package.StoreAssociation.xml
+_pkginfo.txt
+*.appx
+*.appxbundle
+*.appxupload
+
+# Visual Studio cache files
+# files ending in .cache can be ignored
+*.[Cc]ache
+# but keep track of directories ending in .cache
+!?*.[Cc]ache/
+
+# Others
+ClientBin/
+~$*
+*~
+*.dbmdl
+*.dbproj.schemaview
+*.jfm
+*.pfx
+*.publishsettings
+orleans.codegen.cs
+
+# Including strong name files can present a security risk
+# (https://github.com/github/gitignore/pull/2483#issue-259490424)
+#*.snk
+
+# Since there are multiple workflows, uncomment next line to ignore bower_components
+# (https://github.com/github/gitignore/pull/1529#issuecomment-104372622)
+#bower_components/
+
+# RIA/Silverlight projects
+Generated_Code/
+
+# Backup & report files from converting an old project file
+# to a newer Visual Studio version. Backup files are not needed,
+# because we have git ;-)
+_UpgradeReport_Files/
+Backup*/
+UpgradeLog*.XML
+UpgradeLog*.htm
+ServiceFabricBackup/
+*.rptproj.bak
+
+# SQL Server files
+*.mdf
+*.ldf
+*.ndf
+
+# Business Intelligence projects
+*.rdl.data
+*.bim.layout
+*.bim_*.settings
+*.rptproj.rsuser
+*- [Bb]ackup.rdl
+*- [Bb]ackup ([0-9]).rdl
+*- [Bb]ackup ([0-9][0-9]).rdl
+
+# Microsoft Fakes
+FakesAssemblies/
+
+# GhostDoc plugin setting file
+*.GhostDoc.xml
+
+# Node.js Tools for Visual Studio
+.ntvs_analysis.dat
+node_modules/
+
+# Visual Studio 6 build log
+*.plg
+
+# Visual Studio 6 workspace options file
+*.opt
+
+# Visual Studio 6 auto-generated workspace file (contains which files were open etc.)
+*.vbw
+
+# Visual Studio 6 auto-generated project file (contains which files were open etc.)
+*.vbp
+
+# Visual Studio 6 workspace and project file (working project files containing files to include in project)
+*.dsw
+*.dsp
+
+# Visual Studio 6 technical files
+*.ncb
+*.aps
+
+# Visual Studio LightSwitch build output
+**/*.HTMLClient/GeneratedArtifacts
+**/*.DesktopClient/GeneratedArtifacts
+**/*.DesktopClient/ModelManifest.xml
+**/*.Server/GeneratedArtifacts
+**/*.Server/ModelManifest.xml
+_Pvt_Extensions
+
+# Paket dependency manager
+.paket/paket.exe
+paket-files/
+
+# FAKE - F# Make
+.fake/
+
+# CodeRush personal settings
+.cr/personal
+
+# Python Tools for Visual Studio (PTVS)
+__pycache__/
+*.pyc
+
+# Cake - Uncomment if you are using it
+# tools/**
+# !tools/packages.config
+
+# Tabs Studio
+*.tss
+
+# Telerik's JustMock configuration file
+*.jmconfig
+
+# BizTalk build output
+*.btp.cs
+*.btm.cs
+*.odx.cs
+*.xsd.cs
+
+# OpenCover UI analysis results
+OpenCover/
+
+# Azure Stream Analytics local run output
+ASALocalRun/
+
+# MSBuild Binary and Structured Log
+*.binlog
+
+# NVidia Nsight GPU debugger configuration file
+*.nvuser
+
+# MFractors (Xamarin productivity tool) working folder
+.mfractor/
+
+# Local History for Visual Studio
+.localhistory/
+
+# Visual Studio History (VSHistory) files
+.vshistory/
+
+# BeatPulse healthcheck temp database
+healthchecksdb
+
+# Backup folder for Package Reference Convert tool in Visual Studio 2017
+MigrationBackup/
+
+# Ionide (cross platform F# VS Code tools) working folder
+.ionide/
+
+# Fody - auto-generated XML schema
+FodyWeavers.xsd
+
+# VS Code files for those working on multiple tools
+.vscode/*
+!.vscode/settings.json
+!.vscode/tasks.json
+!.vscode/launch.json
+!.vscode/extensions.json
+*.code-workspace
+
+# Local History for Visual Studio Code
+.history/
+
+# Windows Installer files from build outputs
+*.cab
+*.msi
+*.msix
+*.msm
+*.msp
+
+# JetBrains Rider
+*.sln.iml
+.idea
+
+##
+## Visual studio for Mac
+##
+
+
+# globs
+Makefile.in
+*.userprefs
+*.usertasks
+config.make
+config.status
+aclocal.m4
+install-sh
+autom4te.cache/
+*.tar.gz
+tarballs/
+test-results/
+
+# Mac bundle stuff
+*.dmg
+*.app
+
+# content below from: https://github.com/github/gitignore/blob/master/Global/macOS.gitignore
+# General
+.DS_Store
+.AppleDouble
+.LSOverride
+
+# Icon must end with two \r
+Icon
+
+
+# Thumbnails
+._*
+
+# Files that might appear in the root of a volume
+.DocumentRevisions-V100
+.fseventsd
+.Spotlight-V100
+.TemporaryItems
+.Trashes
+.VolumeIcon.icns
+.com.apple.timemachine.donotpresent
+
+# Directories potentially created on remote AFP share
+.AppleDB
+.AppleDesktop
+Network Trash Folder
+Temporary Items
+.apdisk
+
+# content below from: https://github.com/github/gitignore/blob/master/Global/Windows.gitignore
+# Windows thumbnail cache files
+Thumbs.db
+ehthumbs.db
+ehthumbs_vista.db
+
+# Dump file
+*.stackdump
+
+# Folder config file
+[Dd]esktop.ini
+
+# Recycle Bin used on file shares
+$RECYCLE.BIN/
+
+# Windows Installer files
+*.cab
+*.msi
+*.msix
+*.msm
+*.msp
+
+# Windows shortcuts
+*.lnk
+
+# Vim temporary swap files
+*.swp

CursorLang.Agent.Tests/CursorLang.Agent.Tests.csproj

@@ -0,0 +1,43 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0-windows10.0.19041.0</TargetFramework>
+        <SupportedOSPlatformVersion>10.0.17763.0</SupportedOSPlatformVersion>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <OutputType>Exe</OutputType>
+        <PlatformTarget>AnyCPU</PlatformTarget>
+        <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
+        <GenerateDocumentationFile>true</GenerateDocumentationFile>
+        <NoWarn>$(NoWarn);CS1591</NoWarn>
+        <IsPackable>false</IsPackable>
+        <IsTestProject>true</IsTestProject>
+        <RootNamespace>CursorLang.Agent.Tests</RootNamespace>
+        <AssemblyName>CursorLang.Agent.Tests</AssemblyName>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <Using Include="Xunit"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <PackageReference Include="xunit.v3" Version="3.2.2"/>
+        <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5"/>
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.8.1"/>
+        <PackageReference Include="coverlet.collector" Version="10.0.1"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\CursorLang.Core\CursorLang.Core.csproj"/>
+        <ProjectReference Include="..\CursorLang.Tests.Shared\CursorLang.Tests.Shared.csproj"/>
+        <ProjectReference Include="..\CursorLang.Agent\CursorLang.Agent.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <AssemblyAttribute Include="System.Reflection.AssemblyMetadataAttribute">
+            <_Parameter1>CursorLangExecutable</_Parameter1>
+            <_Parameter2>$(MSBuildThisFileDirectory)..\CursorLang.Agent\bin\$(Configuration)\$(TargetFramework)\CursorLang.exe</_Parameter2>
+        </AssemblyAttribute>
+    </ItemGroup>
+
+</Project>

CursorLang.Agent.Tests/EndToEndTests.cs

@@ -0,0 +1,317 @@
+using System.Diagnostics;
+using System.Reflection;
+using System.Runtime.InteropServices;
+using System.Text;
+using System.Text.RegularExpressions;
+using CursorLang.Core.Services;
+
+namespace CursorLang.Agent.Tests;
+
+/// <summary>
+/// The application as a whole: the agent starting, the settings window it opens, the
+/// single instance and the exit.
+/// </summary>
+/// <remarks>
+/// None of this can be built inside the tests — the agent installs a system hook and
+/// takes the place of the single instance for the whole session. It is therefore
+/// started the way the user starts it: as a separate process.
+///
+/// If the application is already running in this session, the checks skip themselves:
+/// meddling with someone else's running instance is not their business. They skip
+/// themselves where there is no desktop to show a window on either — on a build agent
+/// living as a Windows service, for one.
+/// </remarks>
+public sealed partial class EndToEndTests
+{
+    private static readonly TimeSpan StartTimeout = TimeSpan.FromSeconds(30);
+    private static readonly TimeSpan ExitTimeout = TimeSpan.FromSeconds(15);
+
+    /// <summary>How long "the application went on working" is worth watching for.</summary>
+    private static readonly TimeSpan StayTimeout = TimeSpan.FromSeconds(3);
+
+    /// <summary>
+    /// The whole point of the background process, as a test rather than as a promise.
+    /// </summary>
+    /// <remarks>
+    /// The agent must not load the WPF rendering stack. It is checked on the running
+    /// process, not on its references: a reference costs nothing, a load costs the
+    /// hundred megabytes the split exists to avoid.
+    ///
+    /// UI Automation and the assemblies behind it — WindowsBase and PresentationCore —
+    /// are deliberately not in the pattern: the caret fallback pulls them in on purpose
+    /// and only when it runs. What must never appear is the renderer itself.
+    /// </remarks>
+    [Fact]
+    public void The_agent_runs_without_the_rendering_stack()
+    {
+        using Launch launch = Launch.Start(StartupLaunch.Argument);
+
+        Assert.False(launch.Process.WaitForExit(StayTimeout), "the agent ended by itself");
+        launch.Process.Refresh();
+
+        var loaded = launch.Process.Modules
+            .Cast<ProcessModule>()
+            .Select(module => module.ModuleName)
+            .Where(name => RenderingStack().IsMatch(name))
+            .ToList();
+
+        Assert.True(loaded.Count == 0, $"the agent loaded {string.Join(", ", loaded)}");
+    }
+
+    [GeneratedRegex("PresentationFramework|wpfgfx|milcore|PresentationNative", RegexOptions.IgnoreCase)]
+    private static partial Regex RenderingStack();
+
+    /// <summary>
+    /// Started by the user, the application shows the settings window — which lives in
+    /// a process of its own and is started by the agent.
+    /// </summary>
+    [Fact]
+    public void A_launch_by_the_user_opens_the_settings_window()
+    {
+        using Launch launch = Launch.Start();
+
+        Assert.NotEqual(IntPtr.Zero, launch.WaitForSettingsWindow());
+        Assert.False(launch.Process.HasExited);
+    }
+
+    /// <summary>
+    /// Started by Windows itself, the application goes straight to the tray: the
+    /// user asked for it to be there, not for a window to greet them.
+    /// </summary>
+    [Fact]
+    public void A_launch_by_Windows_shows_no_window()
+    {
+        using Launch launch = Launch.Start(StartupLaunch.Argument);
+
+        // Nothing is expected to appear, so the wait is for the whole time
+        Assert.False(launch.Process.WaitForExit(StayTimeout), "the agent ended by itself");
+        Assert.Empty(Launch.SettingsProcesses());
+    }
+
+    // A second run raises no second agent but asks the running one for the window
+    [Fact]
+    public void The_second_run_ends_by_itself()
+    {
+        using Launch launch = Launch.Start(StartupLaunch.Argument);
+        Assert.False(launch.Process.WaitForExit(StayTimeout), "the agent ended by itself");
+
+        using Process second = Launch.StartProcess();
+
+        Assert.True(second.WaitForExit(ExitTimeout), "the second run did not end by itself");
+        Assert.Equal(0, second.ExitCode);
+
+        // And the first one keeps running
+        Assert.False(launch.Process.HasExited);
+    }
+
+    /// <summary>
+    /// Closing the settings window ends that process and leaves the agent alone.
+    /// </summary>
+    /// <remarks>
+    /// This is the behaviour the split was for. The window used to hide itself into the
+    /// tray, because closing it would have thrown away a visual tree the background half
+    /// was still using; now there is nothing shared to throw away, and the memory the
+    /// window took goes back to the system.
+    /// </remarks>
+    [Fact]
+    public void Closing_the_settings_window_leaves_the_agent_running()
+    {
+        using Launch launch = Launch.Start();
+        launch.WaitForSettingsWindow();
+
+        Process settings = Launch.SettingsProcesses().Single();
+        try
+        {
+            Assert.True(settings.CloseMainWindow(), "the window did not accept the request to close");
+            Assert.True(settings.WaitForExit(ExitTimeout), "the settings window outlived its own closing");
+        }
+        finally
+        {
+            settings.Dispose();
+        }
+
+        Assert.False(launch.Process.HasExited);
+    }
+
+    /// <summary>
+    /// "Exit" in the tray menu ends the application whole: the settings window goes with
+    /// the agent instead of staying on the screen belonging to nothing.
+    /// </summary>
+    /// <remarks>
+    /// The menu itself is out of reach of a test — it is a <c>TrackPopupMenuEx</c> menu
+    /// with a modal loop of its own — so what is checked is the request the menu makes.
+    /// The settings window here is the real one, started by the agent, and it must be
+    /// listening by the time it is on the screen.
+    /// </remarks>
+    [Fact]
+    public void The_agents_exit_closes_the_settings_window()
+    {
+        using Launch launch = Launch.Start();
+        launch.WaitForSettingsWindow();
+
+        Process settings = Launch.SettingsProcesses().Single();
+        try
+        {
+            Assert.True(SettingsCloseSignal.RequestClose(), "the settings window was not listening");
+            Assert.True(settings.WaitForExit(ExitTimeout), "the settings window outlived the agent's exit");
+        }
+        finally
+        {
+            settings.Dispose();
+        }
+    }
+
+    /// <summary>A started application that shuts down together with the check.</summary>
+    private sealed class Launch : IDisposable
+    {
+        private const int UOI_NAME = 2;
+        private const string InteractiveWindowStation = "WinSta0";
+
+        [DllImport("user32.dll")]
+        private static extern IntPtr GetProcessWindowStation();
+
+        [DllImport("user32.dll", CharSet = CharSet.Unicode)]
+        private static extern bool GetUserObjectInformation(IntPtr hObj, int nIndex,
+            StringBuilder pvInfo, int nLength, out int lpnLengthNeeded);
+
+        private Launch(Process process) => Process = process;
+
+        internal Process Process { get; }
+
+        /// <summary>Starts the agent first — making sure the place is free.</summary>
+        internal static Launch Start(params string[] arguments)
+        {
+            if (!HasInteractiveDesktop())
+            {
+                Assert.Skip("There is no interactive desktop here — the application has nowhere to show its window");
+            }
+
+            if (Process.GetProcessesByName("CursorLang").Length > 0 || SettingsProcesses().Length > 0)
+            {
+                Assert.Skip("The application is already running — this check keeps out of someone else's run");
+            }
+
+            return new Launch(StartProcess(arguments));
+        }
+
+        /// <summary>Starts the agent the way the user — or Windows — does.</summary>
+        internal static Process StartProcess(params string[] arguments)
+        {
+            string path = ExecutablePath();
+
+            if (!File.Exists(path))
+            {
+                Assert.Skip($"The application is not built: {path}");
+            }
+
+            var start = new ProcessStartInfo(path) { UseShellExecute = true };
+
+            foreach (string argument in arguments)
+            {
+                start.ArgumentList.Add(argument);
+            }
+
+            return Process.Start(start)!;
+        }
+
+        /// <summary>The settings window processes running right now, if any.</summary>
+        internal static Process[] SettingsProcesses() => Process.GetProcessesByName("CursorLang.Settings");
+
+        /// <summary>
+        /// Waits for the settings window. It belongs to another process now, so the
+        /// wait is for that process to appear and put a window on the screen.
+        /// </summary>
+        internal IntPtr WaitForSettingsWindow()
+        {
+            DateTime deadline = DateTime.UtcNow + StartTimeout;
+
+            while (DateTime.UtcNow < deadline)
+            {
+                Process.Refresh();
+
+                if (Process.HasExited)
+                {
+                    Assert.Fail($"The agent exited while starting with code {Process.ExitCode}");
+                }
+
+                foreach (Process settings in SettingsProcesses())
+                {
+                    settings.Refresh();
+                    IntPtr window = settings.MainWindowHandle;
+                    settings.Dispose();
+
+                    if (window != IntPtr.Zero)
+                    {
+                        return window;
+                    }
+                }
+
+                Thread.Sleep(100);
+            }
+
+            Assert.Fail("The settings window never appeared");
+            return IntPtr.Zero;
+        }
+
+        /// <summary>
+        /// Whether there is a desktop here to show a window on. A service gets a
+        /// window station of its own — "Service-0x0-3e7$" and the like: a window can
+        /// be created there, yet nothing shows it. Only "WinSta0" is the interactive one.
+        /// </summary>
+        private static bool HasInteractiveDesktop()
+        {
+            IntPtr station = GetProcessWindowStation();
+            if (station == IntPtr.Zero)
+            {
+                return false;
+            }
+
+            var name = new StringBuilder(256);
+
+            return GetUserObjectInformation(station, UOI_NAME, name, name.Capacity * sizeof(char), out _)
+                   && name.ToString().Equals(InteractiveWindowStation, StringComparison.OrdinalIgnoreCase);
+        }
+
+        private static string ExecutablePath()
+        {
+            string configured = Assembly.GetExecutingAssembly()
+                .GetCustomAttributes<AssemblyMetadataAttribute>()
+                .Single(attribute => attribute.Key == "CursorLangExecutable")
+                .Value!;
+
+            return Path.GetFullPath(configured);
+        }
+
+        public void Dispose()
+        {
+            foreach (Process settings in SettingsProcesses())
+            {
+                Kill(settings);
+            }
+
+            Kill(Process);
+        }
+
+        // The exit lives in a tray menu no test can reach, and the settings are saved
+        // as they change, so nothing is lost by ending the processes outright
+        private static void Kill(Process process)
+        {
+            try
+            {
+                if (!process.HasExited)
+                {
+                    process.Kill(entireProcessTree: true);
+                    process.WaitForExit(ExitTimeout);
+                }
+            }
+            catch (InvalidOperationException)
+            {
+                // The process has already ended on its own
+            }
+            finally
+            {
+                process.Dispose();
+            }
+        }
+    }
+}

CursorLang.Agent.Tests/Services/CapsLockHotkeyServiceTests.cs

@@ -0,0 +1,305 @@
+using System.Collections.Concurrent;
+using CursorLang.Agent.Services;
+using CursorLang.Core.Models;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Agent.Tests.Services;
+
+/// <summary>
+/// Making sense of Caps Lock presses: a short one differs from a long one only
+/// by when the key was released.
+/// </summary>
+public sealed class CapsLockHotkeyServiceTests
+{
+    private const int CapsLock = 0x14;
+    private const int LetterA = 0x41;
+
+    [Fact]
+    public void A_Caps_Lock_press_is_not_passed_on()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 10_000);
+
+        Assert.True(Pump.Run(() => harness.Service.HandleKeyEvent(CapsLock, isKeyDown: true)));
+        Assert.True(Pump.Run(() => harness.Service.HandleKeyEvent(CapsLock, isKeyDown: false)));
+    }
+
+    [Fact]
+    public void The_other_keys_go_through_as_usual()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 10_000);
+
+        Assert.False(Pump.Run(() => harness.Service.HandleKeyEvent(LetterA, isKeyDown: true)));
+        Assert.False(Pump.Run(() => harness.Service.HandleKeyEvent(LetterA, isKeyDown: false)));
+        Assert.Empty(harness.Events);
+    }
+
+    [Fact]
+    public void A_short_press_yields_a_single_event()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 10_000);
+
+        harness.Press();
+        harness.Release();
+
+        Pump.WaitFor(() => harness.Events.Count == 1, "the short press event arrived");
+        Assert.Equal(["tap"], harness.Events);
+    }
+
+    [Fact]
+    public void A_hold_is_announced_once_the_threshold_is_past()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 20);
+
+        harness.Press();
+
+        Pump.WaitFor(() => harness.Events.Contains("hold-start"), "the hold threshold was passed");
+        Assert.Equal(["hold-start"], harness.Events);
+    }
+
+    [Fact]
+    public void A_release_after_a_hold_yields_an_end_rather_than_a_press()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 20);
+
+        harness.Press();
+        Pump.WaitFor(() => harness.Events.Contains("hold-start"), "the hold threshold was passed");
+
+        harness.Release();
+
+        Pump.WaitFor(() => harness.Events.Count == 2, "the end of the hold arrived");
+        Assert.Equal(["hold-start", "hold-end"], harness.Events);
+    }
+
+    // While the key is held down Windows repeats the press: the count runs from the first one
+    [Fact]
+    public void Auto_repeat_does_not_reset_the_countdown()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 60);
+
+        harness.Press();
+
+        for (int i = 0; i < 10; i++)
+        {
+            Pump.Pause(TimeSpan.FromMilliseconds(10));
+            harness.Press();
+        }
+
+        Pump.WaitFor(() => harness.Events.Contains("hold-start"), "the hold was announced despite the repeats");
+    }
+
+    [Fact]
+    public void A_quick_press_does_not_count_as_a_hold()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 300);
+
+        harness.Press();
+        harness.Release();
+
+        Pump.Pause(TimeSpan.FromMilliseconds(400));
+
+        Assert.Equal(["tap"], harness.Events);
+    }
+
+    // The interception may be removed with the key still down — by unticking
+    // the setting, for one. The tooltip has to go away in that case
+    [Fact]
+    public void Removing_the_interception_during_a_hold_announces_its_end()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 20);
+
+        harness.Press();
+        Pump.WaitFor(() => harness.Events.Contains("hold-start"), "the hold threshold was passed");
+
+        Pump.Run(harness.Service.Stop);
+
+        Pump.WaitFor(() => harness.Events.Count == 2, "the end of the hold was announced");
+        Assert.Equal(["hold-start", "hold-end"], harness.Events);
+    }
+
+    [Fact]
+    public void Removing_the_interception_without_a_hold_yields_no_events()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 10_000);
+
+        harness.Press();
+        Pump.Run(harness.Service.Stop);
+        Pump.Pause(TimeSpan.FromMilliseconds(50));
+
+        Assert.Empty(harness.Events);
+    }
+
+    // After the interception is removed the hold countdown must not keep running
+    [Fact]
+    public void Removing_the_interception_stops_the_countdown()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 40);
+
+        harness.Press();
+        Pump.Run(harness.Service.Stop);
+
+        Pump.Pause(TimeSpan.FromMilliseconds(120));
+
+        Assert.Empty(harness.Events);
+    }
+
+    [Fact]
+    public void After_the_interception_is_removed_presses_count_from_scratch()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 10_000);
+
+        harness.Press();
+        Pump.Run(harness.Service.Stop);
+
+        harness.Press();
+        harness.Release();
+
+        Pump.WaitFor(() => harness.Events.Count == 1, "a short press after the restart");
+        Assert.Equal(["tap"], harness.Events);
+    }
+
+    // When the application is shutting down nobody is waiting for events anymore
+    [Fact]
+    public void Closing_the_service_sends_out_no_events()
+    {
+        var harness = Harness.Create(holdMilliseconds: 20);
+
+        harness.Press();
+        Pump.WaitFor(() => harness.Events.Contains("hold-start"), "the hold threshold was passed");
+
+        Pump.Run(harness.Service.Dispose);
+        Pump.Pause(TimeSpan.FromMilliseconds(80));
+
+        Assert.Equal(["hold-start"], harness.Events);
+    }
+
+    [Fact]
+    public void The_hold_threshold_is_read_on_every_press()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 10_000);
+
+        harness.Press();
+        harness.Release();
+        Pump.WaitFor(() => harness.Events.Count == 1, "the first press counted as short");
+
+        harness.Settings.CapsLockHoldMilliseconds = 20;
+        harness.Press();
+
+        Pump.WaitFor(() => harness.Events.Contains("hold-start"), "the new threshold took effect");
+    }
+
+    [Fact]
+    public void Before_the_start_there_is_no_interception()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 10_000);
+
+        Assert.False(harness.Service.IsRunning);
+    }
+
+    // The real system hook is installed and removed on the interface thread
+    [Fact]
+    public void The_interception_is_installed_and_removed()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 10_000);
+        Pump.Run(() =>
+        {
+            harness.Service.Start();
+            Assert.True(harness.Service.IsRunning);
+
+            // Starting again breaks nothing
+            harness.Service.Start();
+            Assert.True(harness.Service.IsRunning);
+
+            harness.Service.Stop();
+            Assert.False(harness.Service.IsRunning);
+
+            // Nor does stopping again
+            harness.Service.Stop();
+            Assert.False(harness.Service.IsRunning);
+        });
+    }
+
+    [Fact]
+    public void Closing_removes_the_interception()
+    {
+        var harness = Harness.Create(holdMilliseconds: 10_000);
+
+        Pump.Run(() =>
+        {
+            harness.Service.Start();
+            harness.Service.Dispose();
+
+            Assert.False(harness.Service.IsRunning);
+        });
+    }
+
+    /// <summary>
+    /// A hold is never announced for a key that has already been let go.
+    /// </summary>
+    /// <remarks>
+    /// The countdown started by the press can still be delivered just after the
+    /// release: Windows does not withdraw a WM_TIMER it has already posted. Taken at
+    /// face value it turned a tap into a hold — the popup came up showing the layout
+    /// the tap was about to change away from, and the switch followed it.
+    ///
+    /// The tick is driven straight in here rather than waited for, because the point is
+    /// the one ordering a real clock will not reproduce on demand.
+    /// </remarks>
+    [Fact]
+    public void A_hold_is_not_announced_after_the_key_has_been_released()
+    {
+        using var harness = Harness.Create(holdMilliseconds: 10_000);
+
+        harness.Press();
+        harness.Release();
+
+        harness.ForceHoldTick();
+        Pump.Drain();
+
+        Assert.Equal(["tap"], harness.Events);
+    }
+
+    /// <summary>
+    /// The service together with its settings and the list of events that happened.
+    /// </summary>
+    private sealed class Harness : IDisposable
+    {
+        private Harness(CapsLockHotkeyService service, AppSettings settings)
+        {
+            Service = service;
+            Settings = settings;
+        }
+
+        internal CapsLockHotkeyService Service { get; }
+
+        internal AppSettings Settings { get; }
+
+        /// <summary>Events arrive from the interface thread and are read by the test thread.</summary>
+        internal ConcurrentQueue<string> Events { get; } = new();
+
+        internal static Harness Create(double holdMilliseconds)
+        {
+            var settings = new AppSettings { CapsLockHoldMilliseconds = holdMilliseconds };
+
+            // Events reach their subscribers the way they do in the agent: posted back
+            // to the message loop, after the hook procedure has returned
+            CapsLockHotkeyService service =
+                Pump.Run(() => new CapsLockHotkeyService(settings, Pump.Post));
+            var harness = new Harness(service, settings);
+
+            service.Tapped += (_, _) => harness.Events.Enqueue("tap");
+            service.HoldStarted += (_, _) => harness.Events.Enqueue("hold-start");
+            service.HoldEnded += (_, _) => harness.Events.Enqueue("hold-end");
+
+            return harness;
+        }
+
+        internal void Press() => Pump.Run(() => Service.HandleKeyEvent(CapsLock, isKeyDown: true));
+
+        internal void Release() => Pump.Run(() => Service.HandleKeyEvent(CapsLock, isKeyDown: false));
+
+        /// <summary>Delivers the hold countdown by hand, the way a late WM_TIMER does.</summary>
+        internal void ForceHoldTick() => Pump.Run(Service.HandleHoldElapsed);
+
+        public void Dispose() => Pump.Run(Service.Dispose);
+    }
+}

CursorLang.Agent.Tests/Services/LayoutPopupServiceTests.cs

@@ -0,0 +1,201 @@
+using CursorLang.Agent.Services;
+using CursorLang.Core.Models;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Agent.Tests.Services;
+
+/// <summary>
+/// The lifetime of the tooltip. Its timer ticks on the message loop,
+/// so everything happens on the pump thread as well.
+/// </summary>
+public sealed class LayoutPopupServiceTests
+{
+    private static readonly KeyboardLayout Russian = KeyboardLayout.FromLocaleId(0x0419);
+    private static readonly KeyboardLayout English = KeyboardLayout.FromLocaleId(0x0409);
+
+    [Fact]
+    public void Showing_puts_out_the_short_name_of_the_layout()
+    {
+        var window = new FakeLayoutPopupWindow();
+        var settings = new AppSettings { DurationMilliseconds = 10_000 };
+
+        Pump.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, settings);
+            service.Show(Russian);
+        });
+
+        Assert.Equal("RU", window.ShownText);
+        Assert.Equal(1, window.ShowCalls);
+    }
+
+    [Fact]
+    public void The_tooltip_goes_away_once_its_time_is_up()
+    {
+        var window = new FakeLayoutPopupWindow();
+        var settings = new AppSettings { DurationMilliseconds = 30 };
+
+        Pump.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, settings);
+            service.Show(Russian);
+
+            Assert.Equal(0, window.HideCalls);
+            Pump.WaitFor(() => window.HideCalls == 1, "the tooltip hid itself");
+        });
+    }
+
+    // The duration is read on every show: it is edited in the settings on the fly
+    [Fact]
+    public void A_new_duration_takes_effect_from_the_next_show()
+    {
+        var window = new FakeLayoutPopupWindow();
+        var settings = new AppSettings { DurationMilliseconds = 10_000 };
+
+        Pump.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, settings);
+            service.Show(Russian);
+
+            settings.DurationMilliseconds = 30;
+            service.Show(English);
+
+            Pump.WaitFor(() => window.HideCalls == 1, "the tooltip hid by the new duration");
+        });
+    }
+
+    // Quick switching must not cut the tooltip off mid-word
+    [Fact]
+    public void Showing_again_extends_the_time_on_screen()
+    {
+        var window = new FakeLayoutPopupWindow();
+        var settings = new AppSettings { DurationMilliseconds = 60 };
+
+        Pump.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, settings);
+
+            for (int i = 0; i < 5; i++)
+            {
+                service.Show(i % 2 == 0 ? Russian : English);
+                Pump.Pause(TimeSpan.FromMilliseconds(20));
+                Assert.Equal(0, window.HideCalls);
+            }
+
+            Pump.WaitFor(() => window.HideCalls == 1, "the tooltip hid after the last show");
+        });
+    }
+
+    [Fact]
+    public void A_show_until_hidden_does_not_hide_by_itself()
+    {
+        var window = new FakeLayoutPopupWindow();
+        var settings = new AppSettings { DurationMilliseconds = 20 };
+
+        Pump.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, settings);
+            service.ShowUntilHidden(Russian);
+
+            Pump.Pause(TimeSpan.FromMilliseconds(80));
+
+            Assert.Equal(1, window.ShowCalls);
+            Assert.Equal(0, window.HideCalls);
+
+            service.Hide();
+            Assert.Equal(1, window.HideCalls);
+        });
+    }
+
+    // A show until hidden on top of an ordinary one also cancels the countdown
+    [Fact]
+    public void A_show_until_hidden_stops_a_running_countdown()
+    {
+        var window = new FakeLayoutPopupWindow();
+        var settings = new AppSettings { DurationMilliseconds = 30 };
+
+        Pump.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, settings);
+            service.Show(Russian);
+            service.ShowUntilHidden(English);
+
+            Pump.Pause(TimeSpan.FromMilliseconds(100));
+
+            Assert.Equal(0, window.HideCalls);
+            Assert.Equal("EN", window.ShownText);
+        });
+    }
+
+    [Fact]
+    public void Hiding_cancels_a_running_countdown()
+    {
+        var window = new FakeLayoutPopupWindow();
+        var settings = new AppSettings { DurationMilliseconds = 30 };
+
+        Pump.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, settings);
+            service.Show(Russian);
+            service.Hide();
+
+            Pump.Pause(TimeSpan.FromMilliseconds(100));
+
+            // There must be no second hide from the timer
+            Assert.Equal(1, window.HideCalls);
+        });
+    }
+
+    [Fact]
+    public void Closing_the_service_closes_the_window()
+    {
+        var window = new FakeLayoutPopupWindow();
+        var settings = new AppSettings();
+
+        Pump.Run(() =>
+        {
+            var service = new LayoutPopupService(window, settings);
+            service.Show(Russian);
+            service.Dispose();
+
+            Pump.Pause(TimeSpan.FromMilliseconds(50));
+        });
+
+        Assert.Equal(1, window.CloseCalls);
+    }
+
+    [Fact]
+    public void After_the_service_is_closed_the_timer_stays_silent()
+    {
+        var window = new FakeLayoutPopupWindow();
+        var settings = new AppSettings { DurationMilliseconds = 20 };
+
+        Pump.Run(() =>
+        {
+            var service = new LayoutPopupService(window, settings);
+            service.Show(Russian);
+            service.Dispose();
+
+            Pump.Pause(TimeSpan.FromMilliseconds(80));
+
+            Assert.Equal(0, window.HideCalls);
+        });
+    }
+
+    [Fact]
+    public void Showing_always_comes_before_hiding()
+    {
+        var window = new FakeLayoutPopupWindow();
+        var settings = new AppSettings { DurationMilliseconds = 20 };
+
+        Pump.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, settings);
+            service.Show(Russian);
+
+            Pump.WaitFor(() => window.HideCalls == 1, "the tooltip hid itself");
+
+            Assert.Equal(["show", "hide"], window.Calls);
+        });
+    }
+}

CursorLang.Agent.Tests/Windows/SettingsSignalTests.cs

@@ -0,0 +1,57 @@
+using CursorLang.Agent.Windows;
+using CursorLang.Core.Services;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Agent.Tests.Windows;
+
+/// <summary>
+/// The one thing the settings window says to the agent.
+/// </summary>
+/// <remarks>
+/// Both halves of it live apart — the message and the window class name in Core, the
+/// window that answers in the agent — and nothing but a matching pair makes it work.
+/// A renamed class or a renamed message would leave the agent showing yesterday's
+/// settings until it is restarted, and nothing else would complain.
+///
+/// The agent's own window is used rather than a stand-in: what is being checked is
+/// that the window the agent really creates is the one the message reaches.
+/// </remarks>
+public sealed class SettingsSignalTests
+{
+    [Fact]
+    public void The_signal_reaches_the_agents_window()
+    {
+        var delivered = 0;
+
+        using AgentWindow window = Pump.Run(() =>
+        {
+            var created = new AgentWindow();
+            created.AddFilter((message, _, _) =>
+            {
+                if (message != SettingsSignal.Message)
+                {
+                    return false;
+                }
+
+                delivered++;
+                return true;
+            });
+
+            return created;
+        });
+
+        Pump.Run(SettingsSignal.NotifyAgent);
+        Pump.Drain();
+
+        Assert.Equal(1, delivered);
+    }
+
+    // Nobody is listening, and that is a normal state of affairs: the settings window
+    // works perfectly well with no agent behind it
+    [Fact]
+    public void Signalling_with_no_agent_running_passes_without_consequence()
+    {
+        Pump.Run(SettingsSignal.NotifyAgent);
+        Pump.Drain();
+    }
+}

CursorLang.Agent/Agent.cs

@@ -0,0 +1,136 @@
+using System.ComponentModel;
+using CursorLang.Agent.Services;
+using CursorLang.Agent.Windows;
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+using CursorLang.Core.Threading;
+
+namespace CursorLang.Agent;
+
+/// <summary>
+/// The background half of CursorLang: the hook, the layout polling, the popup and the
+/// tray icon, with a Win32 message loop underneath and no WPF anywhere.
+/// </summary>
+/// <remarks>
+/// The services are wired by hand rather than through a container, and that is a
+/// decision rather than an omission: the whole point of this process is how little it
+/// weighs, and a container is a megabyte of assembly and a graph of reflection on the
+/// way to the same object. There are ten of them and they are all listed here.
+/// </remarks>
+internal sealed class Agent : IDisposable
+{
+    private readonly SingleInstanceGate _gate;
+    private readonly AgentWindow _window;
+    private readonly SettingsService _settingsService;
+    private readonly AppSettings _settings;
+    private readonly LocalizationService _localization;
+    private readonly NativePopupWindow _popupWindow;
+    private readonly LayoutPopupService _popupService;
+    private readonly KeyboardLayoutService _layoutService;
+    private readonly CapsLockHotkeyService _hotkeyService;
+    private readonly LayoutNotificationCoordinator _notifications;
+    private readonly CapsLockSwitchCoordinator _capsLock;
+    private readonly NativeTrayIcon _tray;
+
+    internal Agent(SingleInstanceGate gate)
+    {
+        _gate = gate;
+
+        _window = new AgentWindow();
+        _window.AddFilter(OnWindowMessage);
+
+        _settingsService = new SettingsService();
+        _settings = _settingsService.Load();
+
+        _localization = new LocalizationService { CurrentLanguage = _settings.Language };
+        _settings.PropertyChanged += OnSettingsChanged;
+
+        _popupWindow = new NativePopupWindow(_settings);
+        _popupService = new LayoutPopupService(_popupWindow, _settings);
+
+        _layoutService = new KeyboardLayoutService(new KeyboardLayoutOptions());
+        _hotkeyService = new CapsLockHotkeyService(_settings, _window.Post);
+
+        _notifications = new LayoutNotificationCoordinator(_layoutService, _popupService);
+        _capsLock = new CapsLockSwitchCoordinator(_hotkeyService, _layoutService, _popupService, _settings);
+
+        _tray = new NativeTrayIcon(_window, _localization);
+    }
+
+    /// <summary>Starts everything and pumps messages until the user asks to quit.</summary>
+    internal int Run(bool automatic)
+    {
+        _gate.ActivationRequested += OnActivationRequested;
+
+        _tray.OpenRequested += OnOpenRequested;
+        _tray.ExitRequested += OnExitRequested;
+
+        bool hasTray = _tray.Install();
+
+        _notifications.Start();
+        _capsLock.Start();
+
+        if (!automatic || !hasTray)
+        {
+            SettingsLauncher.Open();
+        }
+
+        return MessageLoop.Run();
+    }
+
+    public void Dispose()
+    {
+        _gate.ActivationRequested -= OnActivationRequested;
+
+        _tray.OpenRequested -= OnOpenRequested;
+        _tray.ExitRequested -= OnExitRequested;
+
+        _settings.PropertyChanged -= OnSettingsChanged;
+
+        _capsLock.Dispose();
+        _notifications.Dispose();
+        _hotkeyService.Dispose();
+        _layoutService.Dispose();
+        _popupService.Dispose();
+        _settingsService.Dispose();
+        _tray.Dispose();
+        _window.Dispose();
+    }
+
+    // The settings window has written the file and says so. The write was a single
+    // atomic move, so there is nothing to wait for and nothing half-written to read
+    private bool OnWindowMessage(uint message, IntPtr wParam, IntPtr lParam)
+    {
+        if (message != SettingsSignal.Message)
+        {
+            return false;
+        }
+
+        _settingsService.Reload();
+        return true;
+    }
+
+    private void OnSettingsChanged(object? sender, PropertyChangedEventArgs e)
+    {
+        if (e.PropertyName == nameof(AppSettings.Language))
+        {
+            _localization.CurrentLanguage = _settings.Language;
+        }
+    }
+
+    // A second launch of the agent, from the Start menu for instance. The one already
+    // running answers the way the user expects a second launch to be answered
+    private void OnActivationRequested(object? sender, EventArgs e) =>
+        _window.Post(() => SettingsLauncher.Open());
+
+    private void OnOpenRequested(object? sender, EventArgs e) => SettingsLauncher.Open();
+
+    // "Exit" means the application, not just the background half of it. A settings
+    // window left open would outlive the tray icon it was opened from, so it is asked
+    // to close first — it may be the very window the user is looking at
+    private void OnExitRequested(object? sender, EventArgs e)
+    {
+        SettingsCloseSignal.RequestClose();
+        _window.Quit();
+    }
+}

CursorLang.Agent/CursorLang.Agent.csproj

@@ -0,0 +1,54 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <OutputType>WinExe</OutputType>
+        <TargetFramework>net10.0-windows10.0.19041.0</TargetFramework>
+        <SupportedOSPlatformVersion>10.0.17763.0</SupportedOSPlatformVersion>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
+        <GenerateDocumentationFile>true</GenerateDocumentationFile>
+        <NoWarn>$(NoWarn);CS1591</NoWarn>
+        <RootNamespace>CursorLang.Agent</RootNamespace>
+        <AssemblyName>CursorLang</AssemblyName>
+        <ApplicationManifest>app.manifest</ApplicationManifest>
+        <ApplicationIcon>..\CursorLang.Core\Resources\CursorLang.ico</ApplicationIcon>
+        <PlatformTarget>AnyCPU</PlatformTarget>
+        <RuntimeIdentifiers>win-x64</RuntimeIdentifiers>
+        <SelfContained Condition="'$(RuntimeIdentifier)' != ''">true</SelfContained>
+        <PublishTrimmed>false</PublishTrimmed>
+        <PublishReadyToRun Condition="'$(RuntimeIdentifier)' != ''">true</PublishReadyToRun>
+        <Version>1.0.0</Version>
+        <AssemblyVersion>1.0.0.0</AssemblyVersion>
+        <FileVersion>1.0.0.0</FileVersion>
+        <Product>CursorLang</Product>
+        <Company>Aleksandr Neichev</Company>
+        <Description>Shows the keyboard layout at the cursor</Description>
+        <Copyright>Copyright (c) 2026</Copyright>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\CursorLang.Core\CursorLang.Core.csproj"/>
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\CursorLang.Settings\CursorLang.Settings.csproj"
+                          ReferenceOutputAssembly="false"
+                          Private="false"/>
+    </ItemGroup>
+
+    <Target Name="PlaceTheSettingsWindowBesideTheAgent" AfterTargets="Build">
+        <ItemGroup>
+            <SettingsOutput Include="..\CursorLang.Settings\bin\$(Configuration)\$(TargetFramework)\**\*"/>
+        </ItemGroup>
+
+        <Copy SourceFiles="@(SettingsOutput)"
+              DestinationFolder="$(OutDir)%(RecursiveDir)"
+              SkipUnchangedFiles="true"/>
+    </Target>
+
+    <ItemGroup>
+        <InternalsVisibleTo Include="CursorLang.Agent.Tests"/>
+    </ItemGroup>
+
+</Project>

CursorLang.Agent/Interop/GdiNative.cs

@@ -0,0 +1,161 @@
+using System.Drawing;
+using System.Runtime.InteropServices;
+using CursorLang.Core.Interop;
+
+namespace CursorLang.Agent.Interop;
+
+/// <summary>
+/// Plain GDI: a font, text, and an off-screen bitmap to draw them into.
+/// </summary>
+/// <remarks>
+/// GDI rather than GDI+ on purpose. <c>System.Drawing.Common</c> would make the drawing
+/// code shorter, but it is a separate assembly with a native GDI+ library behind it, and
+/// how little this process weighs is the entire reason it exists apart from the window.
+/// Everything the popup needs — one rounded rectangle and one line of text — GDI can do
+/// on its own, and it draws the text with ClearType, exactly as Windows does everywhere else.
+/// </remarks>
+internal static class GdiNative
+{
+    internal const int TRANSPARENT = 1;
+
+    internal const uint DT_SINGLELINE = 0x00000020;
+    internal const uint DT_CENTER = 0x00000001;
+    internal const uint DT_VCENTER = 0x00000004;
+    internal const uint DT_CALCRECT = 0x00000400;
+    internal const uint DT_NOPREFIX = 0x00000800;
+    internal const uint DT_NOCLIP = 0x00000100;
+
+    private const int DEFAULT_CHARSET = 1;
+    private const int OUT_TT_PRECIS = 4;
+    private const int CLIP_DEFAULT_PRECIS = 0;
+    private const int CLEARTYPE_QUALITY = 5;
+    private const int DEFAULT_PITCH = 0;
+
+    /// <summary>
+    /// The face the popup is written in.
+    /// </summary>
+    /// <remarks>
+    /// WPF is asked for "Segoe UI" at FontWeight SemiBold and resolves that to the
+    /// seguisb.ttf face. To GDI that face is a family of its own — "Segoe UI Semibold" —
+    /// and asking for the "Segoe UI" family at weight 600 lands on Bold instead, which
+    /// is visibly heavier. So the family is named outright and the weight is left to the
+    /// mapper: the family has one member and no synthetic emboldening happens.
+    /// </remarks>
+    private const string SemiBoldFace = "Segoe UI Semibold";
+
+    private const int FW_DONTCARE = 0;
+
+    /// <summary>
+    /// A font of the given size in physical pixels.
+    /// </summary>
+    /// <remarks>
+    /// The size in the settings is in WPF units, that is 1/96 inch, while GDI counts
+    /// pixels — hence the multiplication by the monitor scale. The height is negative:
+    /// that asks for the em size rather than the cell height, which is what a font size
+    /// means everywhere else.
+    /// </remarks>
+    internal static IntPtr CreateFont(double wpfFontSize, double scale)
+    {
+        var height = (int)Math.Round(wpfFontSize * scale);
+
+        return CreateFontW(
+            -height, 0, 0, 0, FW_DONTCARE,
+            false, false, false,
+            DEFAULT_CHARSET, OUT_TT_PRECIS, CLIP_DEFAULT_PRECIS, CLEARTYPE_QUALITY, DEFAULT_PITCH,
+            SemiBoldFace);
+    }
+
+    /// <summary>The size of a single line of text with the font selected into the context.</summary>
+    internal static Size MeasureText(IntPtr deviceContext, string text)
+    {
+        var bounds = default(PopupWindowNative.Rect);
+        DrawText(deviceContext, text, text.Length, ref bounds,
+            DT_CALCRECT | DT_SINGLELINE | DT_NOPREFIX);
+
+        return new Size(bounds.Right - bounds.Left, bounds.Bottom - bounds.Top);
+    }
+
+    /// <summary>A colour as GDI wants it: 0x00BBGGRR, the alpha carried elsewhere.</summary>
+    internal static uint ToColorRef(Color color) =>
+        (uint)(color.R | (color.G << 8) | (color.B << 16));
+
+    [DllImport("gdi32.dll", CharSet = CharSet.Unicode, EntryPoint = "CreateFontW")]
+    private static extern IntPtr CreateFontW(int cHeight, int cWidth, int cEscapement, int cOrientation,
+        int cWeight, bool bItalic, bool bUnderline, bool bStrikeOut,
+        int iCharSet, int iOutPrecision, int iClipPrecision, int iQuality, int iPitchAndFamily,
+        string pszFaceName);
+
+    /// <summary>
+    /// A 32-bit surface to draw the popup into before anyone can see it.
+    /// </summary>
+    /// <remarks>
+    /// Top-down — a negative height — so that the first row of <paramref name="bits"/>
+    /// is the top row of the picture and the alpha fixing up afterwards can walk the
+    /// memory straight through.
+    /// </remarks>
+    internal static IntPtr CreateSurface(IntPtr deviceContext, int width, int height, out IntPtr bits)
+    {
+        var header = new BitmapInfoHeader
+        {
+            biSize = Marshal.SizeOf<BitmapInfoHeader>(),
+            biWidth = width,
+            biHeight = -height,
+            biPlanes = 1,
+            biBitCount = 32,
+            biCompression = BI_RGB,
+        };
+
+        return CreateDIBSection(deviceContext, ref header, DIB_RGB_COLORS, out bits, IntPtr.Zero, 0);
+    }
+
+    private const uint BI_RGB = 0;
+    private const uint DIB_RGB_COLORS = 0;
+
+    [DllImport("gdi32.dll")]
+    internal static extern IntPtr CreateCompatibleDC(IntPtr hdc);
+
+    [DllImport("gdi32.dll")]
+    internal static extern bool DeleteDC(IntPtr hdc);
+
+    [DllImport("gdi32.dll")]
+    private static extern IntPtr CreateDIBSection(IntPtr hdc, ref BitmapInfoHeader header, uint usage,
+        out IntPtr bits, IntPtr section, uint offset);
+
+    [DllImport("gdi32.dll")]
+    internal static extern IntPtr SelectObject(IntPtr hdc, IntPtr h);
+
+    [DllImport("gdi32.dll")]
+    internal static extern bool DeleteObject(IntPtr ho);
+
+    [DllImport("gdi32.dll")]
+    internal static extern int SetBkMode(IntPtr hdc, int mode);
+
+    [DllImport("gdi32.dll")]
+    internal static extern uint SetTextColor(IntPtr hdc, uint color);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "DrawTextW")]
+    internal static extern int DrawText(IntPtr hdc, string lpchText, int cchText,
+        ref PopupWindowNative.Rect lprc, uint format);
+
+    [DllImport("user32.dll")]
+    internal static extern IntPtr GetDC(IntPtr hWnd);
+
+    [DllImport("user32.dll")]
+    internal static extern int ReleaseDC(IntPtr hWnd, IntPtr hDC);
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct BitmapInfoHeader
+    {
+        public int biSize;
+        public int biWidth;
+        public int biHeight;
+        public short biPlanes;
+        public short biBitCount;
+        public uint biCompression;
+        public uint biSizeImage;
+        public int biXPelsPerMeter;
+        public int biYPelsPerMeter;
+        public uint biClrUsed;
+        public uint biClrImportant;
+    }
+}

CursorLang.Agent/Interop/MenuNative.cs

@@ -0,0 +1,98 @@
+using System.Runtime.InteropServices;
+using CursorLang.Core.Interop;
+
+namespace CursorLang.Agent.Interop;
+
+/// <summary>
+/// The system context menu — the one the tray icon raises.
+/// </summary>
+/// <remarks>
+/// A menu built this way is drawn by Windows, so the theme and the language chosen in
+/// the application no longer reach it. That is the accepted price of leaving WPF: a WPF
+/// <c>ContextMenu</c> costs the whole rendering stack in the background process.
+/// </remarks>
+internal static class MenuNative
+{
+    private const uint MF_STRING = 0x00000000;
+    private const uint MF_SEPARATOR = 0x00000800;
+    private const uint MF_GRAYED = 0x00000001;
+
+    private const uint TPM_LEFTALIGN = 0x0000;
+    private const uint TPM_RIGHTBUTTON = 0x0002;
+    private const uint TPM_RETURNCMD = 0x0100;
+
+    /// <summary>An item of the menu being built.</summary>
+    /// <param name="Id">What <see cref="Track"/> gives back when the item is chosen.</param>
+    /// <param name="Caption">The text, or <c>null</c> for a separator.</param>
+    /// <param name="IsEnabled">A greyed item is shown but cannot be chosen.</param>
+    internal readonly record struct Item(int Id, string? Caption, bool IsEnabled = true)
+    {
+        internal static Item Separator => new(0, null);
+    }
+
+    /// <summary>
+    /// Raises the menu at a screen point and returns the identifier of the chosen item,
+    /// or zero when the user dismissed it.
+    /// </summary>
+    /// <remarks>
+    /// <c>TPM_RETURNCMD</c> means the answer comes back from the call itself instead of
+    /// as a <c>WM_COMMAND</c> later, which keeps the whole menu in one place. The call
+    /// does not return until the user is done with the menu — that is how a modal menu
+    /// works, and the message loop keeps running inside it.
+    ///
+    /// The window is brought to the foreground first and poked with an empty message
+    /// afterwards: without the first the menu never closes on a click elsewhere, and
+    /// without the second it stays on screen after the choice is made. Both are
+    /// long-standing quirks of a menu owned by a window the user cannot see.
+    /// </remarks>
+    internal static int Track(IntPtr owner, PopupWindowNative.Point at, IReadOnlyList<Item> items)
+    {
+        IntPtr menu = CreatePopupMenu();
+        if (menu == IntPtr.Zero)
+        {
+            return 0;
+        }
+
+        try
+        {
+            foreach (Item item in items)
+            {
+                if (item.Caption is null)
+                {
+                    AppendMenu(menu, MF_SEPARATOR, IntPtr.Zero, null);
+                    continue;
+                }
+
+                uint flags = MF_STRING | (item.IsEnabled ? 0 : MF_GRAYED);
+                AppendMenu(menu, flags, new IntPtr(item.Id), item.Caption);
+            }
+
+            TrayIconNative.BringToForeground(owner);
+
+            int chosen = TrackPopupMenuEx(
+                menu, TPM_LEFTALIGN | TPM_RIGHTBUTTON | TPM_RETURNCMD,
+                at.X, at.Y, owner, IntPtr.Zero);
+
+            WindowNative.PostMessage(owner, WindowNative.WM_NULL, IntPtr.Zero, IntPtr.Zero);
+
+            return chosen;
+        }
+        finally
+        {
+            DestroyMenu(menu);
+        }
+    }
+
+    [DllImport("user32.dll")]
+    private static extern IntPtr CreatePopupMenu();
+
+    [DllImport("user32.dll")]
+    private static extern bool DestroyMenu(IntPtr hMenu);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "AppendMenuW")]
+    private static extern bool AppendMenu(IntPtr hMenu, uint uFlags, IntPtr uIDNewItem, string? lpNewItem);
+
+    [DllImport("user32.dll")]
+    private static extern int TrackPopupMenuEx(IntPtr hMenu, uint uFlags, int x, int y,
+        IntPtr hwnd, IntPtr lptpm);
+}

CursorLang.Agent/Interop/TrayIconNative.cs

@@ -0,0 +1,229 @@
+using System.Runtime.InteropServices;
+using CursorLang.Core.Interop;
+
+namespace CursorLang.Agent.Interop;
+
+/// <summary>
+/// Win32 API for the notification area: the icon itself, the messages it sends
+/// and the icon image taken from the executable.
+/// </summary>
+/// <remarks>
+/// The icon is asked for at version 4 of the protocol. It is the only version that
+/// reports a request for the context menu as such — by the keyboard as well as by
+/// the right button — and passes the point of the click along with it. The
+/// notification then arrives in the low word of <c>lParam</c>, and the point in
+/// <c>wParam</c>, which is the opposite of the earlier versions.
+/// </remarks>
+internal static class TrayIconNative
+{
+    /// <summary>The message the icon sends to its window. WM_APP is free for the app.</summary>
+    internal const int CallbackMessage = 0x8000 + 1;
+
+    /// <summary>The user chose the icon: a click of the left button or Enter on it.</summary>
+    internal const int SelectNotification = 0x0400;
+
+    /// <summary>The same by the space bar — Windows tells the two apart.</summary>
+    internal const int KeySelectNotification = 0x0403;
+
+    /// <summary>The context menu is asked for: the right button or the menu key.</summary>
+    internal const int ContextMenuNotification = 0x007B;
+
+    private const int NIM_ADD = 0x00000000;
+    private const int NIM_DELETE = 0x00000002;
+    private const int NIM_SETVERSION = 0x00000004;
+
+    private const uint NIF_MESSAGE = 0x00000001;
+    private const uint NIF_ICON = 0x00000002;
+    private const uint NIF_TIP = 0x00000004;
+    private const uint NIF_SHOWTIP = 0x00000080;
+
+    private const uint NotifyIconVersion4 = 4;
+
+    private const uint IMAGE_ICON = 1;
+    private const uint LR_DEFAULTCOLOR = 0x00000000;
+
+    private const int SM_CXSMICON = 49;
+    private const int SM_CYSMICON = 50;
+
+    /// <summary>The resource the .NET build puts the application icon under.</summary>
+    private const int ApplicationIconResource = 32512;
+
+    /// <summary>
+    /// Explorer says it has restarted this way. The icons of every application are
+    /// gone by then and have to be put back.
+    /// </summary>
+    internal static int TaskbarCreatedMessage { get; } = RegisterWindowMessage("TaskbarCreated");
+
+    /// <summary>Puts the icon into the notification area.</summary>
+    internal static bool Add(IntPtr window, int id, IntPtr icon, string tooltip)
+    {
+        NotifyIconData data = Describe(window, id, icon, tooltip);
+
+        if (!Shell_NotifyIcon(NIM_ADD, ref data))
+        {
+            return false;
+        }
+
+        // The version is asked for after the icon is added and applies to it alone
+        data.uVersion = NotifyIconVersion4;
+        Shell_NotifyIcon(NIM_SETVERSION, ref data);
+
+        return true;
+    }
+
+    /// <summary>Takes the icon away. A forgotten icon stays in the tray until hovered.</summary>
+    internal static void Remove(IntPtr window, int id)
+    {
+        var data = new NotifyIconData
+        {
+            cbSize = Marshal.SizeOf<NotifyIconData>(),
+            hWnd = window,
+            uID = (uint)id,
+        };
+
+        Shell_NotifyIcon(NIM_DELETE, ref data);
+    }
+
+    /// <summary>
+    /// The icon of the application at the size the tray asks for.
+    /// </summary>
+    /// <remarks>
+    /// The image comes from the executable itself, so the tray shows what the user
+    /// sees in Explorer. The build puts the icon under the standard resource; should
+    /// it end up elsewhere, the first icon of the file is taken, and failing that —
+    /// the icon Windows gives to an application without one. An icon is needed
+    /// either way: without it the tray shows an empty spot.
+    /// </remarks>
+    internal static IntPtr LoadApplicationIcon()
+    {
+        int width = GetSystemMetrics(SM_CXSMICON);
+        int height = GetSystemMetrics(SM_CYSMICON);
+
+        IntPtr icon = LoadImage(
+            GetModuleHandle(null), ApplicationIconResource, IMAGE_ICON, width, height, LR_DEFAULTCOLOR);
+
+        if (icon == IntPtr.Zero && Environment.ProcessPath is { Length: > 0 } path)
+        {
+            icon = ExtractIconEx(path, 0, out IntPtr large, out IntPtr small, 1) > 0 ? small : IntPtr.Zero;
+
+            if (large != IntPtr.Zero)
+            {
+                DestroyIcon(large);
+            }
+        }
+
+        return icon != IntPtr.Zero ? icon : LoadIcon(IntPtr.Zero, ApplicationIconResource);
+    }
+
+    /// <summary>Releases an icon loaded by <see cref="LoadApplicationIcon"/>.</summary>
+    internal static void ReleaseIcon(IntPtr icon)
+    {
+        if (icon != IntPtr.Zero)
+        {
+            DestroyIcon(icon);
+        }
+    }
+
+    /// <summary>The notification the icon has sent: it sits in the low word of lParam.</summary>
+    internal static int NotificationOf(IntPtr lParam) => (int)(lParam.ToInt64() & 0xFFFF);
+
+    /// <summary>
+    /// The point of the click. Version 4 of the protocol reports it in screen pixels
+    /// in <c>wParam</c> — exactly what <c>TrackPopupMenuEx</c> expects.
+    /// </summary>
+    internal static PopupWindowNative.Point PointOf(IntPtr wParam) => new()
+    {
+        X = (short)(wParam.ToInt64() & 0xFFFF),
+        Y = (short)((wParam.ToInt64() >> 16) & 0xFFFF),
+    };
+
+    /// <summary>
+    /// Brings the window to the foreground.
+    /// </summary>
+    /// <remarks>
+    /// Windows takes a menu down when its owner window loses the foreground. A tray
+    /// icon belongs to a window that is never shown, so the foreground has to be
+    /// asked for by hand — otherwise the menu stays on screen after the user has
+    /// clicked past it.
+    /// </remarks>
+    internal static void BringToForeground(IntPtr window) => SetForegroundWindow(window);
+
+    private static NotifyIconData Describe(IntPtr window, int id, IntPtr icon, string tooltip) => new()
+    {
+        cbSize = Marshal.SizeOf<NotifyIconData>(),
+        hWnd = window,
+        uID = (uint)id,
+        uFlags = NIF_MESSAGE | NIF_ICON | NIF_TIP | NIF_SHOWTIP,
+        uCallbackMessage = CallbackMessage,
+        hIcon = icon,
+        szTip = tooltip,
+    };
+
+    [DllImport("shell32.dll", CharSet = CharSet.Unicode)]
+    private static extern bool Shell_NotifyIcon(int dwMessage, ref NotifyIconData lpData);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode)]
+    private static extern int RegisterWindowMessage(string lpString);
+
+    [DllImport("user32.dll")]
+    private static extern IntPtr LoadImage(IntPtr hInst, IntPtr name, uint type, int cx, int cy, uint fuLoad);
+
+    private static IntPtr LoadImage(IntPtr hInst, int resource, uint type, int cx, int cy, uint fuLoad) =>
+        LoadImage(hInst, new IntPtr(resource), type, cx, cy, fuLoad);
+
+    [DllImport("user32.dll")]
+    private static extern IntPtr LoadIcon(IntPtr hInstance, IntPtr lpIconName);
+
+    private static IntPtr LoadIcon(IntPtr hInstance, int resource) =>
+        LoadIcon(hInstance, new IntPtr(resource));
+
+    [DllImport("shell32.dll", CharSet = CharSet.Unicode)]
+    private static extern int ExtractIconEx(string lpszFile, int nIconIndex,
+        out IntPtr phiconLarge, out IntPtr phiconSmall, int nIcons);
+
+    [DllImport("user32.dll")]
+    private static extern bool DestroyIcon(IntPtr hIcon);
+
+    [DllImport("user32.dll")]
+    private static extern int GetSystemMetrics(int nIndex);
+
+    [DllImport("user32.dll")]
+    private static extern bool SetForegroundWindow(IntPtr hWnd);
+
+    [DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
+    private static extern IntPtr GetModuleHandle(string? lpModuleName);
+
+    /// <summary>
+    /// NOTIFYICONDATAW. The whole structure is described even though only its first
+    /// half is used: Windows reads its size and refuses one it does not know.
+    /// </summary>
+    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
+    private struct NotifyIconData
+    {
+        public int cbSize;
+        public IntPtr hWnd;
+        public uint uID;
+        public uint uFlags;
+        public int uCallbackMessage;
+        public IntPtr hIcon;
+
+        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 128)]
+        public string szTip;
+
+        public uint dwState;
+        public uint dwStateMask;
+
+        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 256)]
+        public string szInfo;
+
+        /// <summary>A timeout in the older versions and the protocol version here.</summary>
+        public uint uVersion;
+
+        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 64)]
+        public string szInfoTitle;
+
+        public uint dwInfoFlags;
+        public Guid guidItem;
+        public IntPtr hBalloonIcon;
+    }
+}

CursorLang.Agent/Interop/WindowNative.cs

@@ -0,0 +1,180 @@
+using System.Runtime.InteropServices;
+using CursorLang.Core.Interop;
+
+namespace CursorLang.Agent.Interop;
+
+/// <summary>
+/// The Win32 pieces a window needs when there is no framework to make one:
+/// the class, the window itself, the message loop.
+/// </summary>
+internal static class WindowNative
+{
+    /// <summary>The window procedure. Windows keeps the only reference to it.</summary>
+    internal delegate IntPtr WindowProc(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam);
+
+    internal const int WS_POPUP = unchecked((int)0x80000000);
+
+    internal const int WS_EX_LAYERED = 0x00080000;
+    internal const int WS_EX_TOOLWINDOW = 0x00000080;
+    internal const int WS_EX_NOACTIVATE = 0x08000000;
+    internal const int WS_EX_TRANSPARENT = 0x00000020;
+    internal const int WS_EX_TOPMOST = 0x00000008;
+
+    internal const int SW_HIDE = 0;
+    internal const int SW_SHOWNOACTIVATE = 4;
+
+    internal const uint WM_DESTROY = 0x0002;
+    internal const uint WM_CLOSE = 0x0010;
+    internal const uint WM_QUIT = 0x0012;
+    internal const uint WM_NULL = 0x0000;
+    internal const uint WM_ENDSESSION = 0x0016;
+
+    /// <summary>WM_APP and up belong to the application; the tray takes WM_APP + 1.</summary>
+    internal const uint WM_APP = 0x8000;
+
+    /// <summary>
+    /// Registers a window class. A class already there is not an error: the name is
+    /// unique per window kind, and a second agent in the same process would meet its
+    /// own registration.
+    /// </summary>
+    internal static void RegisterClass(string className, WindowProc windowProc)
+    {
+        var description = new WindowClass
+        {
+            cbSize = Marshal.SizeOf<WindowClass>(),
+            lpfnWndProc = windowProc,
+            hInstance = GetModuleHandle(null),
+            lpszClassName = className,
+        };
+
+        if (RegisterClassEx(ref description) == 0 &&
+            Marshal.GetLastWin32Error() != ErrorClassAlreadyExists)
+        {
+            throw new InvalidOperationException(
+                $"RegisterClassExW failed for '{className}': {Marshal.GetLastWin32Error()}");
+        }
+    }
+
+    private const int ErrorClassAlreadyExists = 1410;
+
+    /// <summary>Creates a window of a registered class. It is not shown.</summary>
+    internal static IntPtr CreateWindow(string className, string title, int style, int exStyle)
+    {
+        IntPtr window = CreateWindowEx(
+            exStyle, className, title, style,
+            0, 0, 0, 0,
+            IntPtr.Zero, IntPtr.Zero, GetModuleHandle(null), IntPtr.Zero);
+
+        if (window == IntPtr.Zero)
+        {
+            throw new InvalidOperationException(
+                $"CreateWindowExW failed for '{className}': {Marshal.GetLastWin32Error()}");
+        }
+
+        return window;
+    }
+
+    /// <summary>
+    /// Puts a finished picture into a layered window, together with where it goes and
+    /// how see-through it is.
+    /// </summary>
+    /// <remarks>
+    /// One call replaces moving the window, resizing it, painting it and setting its
+    /// opacity, and it works while the window is still hidden. That is the point: the
+    /// content is ready before anyone can see the window, so it can never be shown
+    /// holding the picture of the previous time.
+    /// </remarks>
+    internal static bool SetContent(
+        IntPtr window, PopupWindowNative.Point at, Size size, IntPtr sourceDc, byte alpha)
+    {
+        var source = new PopupWindowNative.Point { X = 0, Y = 0 };
+        var blend = new BlendFunction
+        {
+            BlendOp = AC_SRC_OVER,
+            SourceConstantAlpha = alpha,
+            AlphaFormat = AC_SRC_ALPHA,
+        };
+
+        return UpdateLayeredWindow(
+            window, IntPtr.Zero, ref at, ref size, sourceDc, ref source, 0, ref blend, ULW_ALPHA);
+    }
+
+    private const byte AC_SRC_OVER = 0;
+    private const byte AC_SRC_ALPHA = 1;
+    private const uint ULW_ALPHA = 0x00000002;
+
+    /// <summary>
+    /// BLENDFUNCTION. <c>BlendFlags</c> is never assigned and must stay all the same:
+    /// Windows reads the structure by its layout, and dropping a byte from the middle
+    /// of it would shift everything after.
+    /// </summary>
+    [StructLayout(LayoutKind.Sequential)]
+    private struct BlendFunction
+    {
+        public byte BlendOp;
+        public byte BlendFlags;
+        public byte SourceConstantAlpha;
+        public byte AlphaFormat;
+    }
+
+    [StructLayout(LayoutKind.Sequential)]
+    internal struct Size
+    {
+        public int Width;
+        public int Height;
+    }
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern bool UpdateLayeredWindow(IntPtr hWnd, IntPtr hdcDst,
+        ref PopupWindowNative.Point pptDst, ref Size psize, IntPtr hdcSrc,
+        ref PopupWindowNative.Point pptSrc, uint crKey, ref BlendFunction pblend, uint dwFlags);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
+    private static extern ushort RegisterClassEx(ref WindowClass lpwcx);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, SetLastError = true, EntryPoint = "CreateWindowExW")]
+    private static extern IntPtr CreateWindowEx(int dwExStyle, string lpClassName, string lpWindowName,
+        int dwStyle, int x, int y, int nWidth, int nHeight,
+        IntPtr hWndParent, IntPtr hMenu, IntPtr hInstance, IntPtr lpParam);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "DefWindowProcW")]
+    internal static extern IntPtr DefWindowProc(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam);
+
+    [DllImport("user32.dll")]
+    internal static extern bool DestroyWindow(IntPtr hWnd);
+
+    [DllImport("user32.dll")]
+    internal static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);
+
+    [DllImport("user32.dll")]
+    internal static extern bool IsWindowVisible(IntPtr hWnd);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "PostMessageW")]
+    internal static extern bool PostMessage(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam);
+
+    [DllImport("user32.dll")]
+    internal static extern void PostQuitMessage(int nExitCode);
+
+    [DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
+    private static extern IntPtr GetModuleHandle(string? lpModuleName);
+
+    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
+    private struct WindowClass
+    {
+        public int cbSize;
+        public uint style;
+
+        [MarshalAs(UnmanagedType.FunctionPtr)]
+        public WindowProc lpfnWndProc;
+
+        public int cbClsExtra;
+        public int cbWndExtra;
+        public IntPtr hInstance;
+        public IntPtr hIcon;
+        public IntPtr hCursor;
+        public IntPtr hbrBackground;
+        public string? lpszMenuName;
+        public string lpszClassName;
+        public IntPtr hIconSm;
+    }
+}

CursorLang.Agent/Program.cs

@@ -0,0 +1,38 @@
+using CursorLang.Core.Services;
+
+namespace CursorLang.Agent;
+
+internal static class Program
+{
+    /// <summary>
+    /// A single-threaded apartment because of the caret: <c>AccessibleObjectFromWindow</c>
+    /// and UI Automation both go through COM, and both expect the thread that calls them
+    /// to be an STA one.
+    /// </summary>
+    [STAThread]
+    private static int Main(string[] arguments)
+    {
+        bool automatic = StartupLaunch.IsAutomatic(arguments);
+
+        var gate = new SingleInstanceGate(SingleInstanceGate.AgentName);
+
+        // A second launch is the user asking for the application, so the one already
+        // running opens the settings window and this one steps aside. A second launch
+        // by Windows at sign-in asks for nothing and gets nothing
+        if (!gate.TryAcquire(showRunningInstance: !automatic))
+        {
+            gate.Dispose();
+            return 0;
+        }
+
+        try
+        {
+            using var agent = new Agent(gate);
+            return agent.Run(automatic);
+        }
+        finally
+        {
+            gate.Dispose();
+        }
+    }
+}

CursorLang.Agent/Properties/launchSettings.json

@@ -0,0 +1,11 @@
+{
+  "profiles": {
+    "Agent": {
+      "commandName": "Project"
+    },
+    "Agent (started by Windows)": {
+      "commandName": "Project",
+      "commandLineArgs": "--startup"
+    }
+  }
+}

CursorLang.Agent/Services/CapsLockHotkeyService.cs

@@ -0,0 +1,167 @@
+using CursorLang.Core.Interop;
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+using CursorLang.Core.Threading;
+
+namespace CursorLang.Agent.Services;
+
+/// <summary>
+/// Holds the system Caps Lock hook and tells a short tap from a hold.
+/// </summary>
+/// <remarks>
+/// They can be told apart only by the key being released, so both events are
+/// intercepted — the press and the release. That is also the only way to cancel the
+/// case change: Windows toggles Caps Lock on the press event, and letting it through
+/// "just in case" is not an option.
+///
+/// Two things changed on the way out of WPF: the hold is timed by
+/// <see cref="MessageTimer"/>, and the event reaches its subscribers through a message
+/// posted to the agent's window rather than through the dispatcher.
+/// </remarks>
+public sealed class CapsLockHotkeyService : ICapsLockHotkeyService, IDisposable
+{
+    private const int VirtualKeyCapsLock = 0x14;
+
+    private readonly AppSettings _settings;
+    private readonly Action<Action> _post;
+    private readonly LowLevelKeyboardHook _hook;
+    private readonly MessageTimer _holdTimer = new();
+
+    private bool _isPressed;
+    private bool _isHolding;
+
+    /// <param name="settings">Where the hold threshold is read from, on every press.</param>
+    /// <param name="post">
+    /// Hands work back to the message loop. Taken as a delegate rather than as the
+    /// agent's window so that the press logic can be checked without one.
+    /// </param>
+    public CapsLockHotkeyService(AppSettings settings, Action<Action> post)
+    {
+        _settings = settings;
+        _post = post;
+        _hook = new LowLevelKeyboardHook(HandleKeyEvent);
+        _holdTimer.Tick += OnHoldTimerTick;
+    }
+
+    public event EventHandler? Tapped;
+
+    public event EventHandler? HoldStarted;
+
+    public event EventHandler? HoldEnded;
+
+    public bool IsRunning => _hook.IsInstalled;
+
+    public void Start() => _hook.Install();
+
+    public void Stop()
+    {
+        _hook.Uninstall();
+        ResetPress();
+    }
+
+    // When the application is closing, nobody is waiting for events any more, so
+    // unlike in Stop the state is reset quietly: the message loop is already gone by
+    // that moment and posted work would never run
+    public void Dispose()
+    {
+        _holdTimer.Tick -= OnHoldTimerTick;
+        _holdTimer.Dispose();
+        _isPressed = false;
+        _isHolding = false;
+        _hook.Dispose();
+    }
+
+    // Called by the system hook, that is, inside message queue processing. Only state
+    // tracking belongs here: showing windows and raising events from here is not
+    // allowed — the handler must return control within a few milliseconds.
+    // In tests the key presses are fed here as well: there is no need to install a
+    // real keyboard hook just to check how presses are interpreted
+    internal bool HandleKeyEvent(int virtualKey, bool isKeyDown)
+    {
+        if (virtualKey != VirtualKeyCapsLock)
+        {
+            return false;
+        }
+
+        if (isKeyDown)
+        {
+            // While the key is held, Windows repeats the press: the hold is counted
+            // from the first event and the repeats are ignored
+            if (!_isPressed)
+            {
+                _isPressed = true;
+
+                // The threshold is read on every press: it is changed in the settings on the fly
+                _holdTimer.Interval = _settings.CapsLockHoldDelay;
+                _holdTimer.Start();
+            }
+
+            return true;
+        }
+
+        _isPressed = false;
+        _holdTimer.Stop();
+
+        if (_isHolding)
+        {
+            _isHolding = false;
+            Notify(HoldEnded);
+        }
+        else
+        {
+            Notify(Tapped);
+        }
+
+        return true;
+    }
+
+    private void OnHoldTimerTick(object? sender, EventArgs e) => HandleHoldElapsed();
+
+    /// <summary>
+    /// The hold countdown has run out.
+    /// </summary>
+    /// <remarks>
+    /// The key being down is checked rather than assumed. A countdown started on the
+    /// press can still be delivered just after the release — Windows does not withdraw a
+    /// WM_TIMER it has already posted — and announcing a hold then would put the popup on
+    /// screen showing the layout the tap is about to change away from.
+    ///
+    /// The tests reach this directly: that ordering is the whole point and a real clock
+    /// will not reproduce it on demand.
+    /// </remarks>
+    internal void HandleHoldElapsed()
+    {
+        _holdTimer.Stop();
+        if (!_isPressed)
+        {
+            return;
+        }
+
+        _isHolding = true;
+        HoldStarted?.Invoke(this, EventArgs.Empty);
+    }
+
+    // The event reaches the subscribers after the hook returns: they are free to show
+    // windows and do anything else without holding up the handling of the key press
+    private void Notify(EventHandler? handler)
+    {
+        if (handler is not null)
+        {
+            _post(() => handler(this, EventArgs.Empty));
+        }
+    }
+
+    // The hook may have been removed with the key held down — by clearing the
+    // checkbox in the settings, for instance. The popup has to be taken down then
+    private void ResetPress()
+    {
+        _isPressed = false;
+        _holdTimer.Stop();
+
+        if (_isHolding)
+        {
+            _isHolding = false;
+            Notify(HoldEnded);
+        }
+    }
+}

CursorLang.Agent/Services/LayoutPopupService.cs

@@ -0,0 +1,64 @@
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+using CursorLang.Core.Threading;
+
+namespace CursorLang.Agent.Services;
+
+/// <summary>
+/// Manages the lifetime of the popup: the window is only responsible for showing it,
+/// while the decision of when to show and when to take it down is made here.
+/// </summary>
+/// <remarks>
+/// The same service it always was, with <c>DispatcherTimer</c> swapped for
+/// <see cref="MessageTimer"/>: both tick on the thread that owns the window, so
+/// nothing else about the logic had to move.
+/// </remarks>
+public sealed class LayoutPopupService : ILayoutPopupService, IDisposable
+{
+    private readonly ILayoutPopupWindow _window;
+    private readonly AppSettings _settings;
+    private readonly MessageTimer _hideTimer = new();
+
+    public LayoutPopupService(ILayoutPopupWindow window, AppSettings settings)
+    {
+        _window = window;
+        _settings = settings;
+
+        _hideTimer.Tick += OnHideTimerTick;
+    }
+
+    public void Show(KeyboardLayout layout)
+    {
+        ShowUntilHidden(layout);
+
+        // The duration is read on every show: it is changed in the settings on the fly.
+        // Restarting the timer also prolongs the show on quick switches
+        _hideTimer.Interval = _settings.Duration;
+        _hideTimer.Start();
+    }
+
+    public void ShowUntilHidden(KeyboardLayout layout)
+    {
+        _hideTimer.Stop();
+        _window.ShowPopup(layout.ShortName);
+    }
+
+    public void Hide()
+    {
+        _hideTimer.Stop();
+        _window.Hide();
+    }
+
+    public void Dispose()
+    {
+        _hideTimer.Tick -= OnHideTimerTick;
+        _hideTimer.Dispose();
+        _window.Close();
+    }
+
+    private void OnHideTimerTick(object? sender, EventArgs e)
+    {
+        _hideTimer.Stop();
+        _window.Hide();
+    }
+}

CursorLang.Agent/Services/SettingsLauncher.cs

@@ -0,0 +1,43 @@
+using System.ComponentModel;
+using System.Diagnostics;
+using CursorLang.Core.Services;
+
+namespace CursorLang.Agent.Services;
+
+/// <summary>
+/// Starts the settings window.
+/// </summary>
+/// <remarks>
+/// The agent does not keep track of whether the window is already open, and does not
+/// need to: the settings process guards a single-instance slot of its own, so a second
+/// launch raises the window already there and exits. That costs a process start to find
+/// out, which is a fraction of the time it takes a person to look at the tray, and it
+/// saves the agent from holding a handle to something it does not own.
+/// </remarks>
+internal static class SettingsLauncher
+{
+    /// <summary>
+    /// Opens the settings window. Returns <c>false</c> when the executable is not
+    /// where it should be — a half-copied installation, or the agent run from a build
+    /// folder of its own.
+    /// </summary>
+    internal static bool Open()
+    {
+        if (AgentExecutable.SettingsPath is not { } path)
+        {
+            return false;
+        }
+
+        try
+        {
+            using Process? started = Process.Start(new ProcessStartInfo(path) { UseShellExecute = false });
+            return started is not null;
+        }
+        catch (Exception e) when (e is Win32Exception or InvalidOperationException)
+        {
+            // Nothing to tell the user with: the agent has no window of its own, and
+            // the one that would have shown the message is the one that failed to start
+            return false;
+        }
+    }
+}

CursorLang.Agent/Windows/AgentWindow.cs

@@ -0,0 +1,85 @@
+using System.Collections.Concurrent;
+using CursorLang.Agent.Interop;
+
+namespace CursorLang.Agent.Windows;
+
+/// <summary>
+/// The window the agent lives around: never shown, but it owns the tray icon and it
+/// is the way back onto the message loop from a callback.
+/// </summary>
+/// <remarks>
+/// A window with no <c>WS_VISIBLE</c> shows nowhere, yet is a window in every other
+/// way. A message-only window would do as well were it not for the news of Explorer
+/// restarting: that one is broadcast, and broadcasts pass such windows by.
+/// </remarks>
+internal sealed class AgentWindow : NativeWindow
+{
+    /// <summary>
+    /// A message hook. Returning <c>true</c> means the message has been dealt with.
+    /// </summary>
+    internal delegate bool MessageFilter(uint message, IntPtr wParam, IntPtr lParam);
+
+    private const string ClassName = "CursorLang.Agent.Window";
+
+    /// <summary>Drain the queue of posted work. WM_APP is free for the application.</summary>
+    private const uint WM_INVOKE = WindowNative.WM_APP + 100;
+
+    private readonly List<MessageFilter> _filters = [];
+    private readonly ConcurrentQueue<Action> _posted = new();
+
+    internal AgentWindow()
+        : base(ClassName, "CursorLang agent", WindowNative.WS_POPUP, WindowNative.WS_EX_TOOLWINDOW)
+    {
+    }
+
+    internal void AddFilter(MessageFilter filter) => _filters.Add(filter);
+
+    /// <summary>
+    /// Runs the action on the message loop, after the current message is done with.
+    /// </summary>
+    /// <remarks>
+    /// This is what the agent has instead of <c>Dispatcher.BeginInvoke</c>. The caller
+    /// that matters is the keyboard hook: Windows removes a hook whose procedure takes
+    /// too long, so the procedure only records what happened and the answer — showing
+    /// the popup, switching the layout — waits for the message after this one.
+    /// </remarks>
+    internal void Post(Action action)
+    {
+        _posted.Enqueue(action);
+        WindowNative.PostMessage(Handle, WM_INVOKE, IntPtr.Zero, IntPtr.Zero);
+    }
+
+    /// <summary>Asks the message loop to finish.</summary>
+    internal void Quit() => WindowNative.PostQuitMessage(0);
+
+    protected override bool OnMessage(uint message, IntPtr wParam, IntPtr lParam, out IntPtr result)
+    {
+        result = IntPtr.Zero;
+
+        if (message == WM_INVOKE)
+        {
+            while (_posted.TryDequeue(out Action? action))
+            {
+                action();
+            }
+
+            return true;
+        }
+
+        if (message is WindowNative.WM_CLOSE or WindowNative.WM_ENDSESSION)
+        {
+            Quit();
+            return true;
+        }
+
+        foreach (MessageFilter filter in _filters)
+        {
+            if (filter(message, wParam, lParam))
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+}

CursorLang.Agent/Windows/NativePopupWindow.cs

@@ -0,0 +1,400 @@
+using System.Drawing;
+using System.Runtime.InteropServices;
+using CursorLang.Agent.Interop;
+using CursorLang.Agent.Services;
+using CursorLang.Core.Interop;
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+
+namespace CursorLang.Agent.Windows;
+
+/// <summary>
+/// The popup with the short name of the layout, drawn by Win32 alone.
+/// </summary>
+/// <remarks>
+/// A like-for-like replacement of the WPF popup this once was: a rounded rectangle of
+/// radius 4 with 10×4 padding, the fill and the text colour from the settings, the whole
+/// thing at the opacity from the settings, the name in Segoe UI SemiBold at the size
+/// from the settings.
+///
+/// The picture is drawn into an off-screen bitmap and handed to the window whole, by
+/// <c>UpdateLayeredWindow</c>. Painting on demand instead — a <c>WM_PAINT</c> after the
+/// window is shown — is what the first version did, and it had the popup appear holding
+/// the picture of the previous show: hiding a window does not throw its content away,
+/// and the content is always the other layout. Here there is nothing to be stale,
+/// because the window is never shown before its picture is in place.
+///
+/// It also does away with two devices the painted version needed: the corners came from
+/// a window region, which cuts without antialiasing, and the opacity from
+/// <c>SetLayeredWindowAttributes</c>. Both are now just pixels in the bitmap.
+///
+/// Responsible only for showing the popup, its size and its place on screen: when to
+/// take it down is decided by <see cref="LayoutPopupService"/>.
+/// </remarks>
+internal sealed class NativePopupWindow : NativeWindow, ILayoutPopupWindow
+{
+    private const string ClassName = "CursorLang.Agent.Popup";
+
+    // The numbers of the XAML: Border CornerRadius="4" Padding="10,4", all in WPF units
+    private const double CornerRadius = 4;
+    private const double PaddingX = 10;
+    private const double PaddingY = 4;
+
+    private readonly AppSettings _settings;
+
+    private IntPtr _font;
+    private double _fontSize;
+    private double _fontScale;
+
+    internal NativePopupWindow(AppSettings settings)
+        : base(
+            ClassName,
+            "CursorLang popup",
+            WindowNative.WS_POPUP,
+            WindowNative.WS_EX_LAYERED | WindowNative.WS_EX_TOOLWINDOW |
+            WindowNative.WS_EX_NOACTIVATE | WindowNative.WS_EX_TRANSPARENT |
+            WindowNative.WS_EX_TOPMOST)
+    {
+        _settings = settings;
+    }
+
+    /// <summary>
+    /// Shows the popup with the given text at the place set by the settings.
+    /// </summary>
+    public void ShowPopup(string text)
+    {
+        if (Handle == IntPtr.Zero)
+        {
+            return;
+        }
+
+        bool atFixedPoint = _settings.PlacementMode == PopupPlacementMode.FixedPoint;
+        PopupWindowNative.Rect work = default;
+        PopupWindowNative.Rect anchor = default;
+        double scale;
+
+        if (atFixedPoint)
+        {
+            (work, scale) = PopupWindowNative.GetActiveMonitorWorkArea();
+        }
+        else
+        {
+            anchor = GetAnchor();
+            scale = PopupWindowNative.GetScaleAt(new PopupWindowNative.Point { X = anchor.Left, Y = anchor.Top });
+        }
+
+        EnsureFont(scale);
+
+        Size measured = MeasureText(text);
+        int width = measured.Width + (2 * PopupLayout.ToPixels(PaddingX, scale));
+        int height = measured.Height + (2 * PopupLayout.ToPixels(PaddingY, scale));
+
+        PopupWindowNative.Point position = atFixedPoint
+            ? PopupLayout.OnScreen(
+                work,
+                _settings.FixedPoint.Position,
+                PopupLayout.ToPixels(_settings.FixedPoint.Offset, scale),
+                width,
+                height)
+            : PopupLayout.NearAnchor(
+                anchor,
+                SideForMode(),
+                PopupLayout.ToPixels(_settings.Current.Offset, scale),
+                width,
+                height);
+
+        if (!Draw(text, position, width, height, PopupLayout.ToPixels(CornerRadius, scale)))
+        {
+            return;
+        }
+
+        if (!WindowNative.IsWindowVisible(Handle))
+        {
+            WindowNative.ShowWindow(Handle, WindowNative.SW_SHOWNOACTIVATE);
+        }
+    }
+
+    public void Hide()
+    {
+        if (Handle != IntPtr.Zero)
+        {
+            WindowNative.ShowWindow(Handle, WindowNative.SW_HIDE);
+        }
+    }
+
+    /// <summary>Destroys the window. The agent only does this on the way out.</summary>
+    public void Close() => Dispose();
+
+    public override void Dispose()
+    {
+        ReleaseFont();
+        base.Dispose();
+    }
+
+    /// <summary>
+    /// Draws the popup off screen and hands the finished picture to the window.
+    /// </summary>
+    /// <remarks>
+    /// The bitmap is thrown away afterwards rather than kept: it is a few tens of
+    /// kilobytes for the length of one call, the popup is shown rarely, and a cached one
+    /// would have to be rebuilt on every change of size, colour or scale anyway.
+    /// </remarks>
+    private bool Draw(string text, PopupWindowNative.Point at, int width, int height, int radius)
+    {
+        IntPtr screen = GdiNative.GetDC(IntPtr.Zero);
+        if (screen == IntPtr.Zero)
+        {
+            return false;
+        }
+
+        IntPtr memory = IntPtr.Zero;
+        IntPtr surface = IntPtr.Zero;
+
+        try
+        {
+            memory = GdiNative.CreateCompatibleDC(screen);
+            if (memory == IntPtr.Zero)
+            {
+                return false;
+            }
+
+            surface = GdiNative.CreateSurface(memory, width, height, out IntPtr bits);
+            if (surface == IntPtr.Zero)
+            {
+                return false;
+            }
+
+            GdiNative.SelectObject(memory, surface);
+
+            Fill(bits, width, height);
+            DrawText(memory, text, width, height);
+
+            // GDI writes nothing into the alpha channel, so the letters it just drew are
+            // sitting at zero alpha and would come out invisible. The inside of the
+            // popup is opaque anyway, so the whole surface is simply declared so — and
+            // the corners are rounded off afterwards, which is the only place alpha
+            // varies
+            MakeOpaque(bits, width, height);
+            RoundTheCorners(bits, width, height, radius);
+
+            var size = new WindowNative.Size { Width = width, Height = height };
+            var alpha = (byte)Math.Clamp(Math.Round(_settings.Current.Opacity * 255), 0, 255);
+
+            return WindowNative.SetContent(Handle, at, size, memory, alpha);
+        }
+        finally
+        {
+            // The context goes first: a bitmap still selected into one cannot be
+            // deleted, and this way that holds however the method was left
+            if (memory != IntPtr.Zero)
+            {
+                GdiNative.DeleteDC(memory);
+            }
+
+            if (surface != IntPtr.Zero)
+            {
+                GdiNative.DeleteObject(surface);
+            }
+
+            GdiNative.ReleaseDC(IntPtr.Zero, screen);
+        }
+    }
+
+    private void Fill(IntPtr bits, int width, int height)
+    {
+        Color background = _settings.Current.BackgroundColor;
+
+        // Straight into the bitmap rather than through a brush: the pixels have to be
+        // written anyway to carry an alpha channel GDI would not touch
+        int packed = (255 << 24) | (background.R << 16) | (background.G << 8) | background.B;
+        var row = new int[width];
+        Array.Fill(row, packed);
+
+        for (var y = 0; y < height; y++)
+        {
+            Marshal.Copy(row, 0, bits + (y * width * 4), width);
+        }
+    }
+
+    private void DrawText(IntPtr deviceContext, string text, int width, int height)
+    {
+        if (_font == IntPtr.Zero || text.Length == 0)
+        {
+            return;
+        }
+
+        var bounds = new PopupWindowNative.Rect { Left = 0, Top = 0, Right = width, Bottom = height };
+
+        IntPtr previousFont = GdiNative.SelectObject(deviceContext, _font);
+        GdiNative.SetBkMode(deviceContext, GdiNative.TRANSPARENT);
+        GdiNative.SetTextColor(deviceContext, GdiNative.ToColorRef(_settings.Current.ForegroundColor));
+
+        GdiNative.DrawText(deviceContext, text, text.Length, ref bounds,
+            GdiNative.DT_SINGLELINE | GdiNative.DT_CENTER | GdiNative.DT_VCENTER |
+            GdiNative.DT_NOPREFIX | GdiNative.DT_NOCLIP);
+
+        GdiNative.SelectObject(deviceContext, previousFont);
+    }
+
+    private static void MakeOpaque(IntPtr bits, int width, int height)
+    {
+        var row = new int[width];
+
+        for (var y = 0; y < height; y++)
+        {
+            IntPtr line = bits + (y * width * 4);
+            Marshal.Copy(line, row, 0, width);
+
+            for (var x = 0; x < width; x++)
+            {
+                row[x] = (int)((uint)row[x] | 0xFF000000);
+            }
+
+            Marshal.Copy(row, 0, line, width);
+        }
+    }
+
+    /// <summary>
+    /// Cuts the four corners to a radius, fading the edge rather than stepping it.
+    /// </summary>
+    /// <remarks>
+    /// The painted version cut them with a window region, which is a yes-or-no mask and
+    /// left a visible staircase at 200% scale. Here the corner pixels carry a partial
+    /// alpha worked out from how far the pixel centre is past the arc, which is what
+    /// antialiasing amounts to. The colours are premultiplied to match, as
+    /// <c>UpdateLayeredWindow</c> expects.
+    /// </remarks>
+    private static void RoundTheCorners(IntPtr bits, int width, int height, int radius)
+    {
+        if (radius <= 0)
+        {
+            return;
+        }
+
+        radius = Math.Min(radius, Math.Min(width, height) / 2);
+
+        var row = new int[width];
+
+        for (var y = 0; y < height; y++)
+        {
+            bool nearTop = y < radius;
+            bool nearBottom = y >= height - radius;
+            if (!nearTop && !nearBottom)
+            {
+                continue;
+            }
+
+            IntPtr line = bits + (y * width * 4);
+            Marshal.Copy(line, row, 0, width);
+
+            double centreY = nearTop ? radius - 0.5 : height - radius - 0.5;
+
+            for (var x = 0; x < width; x++)
+            {
+                bool nearLeft = x < radius;
+                bool nearRight = x >= width - radius;
+                if (!nearLeft && !nearRight)
+                {
+                    continue;
+                }
+
+                double centreX = nearLeft ? radius - 0.5 : width - radius - 0.5;
+                double distance = Math.Sqrt(
+                    ((x - centreX) * (x - centreX)) + ((y - centreY) * (y - centreY)));
+
+                // One pixel of softness across the arc: fully inside, fully outside,
+                // and a ramp in between
+                double coverage = Math.Clamp(radius - distance + 0.5, 0, 1);
+                if (coverage >= 1)
+                {
+                    continue;
+                }
+
+                row[x] = Premultiply(row[x], coverage);
+            }
+
+            Marshal.Copy(row, 0, line, width);
+        }
+    }
+
+    private static int Premultiply(int pixel, double coverage)
+    {
+        var value = (uint)pixel;
+        var alpha = (uint)Math.Round(((value >> 24) & 0xFF) * coverage);
+
+        uint red = (uint)Math.Round(((value >> 16) & 0xFF) * coverage);
+        uint green = (uint)Math.Round(((value >> 8) & 0xFF) * coverage);
+        uint blue = (uint)Math.Round((value & 0xFF) * coverage);
+
+        return (int)((alpha << 24) | (red << 16) | (green << 8) | blue);
+    }
+
+    // The anchor point: the caret in the input field or the mouse cursor. The cursor
+    // is a rectangle of zero size, so the corner computation is shared by both
+    private PopupWindowNative.Rect GetAnchor()
+    {
+        if (_settings.PlacementMode == PopupPlacementMode.AtCaret &&
+            CaretNative.TryGetCaretRect() is { } caret)
+        {
+            return caret;
+        }
+
+        return PopupLayout.AsAnchor(PopupWindowNative.GetCursorPosition());
+    }
+
+    // The caret has two sides to choose from and the cursor has six, so each mode names
+    // its own side in its own terms
+    private AnchorSide SideForMode() => _settings.PlacementMode == PopupPlacementMode.AtCaret
+        ? _settings.AtCaret.Anchor
+        : _settings.AtCursor.Side;
+
+    private Size MeasureText(string text)
+    {
+        IntPtr deviceContext = GdiNative.GetDC(Handle);
+        if (deviceContext == IntPtr.Zero)
+        {
+            return Size.Empty;
+        }
+
+        try
+        {
+            IntPtr previousFont = GdiNative.SelectObject(deviceContext, _font);
+            Size measured = GdiNative.MeasureText(deviceContext, text);
+            GdiNative.SelectObject(deviceContext, previousFont);
+
+            return measured;
+        }
+        finally
+        {
+            GdiNative.ReleaseDC(Handle, deviceContext);
+        }
+    }
+
+    // The font is rebuilt only when the size in the settings or the monitor scale
+    // changes: it is the one expensive thing a show does. A switch of the placement
+    // mode counts as a change of the size, since the size belongs to the mode
+    private void EnsureFont(double scale)
+    {
+        if (_font != IntPtr.Zero &&
+            Math.Abs(_fontSize - _settings.Current.FontSize) < 0.01 &&
+            Math.Abs(_fontScale - scale) < 0.01)
+        {
+            return;
+        }
+
+        ReleaseFont();
+
+        _fontSize = _settings.Current.FontSize;
+        _fontScale = scale;
+        _font = GdiNative.CreateFont(_fontSize, scale);
+    }
+
+    private void ReleaseFont()
+    {
+        if (_font != IntPtr.Zero)
+        {
+            GdiNative.DeleteObject(_font);
+            _font = IntPtr.Zero;
+        }
+    }
+}

CursorLang.Agent/Windows/NativeTrayIcon.cs

@@ -0,0 +1,128 @@
+using CursorLang.Agent.Interop;
+using CursorLang.Core.Interop;
+using CursorLang.Core.Services;
+
+namespace CursorLang.Agent.Windows;
+
+/// <summary>
+/// The icon in the notification area: the way to the settings window and the only way
+/// to quit the application.
+/// </summary>
+/// <remarks>
+/// The interop half is the same as it always was — the icon has never known anything
+/// about the framework. What changed is the menu: a WPF <c>ContextMenu</c> obeyed the
+/// theme and the language chosen in the settings, and a <c>TrackPopupMenuEx</c> menu is
+/// drawn by Windows in the system look. The language still reaches it, because the
+/// captions are ours; the theme does not, and that is the price of taking the rendering
+/// stack out of the background process.
+///
+/// The captions are read each time the menu is raised rather than once: the language is
+/// changed in the settings without a restart, and the menu is built on every click
+/// anyway — a menu costs nothing to build and is asked for rarely.
+/// </remarks>
+internal sealed class NativeTrayIcon : IDisposable
+{
+    /// <summary>Windows shows it under the pointer. The name of the app says enough.</summary>
+    private const string Tooltip = "CursorLang";
+
+    /// <summary>Distinguishes the icon among those of the same window; we have one.</summary>
+    private const int IconId = 1;
+
+    private const int CommandSettings = 1;
+    private const int CommandExit = 2;
+
+    private readonly AgentWindow _window;
+    private readonly ILocalizationService _localization;
+
+    private IntPtr _icon;
+    private bool _isInstalled;
+
+    internal NativeTrayIcon(AgentWindow window, ILocalizationService localization)
+    {
+        _window = window;
+        _localization = localization;
+
+        _window.AddFilter(OnMessage);
+    }
+
+    internal event EventHandler? OpenRequested;
+
+    internal event EventHandler? ExitRequested;
+
+    internal bool Install()
+    {
+        if (_isInstalled)
+        {
+            return true;
+        }
+
+        _icon = TrayIconNative.LoadApplicationIcon();
+        _isInstalled = TrayIconNative.Add(_window.Handle, IconId, _icon, Tooltip);
+
+        return _isInstalled;
+    }
+
+    public void Dispose()
+    {
+        if (_isInstalled)
+        {
+            TrayIconNative.Remove(_window.Handle, IconId);
+            _isInstalled = false;
+        }
+
+        TrayIconNative.ReleaseIcon(_icon);
+        _icon = IntPtr.Zero;
+    }
+
+    private bool OnMessage(uint message, IntPtr wParam, IntPtr lParam)
+    {
+        // Explorer has restarted and taken every icon down with it
+        if (message == (uint)TrayIconNative.TaskbarCreatedMessage && _isInstalled)
+        {
+            TrayIconNative.Add(_window.Handle, IconId, _icon, Tooltip);
+            return true;
+        }
+
+        if (message != TrayIconNative.CallbackMessage)
+        {
+            return false;
+        }
+
+        switch (TrayIconNative.NotificationOf(lParam))
+        {
+            case TrayIconNative.SelectNotification:
+            case TrayIconNative.KeySelectNotification:
+                OpenRequested?.Invoke(this, EventArgs.Empty);
+                return true;
+
+            case TrayIconNative.ContextMenuNotification:
+                ShowMenu(TrayIconNative.PointOf(wParam));
+                return true;
+
+            default:
+                return false;
+        }
+    }
+
+    /// <summary>Raises the menu of the icon where the pointer is.</summary>
+    internal void ShowMenu(PopupWindowNative.Point at)
+    {
+        MenuNative.Item[] items =
+        [
+            new(CommandSettings, _localization["TrayMenuSettings"]),
+            MenuNative.Item.Separator,
+            new(CommandExit, _localization["TrayMenuExit"]),
+        ];
+
+        switch (MenuNative.Track(_window.Handle, at, items))
+        {
+            case CommandSettings:
+                OpenRequested?.Invoke(this, EventArgs.Empty);
+                break;
+
+            case CommandExit:
+                ExitRequested?.Invoke(this, EventArgs.Empty);
+                break;
+        }
+    }
+}

CursorLang.Agent/Windows/NativeWindow.cs

@@ -0,0 +1,104 @@
+using CursorLang.Agent.Interop;
+
+namespace CursorLang.Agent.Windows;
+
+/// <summary>
+/// A window with no framework behind it: a registered class, a handle and a window
+/// procedure that lands in <see cref="OnMessage"/>.
+/// </summary>
+/// <remarks>
+/// Windows knows one procedure per class, so the procedure here is shared and static,
+/// and finds the instance by handle. The very first message of a window arrives while
+/// <c>CreateWindowExW</c> is still running and there is nothing to find yet — that is
+/// what the field holding the instance under construction is for.
+///
+/// Everything is deliberately without locks: the agent has one message loop, every
+/// window belongs to it, and a window procedure can only ever be called on the thread
+/// that created the window.
+/// </remarks>
+internal abstract class NativeWindow : IDisposable
+{
+    private static readonly Dictionary<IntPtr, NativeWindow> Live = [];
+    private static readonly HashSet<string> RegisteredClasses = new(StringComparer.Ordinal);
+
+    // The shared procedure is a static field for the same reason a hook procedure is:
+    // Windows holds the only reference to it and the collector does not see that
+    private static readonly WindowNative.WindowProc SharedProc = StaticWindowProc;
+
+    [ThreadStatic]
+    private static NativeWindow? _creating;
+
+    protected NativeWindow(string className, string title, int style, int exStyle)
+    {
+        if (RegisteredClasses.Add(className))
+        {
+            WindowNative.RegisterClass(className, SharedProc);
+        }
+
+        _creating = this;
+        try
+        {
+            Handle = WindowNative.CreateWindow(className, title, style, exStyle);
+        }
+        finally
+        {
+            _creating = null;
+        }
+
+        Live[Handle] = this;
+    }
+
+    /// <summary>The window handle. Zero once the window is gone.</summary>
+    internal IntPtr Handle { get; private set; }
+
+    public virtual void Dispose()
+    {
+        if (Handle == IntPtr.Zero)
+        {
+            return;
+        }
+
+        IntPtr handle = Handle;
+        Handle = IntPtr.Zero;
+        Live.Remove(handle);
+
+        WindowNative.DestroyWindow(handle);
+    }
+
+    /// <summary>
+    /// A message for this window. Returning <c>false</c> passes it to
+    /// <c>DefWindowProcW</c>, which is what the vast majority of messages want — and
+    /// what all of them want for a window whose content is set from the outside.
+    /// </summary>
+    protected virtual bool OnMessage(uint message, IntPtr wParam, IntPtr lParam, out IntPtr result)
+    {
+        result = IntPtr.Zero;
+        return false;
+    }
+
+    private static IntPtr StaticWindowProc(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam)
+    {
+        if (!Live.TryGetValue(hWnd, out NativeWindow? window))
+        {
+            if (_creating is null)
+            {
+                return WindowNative.DefWindowProc(hWnd, message, wParam, lParam);
+            }
+
+            // The window is being created right now: bind the handle to the instance
+            // so that the rest of its creation messages find their way home
+            window = _creating;
+            window.Handle = hWnd;
+            Live[hWnd] = window;
+        }
+
+        if (message == WindowNative.WM_DESTROY)
+        {
+            Live.Remove(hWnd);
+        }
+
+        return window.OnMessage(message, wParam, lParam, out IntPtr result)
+            ? result
+            : WindowNative.DefWindowProc(hWnd, message, wParam, lParam);
+    }
+}

CursorLang.Agent/app.manifest

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="utf-8"?>
+<assembly manifestVersion="1.0" xmlns="urn:schemas-microsoft-com:asm.v1">
+  <assemblyIdentity version="1.0.0.0" name="CursorLang.Agent.app" />
+
+  <trustInfo xmlns="urn:schemas-microsoft-com:asm.v2">
+    <security>
+      <requestedPrivileges xmlns="urn:schemas-microsoft-com:asm.v3">
+        <requestedExecutionLevel level="asInvoker" uiAccess="false" />
+      </requestedPrivileges>
+    </security>
+  </trustInfo>
+
+  <application xmlns="urn:schemas-microsoft-com:asm.v3">
+    <windowsSettings>
+      <dpiAwareness xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">PerMonitorV2</dpiAwareness>
+      <dpiAware xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">true/pm</dpiAware>
+    </windowsSettings>
+  </application>
+</assembly>

CursorLang.Core.Tests/CursorLang.Core.Tests.csproj

@@ -0,0 +1,35 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0-windows10.0.19041.0</TargetFramework>
+        <SupportedOSPlatformVersion>10.0.17763.0</SupportedOSPlatformVersion>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <OutputType>Exe</OutputType>
+        <PlatformTarget>AnyCPU</PlatformTarget>
+        <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
+        <GenerateDocumentationFile>true</GenerateDocumentationFile>
+        <NoWarn>$(NoWarn);CS1591</NoWarn>
+        <IsPackable>false</IsPackable>
+        <IsTestProject>true</IsTestProject>
+        <RootNamespace>CursorLang.Core.Tests</RootNamespace>
+        <AssemblyName>CursorLang.Core.Tests</AssemblyName>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <Using Include="Xunit" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <PackageReference Include="xunit.v3" Version="3.2.2" />
+        <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.8.1" />
+        <PackageReference Include="coverlet.collector" Version="10.0.1" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\CursorLang.Core\CursorLang.Core.csproj" />
+        <ProjectReference Include="..\CursorLang.Tests.Shared\CursorLang.Tests.Shared.csproj" />
+    </ItemGroup>
+
+</Project>

CursorLang.Core.Tests/Interop/CaretNativeTests.cs

@@ -0,0 +1,106 @@
+using CursorLang.Core.Interop;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Core.Tests.Interop;
+
+/// <summary>
+/// Vetting the caret position. Some applications report it in their own
+/// coordinate system, and such answers have to be sifted out by the bounds
+/// of the input window.
+/// </summary>
+public sealed class CaretNativeTests
+{
+    private static readonly PopupWindowNative.Rect Window = new()
+    {
+        Left = 100,
+        Top = 100,
+        Right = 900,
+        Bottom = 700,
+    };
+
+    [Fact]
+    public void A_caret_inside_the_window_is_taken_as_is()
+    {
+        var caret = new PopupWindowNative.Rect { Left = 200, Top = 300, Right = 202, Bottom = 320 };
+
+        Assert.Equal(caret, CaretNative.Validate(caret, Window, scale: 1.5));
+    }
+
+    // The application reported the coordinates without the screen scale: on
+    // their own they sit above and to the left of the input window, and after
+    // being brought to the scale they land inside it
+    [Fact]
+    public void An_unscaled_caret_is_brought_to_the_screen_scale()
+    {
+        var caret = new PopupWindowNative.Rect { Left = 80, Top = 80, Right = 81, Bottom = 90 };
+
+        PopupWindowNative.Rect? validated = CaretNative.Validate(caret, Window, scale: 1.5);
+
+        Assert.NotNull(validated);
+        Assert.Equal(120, validated.Value.Left);
+        Assert.Equal(120, validated.Value.Top);
+        Assert.Equal(121, validated.Value.Right);
+        Assert.Equal(135, validated.Value.Bottom);
+    }
+
+    [Fact]
+    public void A_caret_far_from_the_window_is_discarded()
+    {
+        var caret = new PopupWindowNative.Rect { Left = 5000, Top = 5000, Right = 5002, Bottom = 5020 };
+
+        Assert.Null(CaretNative.Validate(caret, Window, scale: 1.5));
+    }
+
+    // At the ordinary scale there is nothing to fix: wrong coordinates stay wrong
+    [Fact]
+    public void At_scale_one_a_caret_outside_the_window_is_discarded()
+    {
+        var caret = new PopupWindowNative.Rect { Left = 10, Top = 10, Right = 12, Bottom = 30 };
+
+        Assert.Null(CaretNative.Validate(caret, Window, scale: 1.0));
+    }
+
+    [Theory]
+    [InlineData(100, 100, 900, 700, true)]
+    [InlineData(99, 100, 900, 700, false)]
+    [InlineData(100, 99, 900, 700, false)]
+    [InlineData(100, 100, 901, 700, false)]
+    [InlineData(100, 100, 900, 701, false)]
+    [InlineData(400, 400, 402, 420, true)]
+    public void Inside_the_window_means_entirely_inside(
+        int left, int top, int right, int bottom, bool expected)
+    {
+        var caret = new PopupWindowNative.Rect { Left = left, Top = top, Right = right, Bottom = bottom };
+
+        Assert.Equal(expected, CaretNative.IsInside(caret, Window));
+    }
+
+    // When there is no caret, its rectangle comes back with zero height. Zero
+    // coordinates, on the other hand, are the ordinary start of an empty field
+    [Theory]
+    [InlineData(0, 0, 0, 0, true)]
+    [InlineData(0, 0, 2, 0, true)]
+    [InlineData(0, 10, 2, 5, true)]
+    [InlineData(0, 0, 0, 1, false)]
+    [InlineData(0, 0, 0, 20, false)]
+    public void A_caret_without_height_counts_as_empty(
+        int left, int top, int right, int bottom, bool expected)
+    {
+        var rect = new PopupWindowNative.Rect { Left = left, Top = top, Right = right, Bottom = bottom };
+
+        Assert.Equal(expected, CaretNative.IsEmpty(rect));
+    }
+
+    // The answer depends on what is on screen right now, but it has no right to
+    // throw: the tooltip is shown whatever the answer
+    [Fact]
+    public void Asking_the_system_for_the_caret_goes_without_errors()
+    {
+        PopupWindowNative.Rect? caret = Pump.Run(CaretNative.TryGetCaretRect);
+
+        if (caret is not null)
+        {
+            Assert.True(caret.Value.Bottom > caret.Value.Top);
+        }
+    }
+}

CursorLang.Core.Tests/Interop/LowLevelKeyboardHookTests.cs

@@ -0,0 +1,209 @@
+using System.Reflection;
+using System.Runtime.InteropServices;
+using CursorLang.Core.Interop;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Core.Tests.Interop;
+
+/// <summary>
+/// Making sense of the events of the system keyboard hook.
+/// </summary>
+/// <remarks>
+/// The events are fed straight into the handler the way Windows sends them:
+/// the tests have no right to press keys for real — the interception is shared
+/// by the whole system, and a real press would land in someone else's window.
+/// </remarks>
+public sealed class LowLevelKeyboardHookTests
+{
+    private const int HcAction = 0;
+    private const int WmKeyDown = 0x0100;
+    private const int WmKeyUp = 0x0101;
+    private const int WmSysKeyDown = 0x0104;
+    private const int WmSysKeyUp = 0x0105;
+    private const int WmMouseMove = 0x0200;
+
+    private const uint Injected = 0x10;
+    private const int CapsLock = 0x14;
+
+    [Theory]
+    [InlineData(WmKeyDown, true)]
+    [InlineData(WmSysKeyDown, true)]
+    [InlineData(WmKeyUp, false)]
+    [InlineData(WmSysKeyUp, false)]
+    public void Presses_and_releases_reach_the_handler(int message, bool expectedKeyDown)
+    {
+        List<(int Key, bool IsDown)> events = [];
+        var hook = new LowLevelKeyboardHook((key, isDown) =>
+        {
+            events.Add((key, isDown));
+            return false;
+        });
+
+        using (hook)
+        {
+            Send(hook, HcAction, message, CapsLock, flags: 0);
+        }
+
+        Assert.Equal([(CapsLock, expectedKeyDown)], events);
+    }
+
+    [Fact]
+    public void A_swallowed_event_goes_no_further()
+    {
+        using var hook = new LowLevelKeyboardHook(static (_, _) => true);
+
+        IntPtr result = Send(hook, HcAction, WmKeyDown, CapsLock, flags: 0);
+
+        // A non-zero answer breaks the chain: neither the application nor the
+        // case handler in Windows will see the event
+        Assert.Equal(new IntPtr(1), result);
+    }
+
+    // Synthetic input comes from on-screen keyboards and automation tools
+    [Fact]
+    public void Synthetic_input_is_not_intercepted()
+    {
+        List<int> keys = [];
+        using var hook = new LowLevelKeyboardHook((key, _) =>
+        {
+            keys.Add(key);
+            return true;
+        });
+
+        IntPtr result = Send(hook, HcAction, WmKeyDown, CapsLock, Injected);
+
+        Assert.Empty(keys);
+        Assert.NotEqual(new IntPtr(1), result);
+    }
+
+    // Windows asks for events below zero not to be inspected but simply passed on
+    [Fact]
+    public void Events_not_meant_for_inspection_are_passed_on()
+    {
+        List<int> keys = [];
+        using var hook = new LowLevelKeyboardHook((key, _) =>
+        {
+            keys.Add(key);
+            return true;
+        });
+
+        Send(hook, code: -1, WmKeyDown, CapsLock, flags: 0);
+
+        Assert.Empty(keys);
+    }
+
+    [Fact]
+    public void The_other_messages_are_not_shown_to_the_handler()
+    {
+        List<int> keys = [];
+        using var hook = new LowLevelKeyboardHook((key, _) =>
+        {
+            keys.Add(key);
+            return true;
+        });
+
+        Send(hook, HcAction, WmMouseMove, CapsLock, flags: 0);
+
+        Assert.Empty(keys);
+    }
+
+    [Fact]
+    public void The_handler_sees_the_code_of_the_pressed_key()
+    {
+        List<int> keys = [];
+        using var hook = new LowLevelKeyboardHook((key, _) =>
+        {
+            keys.Add(key);
+            return false;
+        });
+
+        Send(hook, HcAction, WmKeyDown, virtualKey: 0x41, flags: 0);
+        Send(hook, HcAction, WmKeyDown, virtualKey: 0x1B, flags: 0);
+
+        Assert.Equal([0x41, 0x1B], keys);
+    }
+
+    [Fact]
+    public void The_interception_is_installed_and_removed()
+    {
+        Pump.Run(() =>
+        {
+            using var hook = new LowLevelKeyboardHook(static (_, _) => false);
+
+            Assert.False(hook.IsInstalled);
+
+            Assert.True(hook.Install());
+            Assert.True(hook.IsInstalled);
+
+            hook.Uninstall();
+            Assert.False(hook.IsInstalled);
+        });
+    }
+
+    [Fact]
+    public void Installing_again_changes_nothing()
+    {
+        Pump.Run(() =>
+        {
+            using var hook = new LowLevelKeyboardHook(static (_, _) => false);
+
+            Assert.True(hook.Install());
+            Assert.True(hook.Install());
+            Assert.True(hook.IsInstalled);
+
+            hook.Uninstall();
+        });
+    }
+
+    [Fact]
+    public void Removing_without_installing_passes_silently()
+    {
+        var hook = new LowLevelKeyboardHook(static (_, _) => false);
+
+        hook.Uninstall();
+        hook.Uninstall();
+
+        Assert.False(hook.IsInstalled);
+    }
+
+    [Fact]
+    public void Closing_removes_the_interception()
+    {
+        Pump.Run(() =>
+        {
+            var hook = new LowLevelKeyboardHook(static (_, _) => false);
+            hook.Install();
+
+            hook.Dispose();
+
+            Assert.False(hook.IsInstalled);
+        });
+    }
+
+    // The event arrives from Windows as a structure in unmanaged memory
+    private static IntPtr Send(
+        LowLevelKeyboardHook hook, int code, int message, int virtualKey, uint flags)
+    {
+        // vkCode, scanCode, flags and time take four bytes each, then a pointer
+        const int Size = 24;
+        IntPtr data = Marshal.AllocHGlobal(Size);
+
+        try
+        {
+            Marshal.WriteInt32(data, 0, virtualKey);
+            Marshal.WriteInt32(data, 4, 0);
+            Marshal.WriteInt32(data, 8, (int)flags);
+            Marshal.WriteInt32(data, 12, 0);
+            Marshal.WriteIntPtr(data, 16, IntPtr.Zero);
+
+            MethodInfo handler = typeof(LowLevelKeyboardHook)
+                .GetMethod("OnHookEvent", BindingFlags.Instance | BindingFlags.NonPublic)!;
+
+            return (IntPtr)handler.Invoke(hook, [code, new IntPtr(message), data])!;
+        }
+        finally
+        {
+            Marshal.FreeHGlobal(data);
+        }
+    }
+}

CursorLang.Core.Tests/Models/AppSettingsTests.cs

@@ -0,0 +1,319 @@
+using System.ComponentModel;
+using System.Drawing;
+using System.Reflection;
+using System.Text.Json;
+using CursorLang.Core.Models;
+
+namespace CursorLang.Core.Tests.Models;
+
+public sealed class AppSettingsTests
+{
+    [Fact]
+    public void Default_values_describe_a_tooltip_at_the_cursor()
+    {
+        var settings = new AppSettings();
+
+        Assert.Equal("en", settings.Language);
+        Assert.Equal(AppTheme.System, settings.Theme);
+        Assert.Equal(PopupPlacementMode.AtCursor, settings.PlacementMode);
+        Assert.Equal(500, settings.DurationMilliseconds);
+        Assert.Equal(300, settings.CapsLockHoldMilliseconds);
+    }
+
+    [Fact]
+    public void Every_mode_starts_out_looking_the_same()
+    {
+        var settings = new AppSettings();
+
+        Assert.Equal(AnchorSide.BottomRight, settings.AtCursor.Side);
+        Assert.Equal(16, settings.AtCursor.Offset);
+        Assert.Equal(CaretSide.Right, settings.AtCaret.Side);
+        Assert.Equal(16, settings.AtCaret.Offset);
+
+        // The middle of the monitor, where there is no edge to stand off from
+        Assert.Equal(ScreenPosition.Center, settings.FixedPoint.Position);
+        Assert.Equal(0, settings.FixedPoint.Offset);
+
+        PopupModeSettings[] modes = [settings.AtCursor, settings.AtCaret, settings.FixedPoint];
+
+        Assert.All(modes, mode =>
+        {
+            Assert.Equal(20, mode.FontSize);
+            Assert.Equal(0.9, mode.Opacity);
+            Assert.Equal(Color.FromArgb(0x20, 0x20, 0x20), mode.BackgroundColor);
+            Assert.Equal(Color.FromArgb(0xFF, 0xFF, 0xFF), mode.ForegroundColor);
+        });
+
+        // Three modes and three sets of settings: none of them is shared
+        Assert.Equal(3, modes.Distinct().Count());
+    }
+
+    [Theory]
+    [InlineData(PopupPlacementMode.AtCursor)]
+    [InlineData(PopupPlacementMode.AtCaret)]
+    [InlineData(PopupPlacementMode.FixedPoint)]
+    public void The_chosen_mode_is_the_one_handed_out(PopupPlacementMode mode)
+    {
+        var settings = new AppSettings { PlacementMode = mode };
+
+        PopupModeSettings expected = mode switch
+        {
+            PopupPlacementMode.AtCaret => settings.AtCaret,
+            PopupPlacementMode.FixedPoint => settings.FixedPoint,
+            _ => settings.AtCursor,
+        };
+
+        Assert.Same(expected, settings.Current);
+    }
+
+    /// <summary>
+    /// The settings of a mode are its own: setting one up leaves the others alone.
+    /// </summary>
+    /// <remarks>
+    /// This is the whole point of keeping them per mode. A look shared by the modes
+    /// meant setting it up again after every switch, and switching modes to see what
+    /// they do undid what had just been set up.
+    /// </remarks>
+    [Fact]
+    public void Setting_one_mode_up_leaves_the_others_where_they_were()
+    {
+        var settings = new AppSettings();
+
+        settings.AtCaret.FontSize = 12;
+        settings.AtCaret.Side = CaretSide.Left;
+        settings.AtCaret.BackgroundColor = Color.FromArgb(0x11, 0x22, 0x33);
+
+        Assert.Equal(20, settings.AtCursor.FontSize);
+        Assert.Equal(AnchorSide.BottomRight, settings.AtCursor.Side);
+        Assert.Equal(20, settings.FixedPoint.FontSize);
+        Assert.Equal(Color.FromArgb(0x20, 0x20, 0x20), settings.FixedPoint.BackgroundColor);
+    }
+
+    // The mode in force decides what the look means, so a switch of it is a change of
+    // everything the window shows through the current mode
+    [Fact]
+    public void A_switch_of_the_mode_is_announced_as_a_change_of_the_current_one()
+    {
+        var settings = new AppSettings();
+        List<string?> changed = [];
+        settings.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        settings.PlacementMode = PopupPlacementMode.FixedPoint;
+
+        Assert.Contains(nameof(AppSettings.PlacementMode), changed);
+        Assert.Contains(nameof(AppSettings.Current), changed);
+    }
+
+    /// <summary>
+    /// A change inside a mode is passed on as a change of the settings.
+    /// </summary>
+    /// <remarks>
+    /// The service that writes the file listens to the settings alone. Without this, a
+    /// font size dragged in the window would never reach the disk.
+    /// </remarks>
+    [Fact]
+    public void A_change_inside_a_mode_is_announced_by_the_settings()
+    {
+        var settings = new AppSettings();
+        List<string?> changed = [];
+        settings.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        settings.AtCaret.FontSize = 42;
+        settings.FixedPoint.Position = ScreenPosition.Bottom;
+        settings.AtCursor.Offset = 8;
+
+        Assert.Contains("AtCaret.FontSize", changed);
+        Assert.Contains("FixedPoint.Position", changed);
+        Assert.Contains("AtCursor.Offset", changed);
+    }
+
+    // The app must not change how the system behaves until it is asked to
+    [Fact]
+    public void Caps_Lock_interception_is_off_by_default()
+    {
+        Assert.False(new AppSettings().UseCapsLockHotkey);
+    }
+
+    [Fact]
+    public void The_time_on_screen_is_derived_from_milliseconds()
+    {
+        var settings = new AppSettings { DurationMilliseconds = 1250 };
+
+        Assert.Equal(TimeSpan.FromMilliseconds(1250), settings.Duration);
+    }
+
+    [Fact]
+    public void The_hold_threshold_is_derived_from_milliseconds()
+    {
+        var settings = new AppSettings { CapsLockHoldMilliseconds = 400 };
+
+        Assert.Equal(TimeSpan.FromMilliseconds(400), settings.CapsLockHoldDelay);
+    }
+
+    [Theory]
+    [MemberData(nameof(WritableProperties))]
+    public void A_changed_setting_is_announced_to_subscribers(string propertyName)
+    {
+        var settings = new AppSettings();
+        List<string?> changed = [];
+        settings.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        SetDifferentValue(settings, propertyName);
+
+        Assert.Contains(propertyName, changed);
+    }
+
+    [Theory]
+    [MemberData(nameof(WritableProperties))]
+    public void Writing_the_same_value_leaves_subscribers_alone(string propertyName)
+    {
+        var settings = new AppSettings();
+        PropertyInfo property = typeof(AppSettings).GetProperty(propertyName)!;
+        object? value = property.GetValue(settings);
+
+        List<string?> changed = [];
+        settings.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        property.SetValue(settings, value);
+
+        Assert.Empty(changed);
+    }
+
+    /// <summary>
+    /// Pouring another instance in fills the modes in place rather than replacing them.
+    /// </summary>
+    /// <remarks>
+    /// This is the agent's side of the connection: the fresh values arrive as a freshly
+    /// parsed instance. Replacing the mode objects would leave the popup bound to the
+    /// old ones.
+    /// </remarks>
+    [Fact]
+    public void Taking_other_settings_over_fills_the_modes_that_are_already_there()
+    {
+        var settings = new AppSettings();
+        CursorModeSettings cursor = settings.AtCursor;
+
+        var other = new AppSettings
+        {
+            Language = "ru",
+            Theme = AppTheme.Dark,
+            PlacementMode = PopupPlacementMode.FixedPoint,
+            DurationMilliseconds = 900,
+            UseCapsLockHotkey = true,
+            CapsLockHoldMilliseconds = 450,
+        };
+
+        other.AtCursor.Side = AnchorSide.TopLeft;
+        other.AtCursor.Offset = 5;
+        other.AtCursor.FontSize = 11;
+        other.AtCaret.Side = CaretSide.Left;
+        other.AtCaret.ForegroundColor = Color.FromArgb(0x0A, 0x0B, 0x0C);
+        other.FixedPoint.Position = ScreenPosition.Top;
+        other.FixedPoint.Offset = 64;
+
+        settings.CopyFrom(other);
+
+        Assert.Same(cursor, settings.AtCursor);
+
+        Assert.Equal("ru", settings.Language);
+        Assert.Equal(AppTheme.Dark, settings.Theme);
+        Assert.Equal(PopupPlacementMode.FixedPoint, settings.PlacementMode);
+        Assert.Equal(900, settings.DurationMilliseconds);
+        Assert.True(settings.UseCapsLockHotkey);
+        Assert.Equal(450, settings.CapsLockHoldMilliseconds);
+
+        Assert.Equal(AnchorSide.TopLeft, settings.AtCursor.Side);
+        Assert.Equal(5, settings.AtCursor.Offset);
+        Assert.Equal(11, settings.AtCursor.FontSize);
+        Assert.Equal(CaretSide.Left, settings.AtCaret.Side);
+        Assert.Equal(Color.FromArgb(0x0A, 0x0B, 0x0C), settings.AtCaret.ForegroundColor);
+        Assert.Equal(ScreenPosition.Top, settings.FixedPoint.Position);
+        Assert.Equal(64, settings.FixedPoint.Offset);
+    }
+
+    // Derived values and the modes handed out for convenience are computed from what
+    // is stored and have no business being in the file themselves
+    [Fact]
+    public void Derived_values_stay_out_of_the_file()
+    {
+        using JsonDocument document = JsonSerializer.SerializeToDocument(new AppSettings());
+
+        List<string> names = [.. document.RootElement.EnumerateObject().Select(property => property.Name)];
+
+        Assert.DoesNotContain(nameof(AppSettings.Duration), names);
+        Assert.DoesNotContain(nameof(AppSettings.CapsLockHoldDelay), names);
+        Assert.DoesNotContain(nameof(AppSettings.Current), names);
+
+        // What they are derived from, on the other hand, has to be stored
+        Assert.Contains(nameof(AppSettings.DurationMilliseconds), names);
+        Assert.Contains(nameof(AppSettings.CapsLockHoldMilliseconds), names);
+        Assert.Contains(nameof(AppSettings.AtCursor), names);
+        Assert.Contains(nameof(AppSettings.AtCaret), names);
+        Assert.Contains(nameof(AppSettings.FixedPoint), names);
+    }
+
+    [Fact]
+    public void Settings_report_changes_as_INotifyPropertyChanged()
+    {
+        Assert.IsAssignableFrom<INotifyPropertyChanged>(new AppSettings());
+    }
+
+    public static TheoryData<string> WritableProperties()
+    {
+        var data = new TheoryData<string>();
+
+        foreach (string name in WritablePropertyNames())
+        {
+            data.Add(name);
+        }
+
+        return data;
+    }
+
+    /// <summary>Names of the settings the user is able to change.</summary>
+    /// <remarks>
+    /// The modes are handed out rather than assigned — they are filled in place — so
+    /// they are not among these. What is inside them is checked by
+    /// <see cref="PopupModeSettingsTests"/>.
+    /// </remarks>
+    internal static IEnumerable<string> WritablePropertyNames() =>
+        typeof(AppSettings).GetProperties()
+            .Where(property => property.CanWrite)
+            .Select(property => property.Name);
+
+    // A value guaranteed to differ from the current one: each kind of setting
+    // has its own way of differing
+    private static void SetDifferentValue(AppSettings settings, string propertyName)
+    {
+        PropertyInfo property = typeof(AppSettings).GetProperty(propertyName)!;
+        object? current = property.GetValue(settings);
+
+        object next = current switch
+        {
+            string text => text + "-other",
+            double number => number + 1,
+            bool flag => !flag,
+            Color color => Color.FromArgb((byte)(color.R + 1), color.G, color.B),
+            Enum value => NextEnumValue(value),
+
+            _ => throw new NotSupportedException($"Unknown kind of setting: {property.PropertyType}"),
+        };
+
+        property.SetValue(settings, next);
+    }
+
+    private static object NextEnumValue(Enum current)
+    {
+        Array values = Enum.GetValues(current.GetType());
+
+        foreach (object? value in values)
+        {
+            if (!Equals(value, current))
+            {
+                return value!;
+            }
+        }
+
+        throw new NotSupportedException($"{current.GetType()} has a single value");
+    }
+}

CursorLang.Core.Tests/Models/KeyboardLayoutTests.cs

@@ -0,0 +1,76 @@
+using System.Globalization;
+using CursorLang.Core.Models;
+
+namespace CursorLang.Core.Tests.Models;
+
+public sealed class KeyboardLayoutTests
+{
+    [Theory]
+    [InlineData(0x0409, "EN")]
+    [InlineData(0x0419, "RU")]
+    [InlineData(0x040C, "FR")]
+    [InlineData(0x0407, "DE")]
+    public void The_short_name_comes_from_the_language_code(int localeId, string expected)
+    {
+        KeyboardLayout layout = KeyboardLayout.FromLocaleId(localeId);
+
+        Assert.Equal(expected, layout.ShortName);
+        Assert.Equal(localeId, layout.LocaleId);
+    }
+
+    [Fact]
+    public void The_full_name_joins_the_short_name_and_the_native_language_name()
+    {
+        KeyboardLayout layout = KeyboardLayout.FromLocaleId(0x0419);
+
+        Assert.Equal($"RU — {new CultureInfo(0x0419).NativeName}", layout.DisplayName);
+    }
+
+    // A layout may belong to a language the system does not know — that is not an error
+    [Fact]
+    public void An_unknown_locale_is_shown_by_its_own_code()
+    {
+        int unknown = FindUnknownLocaleId();
+
+        KeyboardLayout layout = KeyboardLayout.FromLocaleId(unknown);
+
+        string expected = $"0x{unknown:X4}";
+        Assert.Equal(expected, layout.ShortName);
+        Assert.Equal(expected, layout.DisplayName);
+    }
+
+    [Fact]
+    public void Layouts_of_the_same_locale_are_equal()
+    {
+        Assert.Equal(KeyboardLayout.FromLocaleId(0x0409), KeyboardLayout.FromLocaleId(0x0409));
+        Assert.NotEqual(KeyboardLayout.FromLocaleId(0x0409), KeyboardLayout.FromLocaleId(0x0419));
+    }
+
+    [Fact]
+    public void A_layout_can_also_be_built_directly()
+    {
+        var layout = new KeyboardLayout(1, "XX", "XX — language");
+
+        Assert.Equal(1, layout.LocaleId);
+        Assert.Equal("XX", layout.ShortName);
+        Assert.Equal("XX — language", layout.DisplayName);
+    }
+
+    // An identifier with no culture behind it in Windows
+    private static int FindUnknownLocaleId()
+    {
+        for (int candidate = 0x1000; candidate <= 0xFFFF; candidate++)
+        {
+            try
+            {
+                _ = new CultureInfo(candidate);
+            }
+            catch (CultureNotFoundException)
+            {
+                return candidate;
+            }
+        }
+
+        throw new InvalidOperationException("The system knows every locale identifier");
+    }
+}

CursorLang.Core.Tests/Models/LayoutChangedEventArgsTests.cs

@@ -0,0 +1,27 @@
+using CursorLang.Core.Models;
+
+namespace CursorLang.Core.Tests.Models;
+
+public sealed class LayoutChangedEventArgsTests
+{
+    [Theory]
+    [InlineData(LayoutChangeReason.UserSwitched)]
+    [InlineData(LayoutChangeReason.ApplicationSwitched)]
+    public void The_event_carries_the_layout_and_the_reason(LayoutChangeReason reason)
+    {
+        KeyboardLayout layout = KeyboardLayout.FromLocaleId(0x0419);
+
+        var args = new LayoutChangedEventArgs(layout, reason);
+
+        Assert.Same(layout, args.Layout);
+        Assert.Equal(reason, args.Reason);
+    }
+
+    [Fact]
+    public void The_event_stays_an_ordinary_dotnet_event()
+    {
+        var args = new LayoutChangedEventArgs(KeyboardLayout.FromLocaleId(0x0409), LayoutChangeReason.UserSwitched);
+
+        Assert.IsAssignableFrom<EventArgs>(args);
+    }
+}

CursorLang.Core.Tests/Models/PopupModeSettingsTests.cs

@@ -0,0 +1,289 @@
+using System.ComponentModel;
+using System.Drawing;
+using System.Reflection;
+using System.Text.Json;
+using CursorLang.Core.Models;
+
+namespace CursorLang.Core.Tests.Models;
+
+/// <summary>
+/// The settings of a single placement mode: what the popup looks like there and how
+/// far from its anchor it sits.
+/// </summary>
+public sealed class PopupModeSettingsTests
+{
+    [Fact]
+    public void The_cursor_mode_starts_out_below_and_right_of_the_pointer()
+    {
+        var mode = new CursorModeSettings();
+
+        Assert.Equal(AnchorSide.BottomRight, mode.Side);
+        Assert.Equal(16, mode.Offset);
+    }
+
+    /// <summary>
+    /// Next to the caret the popup goes beside it, and to the right by default.
+    /// </summary>
+    /// <remarks>
+    /// Above or below the caret is where the next line of the text is, so those sides
+    /// are not on offer at all — the type has the two of them and no more.
+    /// </remarks>
+    [Fact]
+    public void The_caret_mode_starts_out_to_the_right_of_the_caret()
+    {
+        var mode = new CaretModeSettings();
+
+        Assert.Equal(CaretSide.Right, mode.Side);
+        Assert.Equal(16, mode.Offset);
+        Assert.Equal([CaretSide.Left, CaretSide.Right], Enum.GetValues<CaretSide>());
+    }
+
+    [Theory]
+    [InlineData(CaretSide.Left, AnchorSide.Left)]
+    [InlineData(CaretSide.Right, AnchorSide.Right)]
+    public void The_side_of_the_caret_lines_the_popup_up_with_it(CaretSide side, AnchorSide expected)
+    {
+        var mode = new CaretModeSettings { Side = side };
+
+        Assert.Equal(expected, mode.Anchor);
+    }
+
+    [Fact]
+    public void A_changed_side_of_the_caret_is_a_change_of_what_it_lines_up_with()
+    {
+        var mode = new CaretModeSettings();
+        List<string?> changed = [];
+        mode.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        mode.Side = CaretSide.Left;
+
+        Assert.Contains(nameof(CaretModeSettings.Anchor), changed);
+    }
+
+    /// <summary>
+    /// The fixed point starts out in the middle of the monitor, where there is no edge
+    /// to stand off from.
+    /// </summary>
+    [Fact]
+    public void The_fixed_point_starts_out_in_the_middle_with_no_offset()
+    {
+        var mode = new FixedPointModeSettings();
+
+        Assert.Equal(ScreenPosition.Center, mode.Position);
+        Assert.Equal(0, mode.Offset);
+        Assert.False(mode.IsAtAnEdge);
+    }
+
+    // A setting that is not shown must not be one that still applies
+    [Fact]
+    public void Moving_the_fixed_point_to_the_middle_drops_the_offset()
+    {
+        var mode = new FixedPointModeSettings { Position = ScreenPosition.TopLeft, Offset = 40 };
+
+        Assert.True(mode.IsAtAnEdge);
+
+        mode.Position = ScreenPosition.Center;
+
+        Assert.Equal(0, mode.Offset);
+        Assert.False(mode.IsAtAnEdge);
+    }
+
+    [Theory]
+    [InlineData(ScreenPosition.TopLeft)]
+    [InlineData(ScreenPosition.Top)]
+    [InlineData(ScreenPosition.TopRight)]
+    [InlineData(ScreenPosition.BottomLeft)]
+    [InlineData(ScreenPosition.Bottom)]
+    [InlineData(ScreenPosition.BottomRight)]
+    public void Away_from_the_middle_the_offset_is_kept(ScreenPosition position)
+    {
+        var mode = new FixedPointModeSettings { Position = position, Offset = 40 };
+
+        Assert.Equal(40, mode.Offset);
+        Assert.True(mode.IsAtAnEdge);
+    }
+
+    [Fact]
+    public void A_changed_place_of_the_fixed_point_is_a_change_of_having_an_edge()
+    {
+        var mode = new FixedPointModeSettings();
+        List<string?> changed = [];
+        mode.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        mode.Position = ScreenPosition.Bottom;
+
+        Assert.Contains(nameof(FixedPointModeSettings.IsAtAnEdge), changed);
+    }
+
+    [Theory]
+    [MemberData(nameof(WritableProperties), typeof(CursorModeSettings))]
+    [MemberData(nameof(WritableProperties), typeof(CaretModeSettings))]
+    [MemberData(nameof(WritableProperties), typeof(FixedPointModeSettings))]
+    public void A_changed_setting_is_announced_to_subscribers(Type type, string propertyName)
+    {
+        PopupModeSettings mode = Create(type);
+        List<string?> changed = [];
+        mode.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        SetDifferentValue(mode, propertyName);
+
+        Assert.Contains(propertyName, changed);
+    }
+
+    [Theory]
+    [MemberData(nameof(WritableProperties), typeof(CursorModeSettings))]
+    [MemberData(nameof(WritableProperties), typeof(CaretModeSettings))]
+    [MemberData(nameof(WritableProperties), typeof(FixedPointModeSettings))]
+    public void Writing_the_same_value_leaves_subscribers_alone(Type type, string propertyName)
+    {
+        PopupModeSettings mode = Create(type);
+        PropertyInfo property = type.GetProperty(propertyName)!;
+
+        List<string?> changed = [];
+        mode.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        property.SetValue(mode, property.GetValue(mode));
+
+        Assert.Empty(changed);
+    }
+
+    [Fact]
+    public void The_cursor_mode_takes_another_one_over_whole()
+    {
+        var mode = new CursorModeSettings();
+        var other = new CursorModeSettings
+        {
+            Side = AnchorSide.Left,
+            Offset = 3,
+            FontSize = 41,
+            Opacity = 0.25,
+            BackgroundColor = Color.FromArgb(0x01, 0x02, 0x03),
+            ForegroundColor = Color.FromArgb(0x04, 0x05, 0x06),
+        };
+
+        mode.CopyFrom(other);
+
+        Assert.Equal(AnchorSide.Left, mode.Side);
+        Assert.Equal(3, mode.Offset);
+        Assert.Equal(41, mode.FontSize);
+        Assert.Equal(0.25, mode.Opacity);
+        Assert.Equal(Color.FromArgb(0x01, 0x02, 0x03), mode.BackgroundColor);
+        Assert.Equal(Color.FromArgb(0x04, 0x05, 0x06), mode.ForegroundColor);
+    }
+
+    [Fact]
+    public void The_caret_mode_takes_another_one_over_whole()
+    {
+        var mode = new CaretModeSettings();
+        var other = new CaretModeSettings { Side = CaretSide.Left, Offset = 2, FontSize = 15 };
+
+        mode.CopyFrom(other);
+
+        Assert.Equal(CaretSide.Left, mode.Side);
+        Assert.Equal(2, mode.Offset);
+        Assert.Equal(15, mode.FontSize);
+    }
+
+    [Fact]
+    public void The_fixed_point_takes_another_one_over_whole()
+    {
+        var mode = new FixedPointModeSettings();
+        var other = new FixedPointModeSettings
+        {
+            Position = ScreenPosition.Bottom,
+            Offset = 120,
+            FontSize = 60,
+            Opacity = 0.5,
+            BackgroundColor = Color.FromArgb(0x07, 0x08, 0x09),
+            ForegroundColor = Color.FromArgb(0x0A, 0x0B, 0x0C),
+        };
+
+        mode.CopyFrom(other);
+
+        Assert.Equal(ScreenPosition.Bottom, mode.Position);
+        Assert.Equal(120, mode.Offset);
+        Assert.Equal(60, mode.FontSize);
+        Assert.Equal(0.5, mode.Opacity);
+        Assert.Equal(Color.FromArgb(0x07, 0x08, 0x09), mode.BackgroundColor);
+        Assert.Equal(Color.FromArgb(0x0A, 0x0B, 0x0C), mode.ForegroundColor);
+    }
+
+    // Taking over a mode that sits in the middle takes its lack of an offset over too
+    [Fact]
+    public void Taking_over_a_fixed_point_in_the_middle_leaves_no_offset()
+    {
+        var mode = new FixedPointModeSettings { Position = ScreenPosition.Top, Offset = 32 };
+
+        mode.CopyFrom(new FixedPointModeSettings());
+
+        Assert.Equal(ScreenPosition.Center, mode.Position);
+        Assert.Equal(0, mode.Offset);
+    }
+
+    // What a mode works out from what it stores is not stored itself
+    [Fact]
+    public void Derived_values_of_a_mode_stay_out_of_the_file()
+    {
+        List<string> caret = Stored(new CaretModeSettings());
+        List<string> fixedPoint = Stored(new FixedPointModeSettings());
+
+        Assert.DoesNotContain(nameof(CaretModeSettings.Anchor), caret);
+        Assert.DoesNotContain(nameof(FixedPointModeSettings.IsAtAnEdge), fixedPoint);
+
+        Assert.Contains(nameof(CaretModeSettings.Side), caret);
+        Assert.Contains(nameof(FixedPointModeSettings.Position), fixedPoint);
+    }
+
+    [Fact]
+    public void A_mode_reports_changes_as_INotifyPropertyChanged()
+    {
+        Assert.IsAssignableFrom<INotifyPropertyChanged>(new CursorModeSettings());
+        Assert.IsAssignableFrom<INotifyPropertyChanged>(new CaretModeSettings());
+        Assert.IsAssignableFrom<INotifyPropertyChanged>(new FixedPointModeSettings());
+    }
+
+    public static TheoryData<Type, string> WritableProperties(Type type)
+    {
+        var data = new TheoryData<Type, string>();
+
+        foreach (string name in WritablePropertyNames(type))
+        {
+            data.Add(type, name);
+        }
+
+        return data;
+    }
+
+    /// <summary>Names of the settings of a mode the user is able to change.</summary>
+    internal static IEnumerable<string> WritablePropertyNames(Type type) =>
+        type.GetProperties().Where(property => property.CanWrite).Select(property => property.Name);
+
+    private static PopupModeSettings Create(Type type) => (PopupModeSettings)Activator.CreateInstance(type)!;
+
+    private static List<string> Stored<TMode>(TMode mode) where TMode : PopupModeSettings
+    {
+        using JsonDocument document = JsonSerializer.SerializeToDocument(mode);
+
+        return [.. document.RootElement.EnumerateObject().Select(property => property.Name)];
+    }
+
+    // A value guaranteed to differ from the current one: each kind of setting has its
+    // own way of differing
+    private static void SetDifferentValue(PopupModeSettings mode, string propertyName)
+    {
+        PropertyInfo property = mode.GetType().GetProperty(propertyName)!;
+
+        object next = property.GetValue(mode) switch
+        {
+            double number => number + 1,
+            Color color => Color.FromArgb((byte)(color.R + 1), color.G, color.B),
+            Enum value => Enum.GetValues(value.GetType())
+                .Cast<object>()
+                .First(other => !Equals(other, value)),
+
+            var other => throw new NotSupportedException($"Unknown kind of setting: {other?.GetType()}"),
+        };
+
+        property.SetValue(mode, next);
+    }
+}

CursorLang.Core.Tests/Resources/StringsTests.cs

@@ -0,0 +1,142 @@
+using System.Collections;
+using System.Globalization;
+using System.Resources;
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+
+namespace CursorLang.Core.Tests.Resources;
+
+/// <summary>
+/// Checks of the resources themselves: they carry every caption in the settings
+/// window, and a missing key only shows on a live window.
+/// </summary>
+public sealed class StringsTests
+{
+    private static readonly ResourceManager Resources =
+        new("CursorLang.Core.Resources.Strings", typeof(LocalizationService).Assembly);
+
+    private static readonly CultureInfo English = CultureInfo.GetCultureInfo("en");
+    private static readonly CultureInfo Russian = CultureInfo.GetCultureInfo("ru");
+
+    [Theory]
+    [MemberData(nameof(EnumKeys))]
+    public void Every_list_value_has_an_English_caption(string key)
+    {
+        Assert.False(string.IsNullOrWhiteSpace(Resources.GetString(key, English)));
+    }
+
+    [Theory]
+    [MemberData(nameof(EnumKeys))]
+    public void Every_list_value_has_a_Russian_caption(string key)
+    {
+        Assert.False(string.IsNullOrWhiteSpace(Resources.GetString(key, Russian)));
+    }
+
+    [Fact]
+    public void The_Russian_translation_covers_every_string()
+    {
+        List<string> missing = [];
+
+        foreach (string key in NeutralKeys())
+        {
+            // An untranslated resource falls back to English, so the Russian set
+            // is asked directly rather than through the string with a fallback
+            if (RussianSet().GetString(key) is null)
+            {
+                missing.Add(key);
+            }
+        }
+
+        Assert.Empty(missing);
+    }
+
+    [Fact]
+    public void The_Russian_translation_has_no_extra_strings()
+    {
+        HashSet<string> neutral = [.. NeutralKeys()];
+        List<string> extra = [];
+
+        foreach (DictionaryEntry entry in RussianSet())
+        {
+            var key = (string)entry.Key;
+            if (!neutral.Contains(key))
+            {
+                extra.Add(key);
+            }
+        }
+
+        Assert.Empty(extra);
+    }
+
+    [Fact]
+    public void There_are_no_empty_strings_in_the_resources()
+    {
+        foreach (string key in NeutralKeys())
+        {
+            Assert.False(string.IsNullOrWhiteSpace(Resources.GetString(key, English)), key);
+            Assert.False(string.IsNullOrWhiteSpace(Resources.GetString(key, Russian)), key);
+        }
+    }
+
+    /// <summary>
+    /// The version is put into the title by the app, so the place for it has to
+    /// be there in both languages: the title is the only place it is shown, and a
+    /// translation without the placeholder would quietly drop it.
+    /// </summary>
+    [Fact]
+    public void The_title_of_the_window_has_room_for_the_version()
+    {
+        Assert.Contains("{0}", Resources.GetString("SettingsTitle", English), StringComparison.Ordinal);
+        Assert.Contains("{0}", Resources.GetString("SettingsTitle", Russian), StringComparison.Ordinal);
+    }
+
+    public static TheoryData<string> EnumKeys()
+    {
+        var data = new TheoryData<string>();
+
+        foreach (string key in EnumKeysOf<AppTheme>())
+        {
+            data.Add(key);
+        }
+
+        foreach (string key in EnumKeysOf<PopupPlacementMode>())
+        {
+            data.Add(key);
+        }
+
+        foreach (string key in EnumKeysOf<AnchorSide>())
+        {
+            data.Add(key);
+        }
+
+        foreach (string key in EnumKeysOf<CaretSide>())
+        {
+            data.Add(key);
+        }
+
+        foreach (string key in EnumKeysOf<ScreenPosition>())
+        {
+            data.Add(key);
+        }
+
+        return data;
+    }
+
+    // A caption key is built from the type name and the value: PopupPlacementMode_AtCursor
+    private static IEnumerable<string> EnumKeysOf<TEnum>() where TEnum : struct, Enum =>
+        Enum.GetValues<TEnum>().Select(value => $"{typeof(TEnum).Name}_{value}");
+
+    private static IEnumerable<string> NeutralKeys()
+    {
+        ResourceSet set = Resources.GetResourceSet(CultureInfo.InvariantCulture, true, true)!;
+
+        foreach (DictionaryEntry entry in set)
+        {
+            yield return (string)entry.Key;
+        }
+    }
+
+    private static ResourceSet RussianSet() =>
+        Resources.GetResourceSet(Russian, createIfNotExists: true, tryParents: false)!;
+
+}

CursorLang.Core.Tests/Services/CapsLockSwitchCoordinatorTests.cs

@@ -0,0 +1,192 @@
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Core.Tests.Services;
+
+/// <summary>
+/// What happens on Caps Lock presses and how the interception follows the setting.
+/// </summary>
+public sealed class CapsLockSwitchCoordinatorTests
+{
+    [Fact]
+    public void With_the_setting_on_the_interception_starts_at_once()
+    {
+        var hotkey = new FakeCapsLockHotkeyService();
+        using CapsLockSwitchCoordinator coordinator = Create(hotkey, new AppSettings { UseCapsLockHotkey = true });
+
+        coordinator.Start();
+
+        Assert.True(hotkey.IsRunning);
+        Assert.Equal(1, hotkey.StartCalls);
+    }
+
+    [Fact]
+    public void With_the_setting_off_there_is_no_interception()
+    {
+        var hotkey = new FakeCapsLockHotkeyService();
+        using CapsLockSwitchCoordinator coordinator = Create(hotkey, new AppSettings { UseCapsLockHotkey = false });
+
+        coordinator.Start();
+
+        Assert.False(hotkey.IsRunning);
+        Assert.Equal(1, hotkey.StopCalls);
+    }
+
+    [Fact]
+    public void Ticking_the_setting_turns_the_interception_on_live()
+    {
+        var hotkey = new FakeCapsLockHotkeyService();
+        var settings = new AppSettings { UseCapsLockHotkey = false };
+        using CapsLockSwitchCoordinator coordinator = Create(hotkey, settings);
+        coordinator.Start();
+
+        settings.UseCapsLockHotkey = true;
+
+        Assert.True(hotkey.IsRunning);
+    }
+
+    [Fact]
+    public void Unticking_the_setting_gives_the_key_its_usual_behaviour_back()
+    {
+        var hotkey = new FakeCapsLockHotkeyService();
+        var settings = new AppSettings { UseCapsLockHotkey = true };
+        using CapsLockSwitchCoordinator coordinator = Create(hotkey, settings);
+        coordinator.Start();
+
+        settings.UseCapsLockHotkey = false;
+
+        Assert.False(hotkey.IsRunning);
+    }
+
+    // The interception only follows its own setting
+    [Fact]
+    public void Other_settings_leave_the_interception_alone()
+    {
+        var hotkey = new FakeCapsLockHotkeyService();
+        var settings = new AppSettings { UseCapsLockHotkey = true };
+        using CapsLockSwitchCoordinator coordinator = Create(hotkey, settings);
+        coordinator.Start();
+
+        int startsBefore = hotkey.StartCalls;
+        int stopsBefore = hotkey.StopCalls;
+
+        settings.Current.FontSize = 44;
+        settings.CapsLockHoldMilliseconds = 700;
+
+        Assert.Equal(startsBefore, hotkey.StartCalls);
+        Assert.Equal(stopsBefore, hotkey.StopCalls);
+    }
+
+    [Fact]
+    public void A_short_press_switches_the_layout()
+    {
+        var hotkey = new FakeCapsLockHotkeyService();
+        var layouts = new FakeKeyboardLayoutService();
+        var popup = new FakeLayoutPopupService();
+
+        using var coordinator = new CapsLockSwitchCoordinator(
+            hotkey, layouts, popup, new AppSettings { UseCapsLockHotkey = true });
+        coordinator.Start();
+
+        hotkey.RaiseTapped();
+
+        Assert.Equal(1, layouts.SwitchCalls);
+        Assert.Empty(popup.Shown);
+        Assert.Empty(popup.ShownUntilHidden);
+    }
+
+    // The layout stays put, but staying silent is not an option either: without
+    // a tooltip a long press looks like a key that did not work
+    [Fact]
+    public void A_long_press_shows_the_current_layout()
+    {
+        var hotkey = new FakeCapsLockHotkeyService();
+        var layouts = new FakeKeyboardLayoutService
+        {
+            CurrentLayout = KeyboardLayout.FromLocaleId(0x0419),
+        };
+        var popup = new FakeLayoutPopupService();
+
+        using var coordinator = new CapsLockSwitchCoordinator(
+            hotkey, layouts, popup, new AppSettings { UseCapsLockHotkey = true });
+        coordinator.Start();
+
+        hotkey.RaiseHoldStarted();
+
+        Assert.Equal([layouts.CurrentLayout], popup.ShownUntilHidden);
+        Assert.Equal(0, layouts.SwitchCalls);
+    }
+
+    [Fact]
+    public void When_the_hold_ends_the_tooltip_goes_away()
+    {
+        var hotkey = new FakeCapsLockHotkeyService();
+        var layouts = new FakeKeyboardLayoutService();
+        var popup = new FakeLayoutPopupService();
+
+        using var coordinator = new CapsLockSwitchCoordinator(
+            hotkey, layouts, popup, new AppSettings { UseCapsLockHotkey = true });
+        coordinator.Start();
+
+        hotkey.RaiseHoldStarted();
+        hotkey.RaiseHoldEnded();
+
+        Assert.Equal(1, popup.HideCalls);
+        Assert.Equal(0, layouts.SwitchCalls);
+    }
+
+    [Fact]
+    public void Before_the_start_presses_do_nothing()
+    {
+        var hotkey = new FakeCapsLockHotkeyService();
+        var layouts = new FakeKeyboardLayoutService();
+        var popup = new FakeLayoutPopupService();
+
+        using var coordinator = new CapsLockSwitchCoordinator(
+            hotkey, layouts, popup, new AppSettings { UseCapsLockHotkey = true });
+
+        hotkey.RaiseTapped();
+        hotkey.RaiseHoldStarted();
+
+        Assert.Equal(0, layouts.SwitchCalls);
+        Assert.Empty(popup.ShownUntilHidden);
+    }
+
+    [Fact]
+    public void Closing_removes_the_interception_and_unsubscribes_from_presses()
+    {
+        var hotkey = new FakeCapsLockHotkeyService();
+        var layouts = new FakeKeyboardLayoutService();
+        var settings = new AppSettings { UseCapsLockHotkey = true };
+
+        var coordinator = new CapsLockSwitchCoordinator(
+            hotkey, layouts, new FakeLayoutPopupService(), settings);
+        coordinator.Start();
+        coordinator.Dispose();
+
+        Assert.False(hotkey.IsRunning);
+        Assert.False(hotkey.HasSubscribers);
+
+        hotkey.RaiseTapped();
+        Assert.Equal(0, layouts.SwitchCalls);
+    }
+
+    [Fact]
+    public void After_closing_the_setting_no_longer_turns_the_interception_on()
+    {
+        var hotkey = new FakeCapsLockHotkeyService();
+        var settings = new AppSettings { UseCapsLockHotkey = false };
+
+        CapsLockSwitchCoordinator coordinator = Create(hotkey, settings);
+        coordinator.Start();
+        coordinator.Dispose();
+
+        settings.UseCapsLockHotkey = true;
+
+        Assert.False(hotkey.IsRunning);
+    }
+
+    private static CapsLockSwitchCoordinator Create(FakeCapsLockHotkeyService hotkey, AppSettings settings) =>
+        new(hotkey, new FakeKeyboardLayoutService(), new FakeLayoutPopupService(), settings);
+}

CursorLang.Core.Tests/Services/KeyboardLayoutServiceTests.cs

@@ -0,0 +1,286 @@
+using System.Collections.Concurrent;
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Core.Tests.Services;
+
+/// <summary>
+/// Watching the layout of the foreground window. The test supplies what the
+/// system reports: what is under test is the decision about what counts as
+/// a layout change.
+/// </summary>
+public sealed class KeyboardLayoutServiceTests
+{
+    private static readonly IntPtr FirstWindow = new(1000);
+    private static readonly IntPtr SecondWindow = new(2000);
+
+    private const int English = 0x0409;
+    private const int Russian = 0x0419;
+
+    [Fact]
+    public void The_current_layout_is_taken_from_the_foreground_window()
+    {
+        using var world = new World { LocaleId = Russian };
+        KeyboardLayoutService service = world.CreateService();
+
+        Assert.Equal("RU", service.Current.ShortName);
+
+        world.LocaleId = English;
+        Assert.Equal("EN", service.Current.ShortName);
+    }
+
+    [Fact]
+    public void Switching_asks_the_system_to_change_the_layout()
+    {
+        using var world = new World();
+        KeyboardLayoutService service = world.CreateService();
+
+        service.SwitchToNext();
+        service.SwitchToNext();
+
+        Assert.Equal(2, world.SwitchRequests);
+    }
+
+    [Fact]
+    public void A_layout_change_in_the_same_window_counts_as_the_users_doing()
+    {
+        using var world = new World { LocaleId = English };
+        KeyboardLayoutService service = world.CreateService();
+
+        Pump.Run(service.Start);
+        world.LocaleId = Russian;
+        Pump.Run(service.Poll);
+
+        LayoutChangedEventArgs change = Assert.Single(world.Changes);
+        Assert.Equal(LayoutChangeReason.UserSwitched, change.Reason);
+        Assert.Equal("RU", change.Layout.ShortName);
+    }
+
+    [Fact]
+    public void Moving_to_another_application_differs_from_switching()
+    {
+        using var world = new World { LocaleId = English };
+        KeyboardLayoutService service = world.CreateService();
+
+        Pump.Run(service.Start);
+
+        world.ForegroundWindow = SecondWindow;
+        world.LocaleId = Russian;
+        Pump.Run(service.Poll);
+
+        LayoutChangedEventArgs change = Assert.Single(world.Changes);
+        Assert.Equal(LayoutChangeReason.ApplicationSwitched, change.Reason);
+    }
+
+    // Moving to an application with the same layout changes nothing
+    [Fact]
+    public void Moving_without_a_layout_change_yields_no_events()
+    {
+        using var world = new World { LocaleId = English };
+        KeyboardLayoutService service = world.CreateService();
+
+        Pump.Run(service.Start);
+
+        world.ForegroundWindow = SecondWindow;
+        Pump.Run(service.Poll);
+
+        Assert.Empty(world.Changes);
+    }
+
+    [Fact]
+    public void An_unchanged_layout_yields_no_events()
+    {
+        using var world = new World { LocaleId = Russian };
+        KeyboardLayoutService service = world.CreateService();
+
+        Pump.Run(service.Start);
+
+        for (int i = 0; i < 5; i++)
+        {
+            Pump.Run(service.Poll);
+        }
+
+        Assert.Empty(world.Changes);
+    }
+
+    // There is no foreground window — during a desktop switch, for one
+    [Fact]
+    public void Without_a_foreground_window_the_poll_is_skipped()
+    {
+        using var world = new World { LocaleId = English };
+        KeyboardLayoutService service = world.CreateService();
+
+        Pump.Run(service.Start);
+
+        world.ForegroundWindow = IntPtr.Zero;
+        world.LocaleId = Russian;
+        Pump.Run(service.Poll);
+
+        Assert.Empty(world.Changes);
+
+        // And the layout was not remembered: the change is noticed once a window is back
+        world.ForegroundWindow = FirstWindow;
+        Pump.Run(service.Poll);
+
+        Assert.Single(world.Changes);
+    }
+
+    [Fact]
+    public void One_change_yields_exactly_one_event()
+    {
+        using var world = new World { LocaleId = English };
+        KeyboardLayoutService service = world.CreateService();
+
+        Pump.Run(service.Start);
+
+        world.LocaleId = Russian;
+        Pump.Run(service.Poll);
+        Pump.Run(service.Poll);
+
+        Assert.Single(world.Changes);
+    }
+
+    [Fact]
+    public void A_layout_change_before_the_watch_starts_goes_unnoticed()
+    {
+        using var world = new World { LocaleId = English };
+        KeyboardLayoutService service = world.CreateService();
+
+        world.LocaleId = Russian;
+        Pump.Run(service.Start);
+        Pump.Run(service.Poll);
+
+        // Start remembered the layout that was in place at that moment
+        Assert.Empty(world.Changes);
+    }
+
+    [Fact]
+    public void The_watch_runs_on_a_timer()
+    {
+        using var world = new World { LocaleId = English };
+        KeyboardLayoutService service = world.CreateService(TimeSpan.FromMilliseconds(15));
+
+        Pump.Run(service.Start);
+        world.LocaleId = Russian;
+
+        Pump.WaitFor(() => !world.Changes.IsEmpty, "the timer noticed the layout change");
+    }
+
+    [Fact]
+    public void Stopping_ends_the_polling()
+    {
+        using var world = new World { LocaleId = English };
+        KeyboardLayoutService service = world.CreateService(TimeSpan.FromMilliseconds(15));
+
+        Pump.Run(service.Start);
+        Pump.Run(service.Stop);
+
+        world.LocaleId = Russian;
+        Pump.Pause(TimeSpan.FromMilliseconds(120));
+
+        Assert.Empty(world.Changes);
+    }
+
+    [Fact]
+    public void Closing_ends_the_polling()
+    {
+        using var world = new World { LocaleId = English };
+        KeyboardLayoutService service = world.CreateService(TimeSpan.FromMilliseconds(15));
+
+        Pump.Run(service.Start);
+        Pump.Run(service.Dispose);
+
+        world.LocaleId = Russian;
+        Pump.Pause(TimeSpan.FromMilliseconds(120));
+
+        Assert.Empty(world.Changes);
+    }
+
+    [Fact]
+    public void The_watch_can_be_resumed()
+    {
+        using var world = new World { LocaleId = English };
+        KeyboardLayoutService service = world.CreateService(TimeSpan.FromMilliseconds(15));
+
+        Pump.Run(service.Start);
+        Pump.Run(service.Stop);
+        Pump.Run(service.Start);
+
+        world.LocaleId = Russian;
+
+        Pump.WaitFor(() => !world.Changes.IsEmpty, "the watch resumed");
+    }
+
+    // The ordinary service asks Windows itself about the layout
+    [Fact]
+    public void The_service_can_work_with_the_real_system()
+    {
+        KeyboardLayoutService service = Pump.Run(() =>
+            new KeyboardLayoutService(new KeyboardLayoutOptions { PollInterval = TimeSpan.FromHours(1) }));
+
+        try
+        {
+            Pump.Run(service.Start);
+            Pump.Run(service.Poll);
+
+            Assert.InRange(service.Current.LocaleId, 1, 0xFFFF);
+
+            Pump.Run(service.Stop);
+        }
+        finally
+        {
+            Pump.Run(service.Dispose);
+        }
+    }
+
+    [Fact]
+    public void By_default_the_poll_runs_more_than_six_times_a_second()
+    {
+        // Any rarer and the tooltip would visibly lag behind the keystroke
+        Assert.True(new KeyboardLayoutOptions().PollInterval <= TimeSpan.FromMilliseconds(150));
+    }
+
+    /// <summary>
+    /// The state of the system as the service sees it, and everything the
+    /// service reported about it.
+    /// </summary>
+    private sealed class World : IDisposable
+    {
+        private KeyboardLayoutService? _service;
+
+        internal IntPtr ForegroundWindow { get; set; } = FirstWindow;
+
+        internal int LocaleId { get; set; } = English;
+
+        internal int SwitchRequests { get; private set; }
+
+        internal ConcurrentQueue<LayoutChangedEventArgs> Changes { get; } = new();
+
+        internal KeyboardLayoutService CreateService(TimeSpan? pollInterval = null)
+        {
+            var options = new KeyboardLayoutOptions
+            {
+                PollInterval = pollInterval ?? TimeSpan.FromHours(1),
+            };
+
+            _service = Pump.Run(() => new KeyboardLayoutService(
+                options,
+                () => ForegroundWindow,
+                () => LocaleId,
+                () => SwitchRequests++));
+
+            _service.LayoutChanged += (_, e) => Changes.Enqueue(e);
+
+            return _service;
+        }
+
+        public void Dispose()
+        {
+            if (_service is not null)
+            {
+                Pump.Run(_service.Dispose);
+            }
+        }
+    }
+}

CursorLang.Core.Tests/Services/LayoutNotificationCoordinatorTests.cs

@@ -0,0 +1,116 @@
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Core.Tests.Services;
+
+/// <summary>
+/// The link between watching the layout and showing the tooltip.
+/// </summary>
+public sealed class LayoutNotificationCoordinatorTests
+{
+    private static readonly KeyboardLayout Russian = KeyboardLayout.FromLocaleId(0x0419);
+
+    [Fact]
+    public void Starting_turns_on_the_layout_watch()
+    {
+        var layouts = new FakeKeyboardLayoutService();
+        var popup = new FakeLayoutPopupService();
+
+        using var coordinator = new LayoutNotificationCoordinator(layouts, popup);
+        coordinator.Start();
+
+        Assert.Equal(1, layouts.StartCalls);
+    }
+
+    [Fact]
+    public void A_layout_switched_by_the_user_shows_the_tooltip()
+    {
+        var layouts = new FakeKeyboardLayoutService();
+        var popup = new FakeLayoutPopupService();
+
+        using var coordinator = new LayoutNotificationCoordinator(layouts, popup);
+        coordinator.Start();
+
+        layouts.RaiseLayoutChanged(Russian, LayoutChangeReason.UserSwitched);
+
+        Assert.Equal([Russian], popup.Shown);
+    }
+
+    // Moving to another application changes the layout with no user involved,
+    // and the tooltip would be intrusive
+    [Fact]
+    public void Switching_applications_shows_no_tooltip()
+    {
+        var layouts = new FakeKeyboardLayoutService();
+        var popup = new FakeLayoutPopupService();
+
+        using var coordinator = new LayoutNotificationCoordinator(layouts, popup);
+        coordinator.Start();
+
+        layouts.RaiseLayoutChanged(Russian, LayoutChangeReason.ApplicationSwitched);
+
+        Assert.Empty(popup.Shown);
+    }
+
+    [Fact]
+    public void Every_switch_shows_its_own_layout()
+    {
+        var layouts = new FakeKeyboardLayoutService();
+        var popup = new FakeLayoutPopupService();
+
+        using var coordinator = new LayoutNotificationCoordinator(layouts, popup);
+        coordinator.Start();
+
+        KeyboardLayout english = KeyboardLayout.FromLocaleId(0x0409);
+        layouts.RaiseLayoutChanged(Russian, LayoutChangeReason.UserSwitched);
+        layouts.RaiseLayoutChanged(english, LayoutChangeReason.UserSwitched);
+
+        Assert.Equal([Russian, english], popup.Shown);
+    }
+
+    [Fact]
+    public void Before_the_start_no_tooltip_is_shown()
+    {
+        var layouts = new FakeKeyboardLayoutService();
+        var popup = new FakeLayoutPopupService();
+
+        using var coordinator = new LayoutNotificationCoordinator(layouts, popup);
+
+        layouts.RaiseLayoutChanged(Russian, LayoutChangeReason.UserSwitched);
+
+        Assert.Empty(popup.Shown);
+    }
+
+    [Fact]
+    public void Closing_stops_the_watch_and_unsubscribes_from_the_event()
+    {
+        var layouts = new FakeKeyboardLayoutService();
+        var popup = new FakeLayoutPopupService();
+
+        var coordinator = new LayoutNotificationCoordinator(layouts, popup);
+        coordinator.Start();
+        coordinator.Dispose();
+
+        Assert.Equal(1, layouts.StopCalls);
+        Assert.False(layouts.HasSubscribers);
+
+        layouts.RaiseLayoutChanged(Russian, LayoutChangeReason.UserSwitched);
+        Assert.Empty(popup.Shown);
+    }
+
+    [Fact]
+    public void The_show_until_hidden_is_not_called_from_here()
+    {
+        var layouts = new FakeKeyboardLayoutService();
+        var popup = new FakeLayoutPopupService();
+
+        using var coordinator = new LayoutNotificationCoordinator(layouts, popup);
+        coordinator.Start();
+
+        layouts.RaiseLayoutChanged(Russian, LayoutChangeReason.UserSwitched);
+
+        Assert.Empty(popup.ShownUntilHidden);
+        Assert.Equal(0, popup.HideCalls);
+    }
+}

CursorLang.Core.Tests/Services/LocalizationServiceTests.cs

@@ -0,0 +1,149 @@
+using System.ComponentModel;
+using System.Globalization;
+using System.Windows.Data;
+using CursorLang.Core.Services;
+
+namespace CursorLang.Core.Tests.Services;
+
+public sealed class LocalizationServiceTests
+{
+    [Fact]
+    public void The_interface_starts_out_in_English()
+    {
+        Assert.Equal("en", new LocalizationService().CurrentLanguage);
+    }
+
+    [Fact]
+    public void A_string_comes_from_the_resources_of_the_chosen_language()
+    {
+        var localization = new LocalizationService();
+
+        string english = localization["SettingsTitle"];
+        localization.CurrentLanguage = "ru";
+        string russian = localization["SettingsTitle"];
+
+        Assert.False(string.IsNullOrWhiteSpace(english));
+        Assert.False(string.IsNullOrWhiteSpace(russian));
+        Assert.NotEqual(english, russian);
+    }
+
+    // A missing key shows in the interface but does not bring the app down
+    [Fact]
+    public void An_unknown_key_comes_back_as_is()
+    {
+        var localization = new LocalizationService();
+
+        Assert.Equal("NoSuchKey", localization["NoSuchKey"]);
+    }
+
+    [Fact]
+    public void A_language_change_is_announced_to_subscribers()
+    {
+        var localization = new LocalizationService();
+        List<string?> changed = [];
+        localization.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        localization.CurrentLanguage = "ru";
+
+        Assert.Contains(nameof(LocalizationService.CurrentLanguage), changed);
+
+        // The indexer is announced separately: that is how the whole text refreshes
+        Assert.Contains(Binding.IndexerName, changed);
+    }
+
+    [Fact]
+    public void The_same_language_is_not_announced_again()
+    {
+        var localization = new LocalizationService { CurrentLanguage = "ru" };
+        List<string?> changed = [];
+        localization.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        localization.CurrentLanguage = "ru";
+
+        Assert.Empty(changed);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("   ")]
+    [InlineData(null)]
+    public void An_empty_language_changes_nothing(string? value)
+    {
+        var localization = new LocalizationService();
+        List<string?> changed = [];
+        localization.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        localization.CurrentLanguage = value!;
+
+        Assert.Equal("en", localization.CurrentLanguage);
+        Assert.Empty(changed);
+    }
+
+    [Fact]
+    public void A_language_with_a_country_falls_back_to_the_language_code()
+    {
+        var localization = new LocalizationService { CurrentLanguage = "ru-RU" };
+
+        Assert.Equal("ru", localization.CurrentLanguage);
+    }
+
+    // Changing the app language has to change the language of the thread as
+    // well: other texts, down to system messages, depend on it
+    [Fact]
+    public void A_language_change_changes_the_language_of_the_thread()
+    {
+        CultureInfo previous = CultureInfo.CurrentUICulture;
+        try
+        {
+            var localization = new LocalizationService { CurrentLanguage = "ru" };
+
+            Assert.Equal("ru", CultureInfo.CurrentUICulture.TwoLetterISOLanguageName);
+            Assert.Equal("ru", localization.CurrentLanguage);
+        }
+        finally
+        {
+            CultureInfo.CurrentUICulture = previous;
+        }
+    }
+
+    [Fact]
+    public void English_and_Russian_are_offered_for_choosing()
+    {
+        IReadOnlyList<LanguageOption> languages = new LocalizationService().AvailableLanguages;
+
+        Assert.Equal(2, languages.Count);
+        Assert.Contains(languages, language => language.Code == "en");
+        Assert.Contains(languages, language => language.Code == "ru");
+    }
+
+    // A language is named in itself: that way it is recognised even by someone
+    // who does not know the current interface language
+    [Fact]
+    public void The_languages_are_named_in_themselves()
+    {
+        IReadOnlyList<LanguageOption> languages = new LocalizationService().AvailableLanguages;
+
+        Assert.Equal("English", languages.Single(language => language.Code == "en").DisplayName);
+        Assert.Equal("Русский", languages.Single(language => language.Code == "ru").DisplayName);
+    }
+
+    // Accessibility tools take the name of a list item from ToString
+    [Fact]
+    public void A_language_presents_itself_by_its_name()
+    {
+        Assert.Equal("Русский", new LanguageOption("ru", "Русский").ToString());
+    }
+
+    [Fact]
+    public void Languages_with_the_same_code_and_name_are_equal()
+    {
+        Assert.Equal(new LanguageOption("ru", "Русский"), new LanguageOption("ru", "Русский"));
+        Assert.NotEqual(new LanguageOption("ru", "Русский"), new LanguageOption("en", "English"));
+    }
+
+    [Fact]
+    public void The_service_reports_changes_as_INotifyPropertyChanged()
+    {
+        Assert.IsAssignableFrom<INotifyPropertyChanged>(new LocalizationService());
+    }
+}

CursorLang.Core.Tests/Services/PopupLayoutTests.cs

@@ -0,0 +1,229 @@
+using CursorLang.Core.Interop;
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+
+namespace CursorLang.Core.Tests.Services;
+
+/// <summary>
+/// The placement maths for the tooltip. This is the easiest place to get a sign
+/// or half a size wrong, and on screen such a mistake is only visible by eye.
+/// </summary>
+public sealed class PopupLayoutTests
+{
+    // The anchor: 100..140 horizontally, 200..220 vertically
+    private static readonly PopupWindowNative.Rect Anchor = new()
+    {
+        Left = 100,
+        Top = 200,
+        Right = 140,
+        Bottom = 220,
+    };
+
+    private const int Offset = 10;
+    private const int Width = 30;
+    private const int Height = 16;
+
+    [Fact]
+    public void Bottom_right_offsets_the_tooltip_from_the_bottom_right_corner()
+    {
+        PopupWindowNative.Point point = Near(AnchorSide.BottomRight);
+
+        Assert.Equal(140 + 10, point.X);
+        Assert.Equal(220 + 10, point.Y);
+    }
+
+    [Fact]
+    public void Bottom_left_fits_the_tooltip_to_the_left_of_the_anchor()
+    {
+        PopupWindowNative.Point point = Near(AnchorSide.BottomLeft);
+
+        Assert.Equal(100 - 10 - Width, point.X);
+        Assert.Equal(220 + 10, point.Y);
+    }
+
+    [Fact]
+    public void Top_right_fits_the_tooltip_above_the_anchor()
+    {
+        PopupWindowNative.Point point = Near(AnchorSide.TopRight);
+
+        Assert.Equal(140 + 10, point.X);
+        Assert.Equal(200 - 10 - Height, point.Y);
+    }
+
+    [Fact]
+    public void Top_left_fits_the_tooltip_both_left_of_and_above_the_anchor()
+    {
+        PopupWindowNative.Point point = Near(AnchorSide.TopLeft);
+
+        Assert.Equal(100 - 10 - Width, point.X);
+        Assert.Equal(200 - 10 - Height, point.Y);
+    }
+
+    // At the sides the tooltip lines up with the middle of the anchor
+    [Fact]
+    public void On_the_right_the_tooltip_lines_up_with_the_anchor()
+    {
+        PopupWindowNative.Point point = Near(AnchorSide.Right);
+
+        Assert.Equal(140 + 10, point.X);
+        Assert.Equal(200 + ((20 - Height) / 2), point.Y);
+    }
+
+    [Fact]
+    public void On_the_left_the_tooltip_lines_up_with_the_anchor()
+    {
+        PopupWindowNative.Point point = Near(AnchorSide.Left);
+
+        Assert.Equal(100 - 10 - Width, point.X);
+        Assert.Equal(200 + ((20 - Height) / 2), point.Y);
+    }
+
+    // A tooltip taller than the input field: the middle is measured from the
+    // anchor, not from zero
+    [Fact]
+    public void At_the_side_a_tooltip_taller_than_the_anchor_rises_above_it()
+    {
+        PopupWindowNative.Point point =
+            PopupLayout.NearAnchor(Anchor, AnchorSide.Right, Offset, Width, height: 40);
+
+        Assert.Equal(200 + ((20 - 40) / 2), point.Y);
+        Assert.True(point.Y < Anchor.Top);
+    }
+
+    [Fact]
+    public void The_cursor_anchors_as_a_rectangle_of_zero_size()
+    {
+        PopupWindowNative.Rect anchor = PopupLayout.AsAnchor(new PopupWindowNative.Point { X = 50, Y = 60 });
+
+        Assert.Equal(50, anchor.Left);
+        Assert.Equal(50, anchor.Right);
+        Assert.Equal(60, anchor.Top);
+        Assert.Equal(60, anchor.Bottom);
+    }
+
+    [Fact]
+    public void At_the_cursor_both_sides_are_measured_from_the_same_point()
+    {
+        PopupWindowNative.Rect cursor = PopupLayout.AsAnchor(new PopupWindowNative.Point { X = 50, Y = 60 });
+
+        PopupWindowNative.Point bottomRight =
+            PopupLayout.NearAnchor(cursor, AnchorSide.BottomRight, Offset, Width, Height);
+        PopupWindowNative.Point topLeft =
+            PopupLayout.NearAnchor(cursor, AnchorSide.TopLeft, Offset, Width, Height);
+
+        Assert.Equal(60, bottomRight.X);
+        Assert.Equal(70, bottomRight.Y);
+        Assert.Equal(50 - 10 - Width, topLeft.X);
+        Assert.Equal(60 - 10 - Height, topLeft.Y);
+    }
+
+    // A monitor to the left of the primary one gives negative coordinates — that is normal
+    [Fact]
+    public void Negative_coordinates_of_a_neighbouring_monitor_are_allowed()
+    {
+        var anchor = new PopupWindowNative.Rect { Left = -800, Top = -200, Right = -800, Bottom = -200 };
+
+        PopupWindowNative.Point point =
+            PopupLayout.NearAnchor(anchor, AnchorSide.BottomRight, Offset, Width, Height);
+
+        Assert.Equal(-790, point.X);
+        Assert.Equal(-190, point.Y);
+    }
+
+    [Fact]
+    public void A_zero_offset_puts_the_tooltip_flush_against_the_anchor()
+    {
+        PopupWindowNative.Point point =
+            PopupLayout.NearAnchor(Anchor, AnchorSide.BottomRight, offset: 0, Width, Height);
+
+        Assert.Equal(Anchor.Right, point.X);
+        Assert.Equal(Anchor.Bottom, point.Y);
+    }
+
+    [Theory]
+    [InlineData(AnchorSide.TopLeft)]
+    [InlineData(AnchorSide.TopRight)]
+    [InlineData(AnchorSide.Left)]
+    [InlineData(AnchorSide.Right)]
+    [InlineData(AnchorSide.BottomLeft)]
+    [InlineData(AnchorSide.BottomRight)]
+    public void No_side_is_left_behind(AnchorSide side)
+    {
+        // The sides are handled by a switch expression with a fallback branch:
+        // each of them has to get its own place, not the shared "bottom right"
+        PopupWindowNative.Point point = Near(side);
+        PopupWindowNative.Point bottomRight = Near(AnchorSide.BottomRight);
+
+        if (side != AnchorSide.BottomRight)
+        {
+            Assert.True(point.X != bottomRight.X || point.Y != bottomRight.Y);
+        }
+    }
+
+    public static TheoryData<ScreenPosition, int, int> ScreenCases() => new()
+    {
+        // A work area of 0..1000 horizontally and 0..800 vertically, margin 20
+        { ScreenPosition.TopLeft, 20, 20 },
+        { ScreenPosition.Top, (1000 - Width) / 2, 20 },
+        { ScreenPosition.TopRight, 1000 - 20 - Width, 20 },
+        { ScreenPosition.Center, (1000 - Width) / 2, (800 - Height) / 2 },
+        { ScreenPosition.BottomLeft, 20, 800 - 20 - Height },
+        { ScreenPosition.Bottom, (1000 - Width) / 2, 800 - 20 - Height },
+        { ScreenPosition.BottomRight, 1000 - 20 - Width, 800 - 20 - Height },
+    };
+
+    [Theory]
+    [MemberData(nameof(ScreenCases))]
+    public void The_place_on_the_monitor_is_measured_from_the_work_area(
+        ScreenPosition position, int expectedX, int expectedY)
+    {
+        var work = new PopupWindowNative.Rect { Left = 0, Top = 0, Right = 1000, Bottom = 800 };
+
+        PopupWindowNative.Point point = PopupLayout.OnScreen(work, position, margin: 20, Width, Height);
+
+        Assert.Equal(expectedX, point.X);
+        Assert.Equal(expectedY, point.Y);
+    }
+
+    // The work area of a second monitor does not start at zero, and the taskbar
+    // takes its bottom away — the place is measured from those bounds
+    [Fact]
+    public void The_place_on_a_neighbouring_monitor_is_measured_from_its_own_bounds()
+    {
+        var work = new PopupWindowNative.Rect { Left = 1920, Top = 0, Right = 3520, Bottom = 860 };
+
+        PopupWindowNative.Point point =
+            PopupLayout.OnScreen(work, ScreenPosition.BottomRight, margin: 20, Width, Height);
+
+        Assert.Equal(3520 - 20 - Width, point.X);
+        Assert.Equal(860 - 20 - Height, point.Y);
+    }
+
+    [Fact]
+    public void In_the_centre_of_the_monitor_the_margin_is_ignored()
+    {
+        var work = new PopupWindowNative.Rect { Left = 0, Top = 0, Right = 1000, Bottom = 800 };
+
+        PopupWindowNative.Point withMargin = PopupLayout.OnScreen(work, ScreenPosition.Center, 20, Width, Height);
+        PopupWindowNative.Point withoutMargin = PopupLayout.OnScreen(work, ScreenPosition.Center, 0, Width, Height);
+
+        Assert.Equal(withoutMargin.X, withMargin.X);
+        Assert.Equal(withoutMargin.Y, withMargin.Y);
+    }
+
+    private static PopupWindowNative.Point Near(AnchorSide side) =>
+        PopupLayout.NearAnchor(Anchor, side, Offset, Width, Height);
+
+    [Theory]
+    [InlineData(16, 1.0, 16)]
+    [InlineData(16, 1.25, 20)]
+    [InlineData(16, 1.5, 24)]
+    [InlineData(16, 2.0, 32)]
+    [InlineData(0, 2.0, 0)]
+    [InlineData(20.4, 1.0, 20)]
+    [InlineData(20.6, 1.0, 21)]
+    public void WPF_units_turn_into_pixels_by_the_scale(double units, double scale, int expected)
+    {
+        Assert.Equal(expected, PopupLayout.ToPixels(units, scale));
+    }
+}

CursorLang.Core.Tests/Services/RegistryStartupTests.cs

@@ -0,0 +1,156 @@
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+using CursorLang.Tests.Shared;
+using Microsoft.Win32;
+
+namespace CursorLang.Core.Tests.Services;
+
+/// <summary>
+/// Startup of a build unpacked into a folder: a value under the Run key. The tests
+/// keep to a root of their own, so the startup list of the machine is untouched.
+/// </summary>
+public sealed class RegistryStartupTests
+{
+    private const string RunPath = @"Software\Microsoft\Windows\CurrentVersion\Run";
+
+    private const string ApprovedPath =
+        @"Software\Microsoft\Windows\CurrentVersion\Explorer\StartupApproved\Run";
+
+    private const string ValueName = "CursorLang";
+
+    private const string Command = @"""C:\Apps\CursorLang\CursorLang.exe""";
+
+    [Fact]
+    public void With_nothing_written_down_startup_is_off()
+    {
+        using var root = new TempRegistryKey();
+
+        Assert.Equal(StartupState.Disabled, new RegistryStartup(root.Key, Command).GetState());
+    }
+
+    [Fact]
+    public void Switching_startup_on_writes_the_path_of_the_app()
+    {
+        using var root = new TempRegistryKey();
+        var startup = new RegistryStartup(root.Key, Command);
+
+        Assert.Equal(StartupState.Enabled, startup.SetEnabled(true));
+        Assert.Equal(Command, ReadRunValue(root));
+    }
+
+    [Fact]
+    public void Switching_startup_off_takes_the_entry_away()
+    {
+        using var root = new TempRegistryKey();
+        var startup = new RegistryStartup(root.Key, Command);
+
+        startup.SetEnabled(true);
+
+        Assert.Equal(StartupState.Disabled, startup.SetEnabled(false));
+        Assert.Null(ReadRunValue(root));
+    }
+
+    // Switching off what is already off is what happens when Windows and the app
+    // disagree about the state, and it is no reason to fail
+    [Fact]
+    public void Switching_off_startup_that_is_already_off_passes_quietly()
+    {
+        using var root = new TempRegistryKey();
+
+        Assert.Equal(StartupState.Disabled, new RegistryStartup(root.Key, Command).SetEnabled(false));
+    }
+
+    [Fact]
+    public void The_path_is_written_afresh_every_time()
+    {
+        using var root = new TempRegistryKey();
+
+        new RegistryStartup(root.Key, @"""C:\Old\CursorLang.exe""").SetEnabled(true);
+        new RegistryStartup(root.Key, Command).SetEnabled(true);
+
+        Assert.Equal(Command, ReadRunValue(root));
+    }
+
+    [Fact]
+    public void An_entry_the_user_has_banned_counts_as_off()
+    {
+        using var root = new TempRegistryKey();
+        var startup = new RegistryStartup(root.Key, Command);
+
+        startup.SetEnabled(true);
+        Ban(root);
+
+        Assert.Equal(StartupState.DisabledByUser, startup.GetState());
+    }
+
+    // The ban outlives the request: the entry is written, and Windows still ignores it
+    [Fact]
+    public void The_ban_of_the_user_survives_a_request_to_switch_startup_on()
+    {
+        using var root = new TempRegistryKey();
+        var startup = new RegistryStartup(root.Key, Command);
+
+        Ban(root);
+
+        Assert.Equal(StartupState.DisabledByUser, startup.SetEnabled(true));
+        Assert.Equal(Command, ReadRunValue(root));
+    }
+
+    [Fact]
+    public void A_verdict_of_the_user_in_favour_leaves_startup_on()
+    {
+        using var root = new TempRegistryKey();
+        var startup = new RegistryStartup(root.Key, Command);
+
+        startup.SetEnabled(true);
+        WriteVerdict(root, 0x02);
+
+        Assert.Equal(StartupState.Enabled, startup.GetState());
+    }
+
+    // An empty blob is not a ban: Windows writes twelve bytes, but a value cut
+    // short says nothing about the will of the user
+    [Fact]
+    public void A_verdict_with_no_bytes_in_it_is_no_ban()
+    {
+        using var root = new TempRegistryKey();
+        var startup = new RegistryStartup(root.Key, Command);
+
+        startup.SetEnabled(true);
+
+        using RegistryKey approved = root.Key.CreateSubKey(ApprovedPath);
+        approved.SetValue(ValueName, Array.Empty<byte>(), RegistryValueKind.Binary);
+
+        Assert.Equal(StartupState.Enabled, startup.GetState());
+    }
+
+    [Fact]
+    public void With_no_path_to_the_app_startup_is_unavailable()
+    {
+        using var root = new TempRegistryKey();
+        var startup = new RegistryStartup(root.Key, command: null);
+
+        Assert.Equal(StartupState.Unavailable, startup.GetState());
+        Assert.Equal(StartupState.Unavailable, startup.SetEnabled(true));
+        Assert.Equal(StartupState.Unavailable, startup.SetEnabled(false));
+        Assert.Null(ReadRunValue(root));
+    }
+
+    private static string? ReadRunValue(TempRegistryKey root)
+    {
+        using RegistryKey? run = root.Key.OpenSubKey(RunPath);
+        return run?.GetValue(ValueName) as string;
+    }
+
+    /// <summary>The mark Windows leaves after the user switches the entry off.</summary>
+    private static void Ban(TempRegistryKey root) => WriteVerdict(root, 0x03);
+
+    private static void WriteVerdict(TempRegistryKey root, byte first)
+    {
+        var verdict = new byte[12];
+        verdict[0] = first;
+
+        using RegistryKey approved = root.Key.CreateSubKey(ApprovedPath);
+        approved.SetValue(ValueName, verdict, RegistryValueKind.Binary);
+    }
+}

CursorLang.Core.Tests/Services/SettingsCloseSignalTests.cs

@@ -0,0 +1,115 @@
+using System.Collections.Concurrent;
+using CursorLang.Core.Services;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Core.Tests.Services;
+
+/// <summary>
+/// The one thing the agent says to the settings window: quit with me.
+/// </summary>
+/// <remarks>
+/// The event names here are the tests' own. The application's name is fixed, and a test
+/// listening on it would answer for a settings window someone is using — or, signalling,
+/// close it.
+/// </remarks>
+public sealed class SettingsCloseSignalTests
+{
+    [Fact]
+    public void The_request_reaches_the_settings_window()
+    {
+        string suffix = UniqueSuffix();
+        var signal = new SettingsCloseSignal(suffix);
+
+        ConcurrentQueue<EventArgs> requests = new();
+        signal.CloseRequested += (_, e) => requests.Enqueue(e);
+
+        try
+        {
+            signal.Listen();
+
+            Assert.True(RequestApart(suffix), "the request found nobody listening");
+            Pump.WaitFor(() => !requests.IsEmpty, "the settings window got the request to close");
+        }
+        finally
+        {
+            signal.Dispose();
+        }
+    }
+
+    // The usual case: the user quits from the tray with no settings window on the screen
+    [Fact]
+    public void A_request_with_no_settings_window_open_passes_without_consequence()
+    {
+        Assert.False(RequestApart(UniqueSuffix()));
+    }
+
+    // The window has closed on its own, and the process is on its way out anyway
+    [Fact]
+    public void No_request_arrives_after_the_window_is_gone()
+    {
+        string suffix = UniqueSuffix();
+        var signal = new SettingsCloseSignal(suffix);
+
+        ConcurrentQueue<EventArgs> requests = new();
+        signal.CloseRequested += (_, e) => requests.Enqueue(e);
+
+        signal.Listen();
+        signal.Dispose();
+
+        RequestApart(suffix);
+        Pump.Pause(TimeSpan.FromMilliseconds(80));
+
+        Assert.Empty(requests);
+    }
+
+    [Fact]
+    public void Listening_twice_leaves_one_listener()
+    {
+        string suffix = UniqueSuffix();
+        var signal = new SettingsCloseSignal(suffix);
+
+        var requests = 0;
+        signal.CloseRequested += (_, _) => Interlocked.Increment(ref requests);
+
+        try
+        {
+            signal.Listen();
+            signal.Listen();
+
+            Assert.True(RequestApart(suffix));
+            Pump.WaitFor(() => Volatile.Read(ref requests) > 0, "the request arrived");
+            Pump.Pause(TimeSpan.FromMilliseconds(80));
+
+            Assert.Equal(1, Volatile.Read(ref requests));
+        }
+        finally
+        {
+            signal.Dispose();
+        }
+    }
+
+    [Fact]
+    public void Closing_without_listening_passes_without_consequence()
+    {
+        var signal = new SettingsCloseSignal(UniqueSuffix());
+
+        signal.Dispose();
+        signal.Dispose();
+    }
+
+    // Every test gets a namespace of kernel objects of its own
+    private static string UniqueSuffix() => "." + Guid.NewGuid().ToString("N");
+
+    /// <summary>
+    /// Asks for the close the way the agent does it — from another process, and here
+    /// from another thread, which is as foreign as a test can get.
+    /// </summary>
+    private static bool RequestApart(string suffix)
+    {
+        var heard = false;
+
+        Pump.RunApart(() => heard = SettingsCloseSignal.RequestClose(suffix));
+
+        return heard;
+    }
+}

CursorLang.Core.Tests/Services/SettingsServiceTests.cs

@@ -0,0 +1,653 @@
+using System.Drawing;
+using System.Globalization;
+using System.Text.Json;
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+using CursorLang.Core.Tests.Models;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Core.Tests.Services;
+
+/// <summary>
+/// Keeping the settings in a file. Only the settings window writes, and it asks for
+/// that with TrackChanges; the agent loads the same file and never saves. Everything happens in a temporary folder:
+/// the tests have no business touching the user's own settings.
+/// </summary>
+public sealed class SettingsServiceTests
+{
+    /// <summary>The deferred write delay in tests: half a second is not worth waiting for.</summary>
+    private static readonly TimeSpan SaveDelay = TimeSpan.FromMilliseconds(20);
+
+    [Fact]
+    public void Without_a_file_the_defaults_are_handed_out()
+    {
+        using var folder = new TempFolder();
+
+        AppSettings settings = Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(AppTheme.System, settings.Theme);
+        Assert.Equal(20, settings.Current.FontSize);
+    }
+
+    [Theory]
+    [InlineData("ru", "ru")]
+    [InlineData("ru-RU", "ru")]
+    [InlineData("en-US", "en")]
+    [InlineData("de-DE", "en")]
+    [InlineData("fr", "en")]
+    public void The_default_language_follows_the_language_of_Windows(string uiCulture, string expected)
+    {
+        using var folder = new TempFolder();
+
+        AppSettings settings = Pump.Run(() =>
+        {
+            CultureInfo previous = CultureInfo.CurrentUICulture;
+            try
+            {
+                CultureInfo.CurrentUICulture = CultureInfo.GetCultureInfo(uiCulture);
+
+                using SettingsService service = Create(folder);
+                return service.Load();
+            }
+            finally
+            {
+                CultureInfo.CurrentUICulture = previous;
+            }
+        });
+
+        Assert.Equal(expected, settings.Language);
+    }
+
+    [Fact]
+    public void Saved_settings_are_read_back()
+    {
+        using var folder = new TempFolder();
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+
+            settings.Theme = AppTheme.Dark;
+            settings.PlacementMode = PopupPlacementMode.AtCaret;
+            settings.Current.FontSize = 42;
+            settings.Current.BackgroundColor = Color.FromArgb(0x11, 0x22, 0x33);
+            settings.UseCapsLockHotkey = true;
+
+            service.Save();
+        });
+
+        AppSettings restored = Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(42, restored.Current.FontSize);
+        Assert.Equal(AppTheme.Dark, restored.Theme);
+        Assert.Equal(PopupPlacementMode.AtCaret, restored.PlacementMode);
+        Assert.Equal(Color.FromArgb(0x11, 0x22, 0x33), restored.Current.BackgroundColor);
+        Assert.True(restored.UseCapsLockHotkey);
+    }
+
+    [Fact]
+    public void The_settings_land_in_the_file_in_a_readable_form()
+    {
+        using var folder = new TempFolder();
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+            settings.Theme = AppTheme.Dark;
+            settings.Current.BackgroundColor = Color.FromArgb(0x20, 0x20, 0x20);
+            service.Save();
+        });
+
+        string json = File.ReadAllText(folder.File("settings.json"));
+
+        // The theme as a word rather than a number; the colour in its usual notation
+        Assert.Contains("\"Theme\": \"Dark\"", json, StringComparison.Ordinal);
+        Assert.Contains("#FF202020", json, StringComparison.Ordinal);
+
+        // And all of it across lines: the file is sometimes edited by hand
+        Assert.Contains('\n', json);
+    }
+
+    [Fact]
+    public void A_changed_setting_saves_itself()
+    {
+        using var folder = new TempFolder();
+        string path = folder.File("settings.json");
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+            service.TrackChanges();
+
+            settings.Current.FontSize = 33;
+
+            // Right after the edit there is nothing on disk yet: the write is deferred
+            Assert.False(File.Exists(path));
+
+            Pump.WaitFor(() => File.Exists(path), "the settings were written by the timer");
+        });
+
+        Assert.Contains("\"FontSize\": 33", File.ReadAllText(path), StringComparison.Ordinal);
+    }
+
+    // A slider changes its value continuously, and writing every move to disk is pointless
+    [Fact]
+    public void A_run_of_edits_defers_the_write_until_a_pause()
+    {
+        using var folder = new TempFolder();
+        string path = folder.File("settings.json");
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = new(path, folder.File("inherited.json"), TimeSpan.FromMilliseconds(150));
+            AppSettings settings = service.Load();
+            service.TrackChanges();
+
+            for (int i = 0; i < 10; i++)
+            {
+                settings.Current.Opacity = 0.5 + (i * 0.01);
+                Assert.False(File.Exists(path));
+                Pump.Pause(TimeSpan.FromMilliseconds(10));
+            }
+
+            Pump.WaitFor(() => File.Exists(path), "the write happened after the pause in edits");
+        });
+    }
+
+    /// <summary>
+    /// Asking to track changes before reading the file still tracks them.
+    /// </summary>
+    /// <remarks>
+    /// The settings window asks in exactly that order: its container hands out the
+    /// service first and the settings only when something needs them. A version of this
+    /// that quietly did nothing when the file had not been read yet left the window
+    /// saving nothing at all — neither while it was open nor when it was closed.
+    /// </remarks>
+    [Fact]
+    public void Tracking_asked_for_before_the_file_is_read_still_saves()
+    {
+        using var folder = new TempFolder();
+        string path = folder.File("settings.json");
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+
+            // Before Load, the way the settings window does it
+            service.TrackChanges();
+
+            AppSettings settings = service.Load();
+            settings.Current.FontSize = 29;
+
+            Pump.WaitFor(() => File.Exists(path), "the settings were written by the timer");
+        });
+
+        Assert.Contains("\"FontSize\": 29", File.ReadAllText(path), StringComparison.Ordinal);
+    }
+
+    // Two reads would mean two instances, and the window would edit one while the
+    // service saved the other
+    [Fact]
+    public void Reading_twice_hands_out_the_same_settings()
+    {
+        using var folder = new TempFolder();
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+
+            Assert.Same(service.Load(), service.Load());
+        });
+    }
+
+    /// <summary>
+    /// Re-reading pours the file into the instance everything is already bound to.
+    /// </summary>
+    /// <remarks>
+    /// This is the agent's whole side of the connection: the settings window writes and
+    /// says so, and the agent calls this. Replacing the instance instead of filling it
+    /// would leave the popup, the hook and the timers bound to the old one.
+    /// </remarks>
+    [Fact]
+    public void Re_reading_lands_in_the_settings_already_in_hand()
+    {
+        using var folder = new TempFolder();
+        string path = folder.File("settings.json");
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+
+            File.WriteAllText(path, """{"AtCursor": {"FontSize": 31, "BackgroundColor": "#FF102030"}}""");
+            service.Reload();
+
+            Assert.Equal(31, settings.Current.FontSize);
+            Assert.Equal(Color.FromArgb(0x10, 0x20, 0x30), settings.Current.BackgroundColor);
+        });
+    }
+
+    // A file that has gone missing or turned to nonsense leaves the settings alone:
+    // showing the popup with yesterday's colours beats showing it with none
+    [Fact]
+    public void Re_reading_an_unreadable_file_keeps_what_was_already_there()
+    {
+        using var folder = new TempFolder();
+        string path = folder.File("settings.json");
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+            settings.Current.FontSize = 44;
+
+            File.WriteAllText(path, "not json at all");
+            service.Reload();
+
+            Assert.Equal(44, settings.Current.FontSize);
+        });
+    }
+
+    [Fact]
+    public void Closing_the_service_saves_the_latest_edits()
+    {
+        using var folder = new TempFolder();
+
+        Pump.Run(() =>
+        {
+            SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+            service.TrackChanges();
+            settings.Current.FontSize = 27;
+
+            service.Dispose();
+        });
+
+        Assert.Contains(
+            "\"FontSize\": 27",
+            File.ReadAllText(folder.File("settings.json")),
+            StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void After_closing_edits_no_longer_reach_the_disk()
+    {
+        using var folder = new TempFolder();
+        string path = folder.File("settings.json");
+
+        Pump.Run(() =>
+        {
+            SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+            service.TrackChanges();
+            service.Dispose();
+
+            string afterDispose = File.ReadAllText(path);
+
+            settings.Current.FontSize = 99;
+            Pump.Pause(TimeSpan.FromMilliseconds(60));
+
+            Assert.Equal(afterDispose, File.ReadAllText(path));
+        });
+    }
+
+    [Fact]
+    public void Settings_of_a_previous_install_are_taken_over()
+    {
+        using var folder = new TempFolder();
+        string inherited = folder.File("inherited.json");
+        string own = folder.File("settings.json");
+
+        File.WriteAllText(inherited, """{"AtCursor": {"FontSize": 31}, "Language": "ru"}""");
+
+        AppSettings settings = Pump.Run(() =>
+        {
+            using SettingsService service = new(own, inherited, SaveDelay);
+            return service.Load();
+        });
+
+        Assert.Equal(31, settings.Current.FontSize);
+        Assert.Equal("ru", settings.Language);
+
+        // What was taken over is pinned to its new place at once rather than on the first edit
+        Assert.True(File.Exists(own));
+        Assert.Contains("\"FontSize\": 31", File.ReadAllText(own), StringComparison.Ordinal);
+    }
+
+    // Both builds may be installed side by side: the other one keeps its settings
+    [Fact]
+    public void The_previous_install_does_not_lose_its_settings()
+    {
+        using var folder = new TempFolder();
+        string inherited = folder.File("inherited.json");
+        string original = """{"AtCursor": {"FontSize": 31}}""";
+
+        File.WriteAllText(inherited, original);
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = new(folder.File("settings.json"), inherited, SaveDelay);
+            _ = service.Load();
+        });
+
+        Assert.Equal(original, File.ReadAllText(inherited));
+    }
+
+    [Fact]
+    public void Own_settings_outweigh_those_of_a_previous_install()
+    {
+        using var folder = new TempFolder();
+        string own = folder.File("settings.json");
+        string inherited = folder.File("inherited.json");
+
+        File.WriteAllText(own, """{"AtCursor": {"FontSize": 12}}""");
+        File.WriteAllText(inherited, """{"AtCursor": {"FontSize": 31}}""");
+
+        AppSettings settings = Pump.Run(() =>
+        {
+            using SettingsService service = new(own, inherited, SaveDelay);
+            return service.Load();
+        });
+
+        Assert.Equal(12, settings.Current.FontSize);
+    }
+
+    // Outside a package both paths are the same, so there is nothing to take over
+    [Fact]
+    public void Without_a_package_no_settings_are_taken_over()
+    {
+        using var folder = new TempFolder();
+        string path = folder.File("settings.json");
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = new(path, path, SaveDelay);
+            _ = service.Load();
+
+            // No file appeared: there was nothing to take over and nowhere to take it from
+            Assert.False(File.Exists(path));
+        });
+    }
+
+    [Fact]
+    public void A_broken_settings_file_does_not_bring_the_app_down()
+    {
+        using var folder = new TempFolder();
+        File.WriteAllText(folder.File("settings.json"), "{this is not json");
+
+        AppSettings settings = Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(20, settings.Current.FontSize);
+    }
+
+    [Fact]
+    public void Settings_with_unknown_fields_are_still_read()
+    {
+        using var folder = new TempFolder();
+        File.WriteAllText(folder.File("settings.json"), """{"AtCursor": {"FontSize": 15}, "SomethingNew": true}""");
+
+        AppSettings settings = Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(15, settings.Current.FontSize);
+    }
+
+    [Theory]
+    [InlineData("\"#FF102030\"", 0x10, 0x20, 0x30)]
+    [InlineData("\"#102030\"", 0x10, 0x20, 0x30)]
+    [InlineData("\"Red\"", 0xFF, 0x00, 0x00)]
+    public void A_colour_is_read_from_its_usual_notation(string stored, byte r, byte g, byte b)
+    {
+        using var folder = new TempFolder();
+        File.WriteAllText(folder.File("settings.json"), $$"""{"AtCursor": {"BackgroundColor": {{stored}} } }""");
+
+        AppSettings settings = Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(Color.FromArgb(r, g, b), settings.Current.BackgroundColor);
+    }
+
+    [Theory]
+    [InlineData("\"\"")]
+    [InlineData("\"   \"")]
+    [InlineData("\"not a colour\"")]
+    public void An_unintelligible_colour_becomes_black(string stored)
+    {
+        using var folder = new TempFolder();
+        File.WriteAllText(folder.File("settings.json"), $$"""{"AtCursor": {"BackgroundColor": {{stored}} } }""");
+
+        AppSettings settings = Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(Color.Black, settings.Current.BackgroundColor);
+    }
+
+    // The service creates the settings folder itself
+    [Fact]
+    public void The_settings_folder_is_created_on_write()
+    {
+        using var folder = new TempFolder();
+        string nested = Path.Combine(folder.Path, "a", "b", "settings.json");
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = new(nested, folder.File("inherited.json"), SaveDelay);
+            _ = service.Load();
+            service.Save();
+        });
+
+        Assert.True(File.Exists(nested));
+    }
+
+    // Settings are not the kind of thing worth bringing the app down for
+    [Fact]
+    public void A_path_that_cannot_be_written_does_not_bring_the_app_down()
+    {
+        using var folder = new TempFolder();
+
+        // A folder sits where the settings file should be: writing there will not work
+        string path = folder.File("settings.json");
+        Directory.CreateDirectory(path);
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = new(path, folder.File("inherited.json"), SaveDelay);
+            AppSettings settings = service.Load();
+
+            settings.Current.FontSize = 18;
+            service.Save();
+        });
+
+        Assert.True(Directory.Exists(path));
+    }
+
+    [Fact]
+    public void Saving_without_loading_writes_nothing()
+    {
+        using var folder = new TempFolder();
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            service.Save();
+        });
+
+        Assert.False(File.Exists(folder.File("settings.json")));
+    }
+
+    [Fact]
+    public void Closing_without_loading_passes_without_consequence()
+    {
+        using var folder = new TempFolder();
+
+        Pump.Run(() =>
+        {
+            SettingsService service = Create(folder);
+            service.Dispose();
+        });
+
+        Assert.False(File.Exists(folder.File("settings.json")));
+    }
+
+    [Fact]
+    public void Every_setting_of_the_app_reaches_the_file()
+    {
+        using var folder = new TempFolder();
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            _ = service.Load();
+            service.Save();
+        });
+
+        using JsonDocument document = JsonDocument.Parse(File.ReadAllText(folder.File("settings.json")));
+        List<string> stored = [.. document.RootElement.EnumerateObject().Select(property => property.Name)];
+
+        foreach (string name in AppSettingsTests.WritablePropertyNames())
+        {
+            Assert.Contains(name, stored);
+        }
+    }
+
+    // The modes are sections of their own, and every setting of a mode has to reach the
+    // file inside its own section
+    [Theory]
+    [InlineData(nameof(AppSettings.AtCursor), typeof(CursorModeSettings))]
+    [InlineData(nameof(AppSettings.AtCaret), typeof(CaretModeSettings))]
+    [InlineData(nameof(AppSettings.FixedPoint), typeof(FixedPointModeSettings))]
+    public void Every_setting_of_a_mode_reaches_its_section_of_the_file(string section, Type mode)
+    {
+        using var folder = new TempFolder();
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            _ = service.Load();
+            service.Save();
+        });
+
+        using JsonDocument document = JsonDocument.Parse(File.ReadAllText(folder.File("settings.json")));
+
+        Assert.True(document.RootElement.TryGetProperty(section, out JsonElement stored));
+
+        foreach (string name in PopupModeSettingsTests.WritablePropertyNames(mode))
+        {
+            Assert.True(stored.TryGetProperty(name, out _), $"{section}.{name}");
+        }
+    }
+
+    /// <summary>
+    /// The modes are stored apart: what is set up in one is still there after a trip
+    /// through the file and the other two.
+    /// </summary>
+    [Fact]
+    public void Each_mode_keeps_its_own_settings()
+    {
+        using var folder = new TempFolder();
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+
+            settings.AtCursor.Side = AnchorSide.TopLeft;
+            settings.AtCursor.FontSize = 14;
+            settings.AtCaret.Side = CaretSide.Left;
+            settings.AtCaret.FontSize = 28;
+            settings.AtCaret.ForegroundColor = Color.FromArgb(0x0A, 0x0B, 0x0C);
+            settings.FixedPoint.Position = ScreenPosition.Top;
+            settings.FixedPoint.Offset = 96;
+            settings.FixedPoint.Opacity = 0.4;
+
+            service.Save();
+        });
+
+        AppSettings restored = Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(AnchorSide.TopLeft, restored.AtCursor.Side);
+        Assert.Equal(14, restored.AtCursor.FontSize);
+        Assert.Equal(CaretSide.Left, restored.AtCaret.Side);
+        Assert.Equal(28, restored.AtCaret.FontSize);
+        Assert.Equal(Color.FromArgb(0x0A, 0x0B, 0x0C), restored.AtCaret.ForegroundColor);
+        Assert.Equal(ScreenPosition.Top, restored.FixedPoint.Position);
+        Assert.Equal(96, restored.FixedPoint.Offset);
+        Assert.Equal(0.4, restored.FixedPoint.Opacity);
+
+        // What was left alone stays at its default rather than following a neighbour
+        Assert.Equal(0.9, restored.AtCursor.Opacity);
+    }
+
+    // A change inside a mode is a change of the settings: the file has to follow the
+    // sliders of the look as well as the ones above them
+    [Fact]
+    public void A_change_inside_a_mode_saves_itself()
+    {
+        using var folder = new TempFolder();
+        string path = folder.File("settings.json");
+
+        Pump.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+            service.TrackChanges();
+
+            settings.AtCaret.FontSize = 37;
+
+            Pump.WaitFor(() => File.Exists(path), "the settings were written by the timer");
+        });
+
+        using JsonDocument document = JsonDocument.Parse(File.ReadAllText(path));
+
+        Assert.Equal(
+            37,
+            document.RootElement
+                .GetProperty(nameof(AppSettings.AtCaret))
+                .GetProperty(nameof(PopupModeSettings.FontSize))
+                .GetDouble());
+    }
+
+    // An ordinary run picks the storage place itself: a package keeps settings
+    // of its own, a separate install keeps them in the user profile
+    [Fact]
+    public void The_storage_place_is_chosen_on_its_own()
+    {
+        Pump.Run(() =>
+        {
+            // Nothing is read and nothing is written: only the fact that a path
+            // gets chosen without error is under test
+            using var service = new SettingsService();
+        });
+    }
+
+    private static SettingsService Create(TempFolder folder) =>
+        new(folder.File("settings.json"), folder.File("inherited.json"), SaveDelay);
+}

CursorLang.Core.Tests/Services/SingleInstanceGateTests.cs

@@ -0,0 +1,207 @@
+using System.Collections.Concurrent;
+using CursorLang.Core.Services;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Core.Tests.Services;
+
+/// <summary>
+/// The place of the single instance. The kernel object names in the tests are
+/// their own: sharing them with a running application is not an option.
+/// </summary>
+public sealed class SingleInstanceGateTests
+{
+    [Fact]
+    public void The_first_run_takes_the_place()
+    {
+        string suffix = UniqueSuffix();
+        SingleInstanceGate gate = Pump.Run(() => new SingleInstanceGate(suffix));
+
+        try
+        {
+            Assert.True(Pump.Run(gate.TryAcquire));
+        }
+        finally
+        {
+            Pump.Run(gate.Dispose);
+        }
+    }
+
+    [Fact]
+    public void The_second_run_does_not_get_the_place()
+    {
+        string suffix = UniqueSuffix();
+        SingleInstanceGate first = Pump.Run(() => new SingleInstanceGate(suffix));
+
+        try
+        {
+            Assert.True(Pump.Run(first.TryAcquire));
+            Assert.False(TryAcquireApart(suffix));
+        }
+        finally
+        {
+            Pump.Run(first.Dispose);
+        }
+    }
+
+    [Fact]
+    public void The_second_run_asks_the_running_one_to_show_its_window()
+    {
+        string suffix = UniqueSuffix();
+        SingleInstanceGate first = Pump.Run(() => new SingleInstanceGate(suffix));
+
+        ConcurrentQueue<EventArgs> requests = new();
+        first.ActivationRequested += (_, e) => requests.Enqueue(e);
+
+        try
+        {
+            Pump.Run(first.TryAcquire);
+            Assert.False(TryAcquireApart(suffix));
+
+            Pump.WaitFor(() => !requests.IsEmpty, "the running instance got the request to show itself");
+        }
+        finally
+        {
+            Pump.Run(first.Dispose);
+        }
+    }
+
+    [Fact]
+    public void Without_a_second_run_no_request_arrives()
+    {
+        string suffix = UniqueSuffix();
+        SingleInstanceGate gate = Pump.Run(() => new SingleInstanceGate(suffix));
+
+        ConcurrentQueue<EventArgs> requests = new();
+        gate.ActivationRequested += (_, e) => requests.Enqueue(e);
+
+        try
+        {
+            Pump.Run(gate.TryAcquire);
+            Pump.Pause(TimeSpan.FromMilliseconds(80));
+
+            Assert.Empty(requests);
+        }
+        finally
+        {
+            Pump.Run(gate.Dispose);
+        }
+    }
+
+    // The place is released on exit — otherwise the app would never start again
+    [Fact]
+    public void After_the_exit_the_place_is_free_again()
+    {
+        string suffix = UniqueSuffix();
+
+        SingleInstanceGate first = Pump.Run(() => new SingleInstanceGate(suffix));
+        Assert.True(Pump.Run(first.TryAcquire));
+        Pump.Run(first.Dispose);
+
+        Assert.True(TryAcquireApart(suffix));
+    }
+
+    [Fact]
+    public void No_requests_arrive_after_the_exit()
+    {
+        string suffix = UniqueSuffix();
+        SingleInstanceGate first = Pump.Run(() => new SingleInstanceGate(suffix));
+
+        ConcurrentQueue<EventArgs> requests = new();
+        first.ActivationRequested += (_, e) => requests.Enqueue(e);
+
+        Pump.Run(first.TryAcquire);
+        Pump.Run(first.Dispose);
+
+        // The place is free, so the new run simply takes it for itself
+        Assert.True(TryAcquireApart(suffix));
+        Pump.Pause(TimeSpan.FromMilliseconds(80));
+
+        Assert.Empty(requests);
+    }
+
+    // The previous instance crashed and did not release the place. It has no
+    // owner any more, which means the place is free
+    [Fact]
+    public void A_place_left_by_a_crash_counts_as_free()
+    {
+        string suffix = UniqueSuffix();
+
+        // A thread that took the mutex and ended without releasing it is exactly
+        // what a crashed application looks like to Windows
+        Pump.RunApart(() =>
+        {
+            var abandoned = new Mutex(initiallyOwned: false, "CursorLang.SingleInstance" + suffix);
+            abandoned.WaitOne(TimeSpan.Zero, exitContext: false);
+        });
+
+        SingleInstanceGate gate = Pump.Run(() => new SingleInstanceGate(suffix));
+
+        try
+        {
+            Assert.True(Pump.Run(gate.TryAcquire));
+        }
+        finally
+        {
+            Pump.Run(gate.Dispose);
+        }
+    }
+
+    [Fact]
+    public void Closing_without_taking_the_place_passes_without_consequence()
+    {
+        SingleInstanceGate gate = Pump.Run(() => new SingleInstanceGate(UniqueSuffix()));
+
+        Pump.Run(gate.Dispose);
+    }
+
+    // Each half of the application takes a place of its own: one background process
+    // and one settings window, and neither gets in the other's way
+    [Theory]
+    [InlineData(SingleInstanceGate.AgentName)]
+    [InlineData(SingleInstanceGate.SettingsName)]
+    public void Each_half_of_the_application_takes_a_place_of_its_own(string name)
+    {
+        SingleInstanceGate gate = Pump.Run(() => new SingleInstanceGate(name));
+
+        // The place may be held by a running application — then it is simply not taken
+        Pump.Run(gate.Dispose);
+    }
+
+    [Fact]
+    public void Closing_twice_passes_without_consequence()
+    {
+        SingleInstanceGate gate = Pump.Run(() => new SingleInstanceGate(UniqueSuffix()));
+
+        Pump.Run(gate.TryAcquire);
+        Pump.Run(gate.Dispose);
+        Pump.Run(gate.Dispose);
+    }
+
+    // Every test gets its own namespace of kernel objects
+    private static string UniqueSuffix() => "." + Guid.NewGuid().ToString("N");
+
+    /// <summary>
+    /// Tries to take the place the way a run started afterwards does it —
+    /// from another thread rather than from the same one.
+    /// </summary>
+    private static bool TryAcquireApart(string suffix)
+    {
+        bool acquired = false;
+
+        Pump.RunApart(() =>
+        {
+            var gate = new SingleInstanceGate(suffix);
+
+            try
+            {
+                acquired = gate.TryAcquire();
+            }
+            finally
+            {
+                gate.Dispose();
+            }
+        });
+
+        return acquired;
+    }
+}

CursorLang.Core.Tests/Services/StartupLaunchTests.cs

@@ -0,0 +1,98 @@
+using CursorLang.Core.Services;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Core.Tests.Services;
+
+/// <summary>
+/// Telling a launch by Windows apart from a launch by the user: the first one goes
+/// to the tray without a window, the second one is what the window is for.
+/// </summary>
+public sealed class StartupLaunchTests
+{
+    [Fact]
+    public void A_launch_by_the_user_carries_no_argument()
+    {
+        Assert.False(StartupLaunch.HasArgument([]));
+        Assert.False(StartupLaunch.IsAutomatic([]));
+    }
+
+    [Fact]
+    public void The_startup_entry_says_so_in_the_command_line()
+    {
+        Assert.True(StartupLaunch.HasArgument([StartupLaunch.Argument]));
+        Assert.True(StartupLaunch.IsAutomatic([StartupLaunch.Argument]));
+    }
+
+    // The argument does not have to come first: Windows may put its own
+    // alongside it one day
+    [Fact]
+    public void The_argument_is_looked_for_among_the_others()
+    {
+        Assert.True(StartupLaunch.HasArgument(["--whatever", StartupLaunch.Argument]));
+    }
+
+    [Fact]
+    public void The_case_of_the_argument_does_not_matter()
+    {
+        Assert.True(StartupLaunch.HasArgument([StartupLaunch.Argument.ToUpperInvariant()]));
+    }
+
+    [Fact]
+    public void Anything_else_is_a_launch_by_the_user()
+    {
+        Assert.False(StartupLaunch.HasArgument(["--startupp", "startup", "-startup"]));
+    }
+
+    /// <summary>
+    /// The command written into the registry carries the argument: that is the whole
+    /// point of the argument.
+    /// </summary>
+    [Fact]
+    public void The_startup_entry_is_written_with_the_argument()
+    {
+        using var agent = new StagedAgentExecutable();
+
+        string? command = RegistryStartup.GetCommand();
+
+        Assert.NotNull(command);
+        Assert.EndsWith(StartupLaunch.Argument, command, StringComparison.Ordinal);
+
+        // And the path itself stays quoted: it has spaces in it more often than not
+        Assert.StartsWith("\"", command, StringComparison.Ordinal);
+    }
+
+    /// <summary>
+    /// Windows must start the agent, whoever asked for it.
+    /// </summary>
+    /// <remarks>
+    /// The checkbox lives in the settings window, which is a process of its own. Were
+    /// the entry written from the path of whoever is running, the startup list would
+    /// hold the settings window — a process that shows a window and exits, instead of
+    /// the one that is supposed to sit in the tray.
+    /// </remarks>
+    [Fact]
+    public void The_startup_entry_names_the_agent_rather_than_whoever_wrote_it()
+    {
+        using var agent = new StagedAgentExecutable();
+
+        string? command = RegistryStartup.GetCommand();
+
+        Assert.NotNull(command);
+        Assert.Contains("CursorLang.exe", command, StringComparison.OrdinalIgnoreCase);
+        Assert.DoesNotContain("CursorLang.Settings.exe", command, StringComparison.OrdinalIgnoreCase);
+    }
+
+    // Without an agent on disk there is nothing to put in the startup list, and
+    // pointing Windows at a file that is not there would be worse than saying nothing
+    [Fact]
+    public void Without_an_agent_on_disk_there_is_no_entry_to_write()
+    {
+        string agent = Path.Combine(AppContext.BaseDirectory, "CursorLang.exe");
+        if (File.Exists(agent))
+        {
+            Assert.Skip("The agent is built into the test output folder — nothing to check here");
+        }
+
+        Assert.Null(RegistryStartup.GetCommand());
+    }
+}

CursorLang.Core.Tests/Services/StartupServiceTests.cs

@@ -0,0 +1,58 @@
+using CursorLang.Core.Interop;
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+using CursorLang.Tests.Shared;
+using Windows.ApplicationModel;
+
+namespace CursorLang.Core.Tests.Services;
+
+/// <summary>
+/// Startup by way of Windows. The tests run outside an MSIX package — as does any
+/// run of the app from a folder — so the answer they get comes from the registry.
+/// </summary>
+public sealed class StartupServiceTests
+{
+    [Fact]
+    public async Task Outside_a_package_the_state_comes_from_the_registry()
+    {
+        if (PackageIdentityNative.IsPackaged)
+        {
+            // The tests are running as a package: the answer comes from the task instead
+            return;
+        }
+
+        using var agent = new StagedAgentExecutable();
+
+        // Whether startup is on depends on the machine; what matters is that the
+        // question is answered at all and the setting is not hidden
+        Assert.NotEqual(StartupState.Unavailable, await new StartupService().GetStateAsync());
+    }
+
+    [Theory]
+    [InlineData(StartupTaskState.Enabled, StartupState.Enabled)]
+    [InlineData(StartupTaskState.Disabled, StartupState.Disabled)]
+    [InlineData(StartupTaskState.DisabledByUser, StartupState.DisabledByUser)]
+    [InlineData(StartupTaskState.DisabledByPolicy, StartupState.DisabledByPolicy)]
+    [InlineData(StartupTaskState.EnabledByPolicy, StartupState.EnabledByPolicy)]
+    public void A_Windows_task_state_translates_into_an_app_state(
+        StartupTaskState windows, StartupState expected)
+    {
+        Assert.Equal(expected, StartupService.Translate(windows));
+    }
+
+    // Windows may grow a state the app knows nothing about
+    [Fact]
+    public void An_unfamiliar_state_counts_as_unavailable()
+    {
+        Assert.Equal(StartupState.Unavailable, StartupService.Translate((StartupTaskState)999));
+    }
+
+    [Fact]
+    public void No_Windows_state_is_left_behind()
+    {
+        foreach (StartupTaskState state in Enum.GetValues<StartupTaskState>())
+        {
+            Assert.NotEqual(StartupState.Unavailable, StartupService.Translate(state));
+        }
+    }
+}

CursorLang.Core.Tests/Threading/MessageTimerTests.cs

@@ -0,0 +1,129 @@
+using CursorLang.Core.Threading;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Core.Tests.Threading;
+
+/// <summary>
+/// The timer the agent has instead of a dispatcher timer.
+/// </summary>
+/// <remarks>
+/// It ticks on the message loop, so everything here runs on the pump thread — a timer
+/// started on one thread and awaited on another would never be seen to fire.
+/// </remarks>
+public sealed class MessageTimerTests
+{
+    [Fact]
+    public void A_started_timer_ticks()
+    {
+        int ticks = 0;
+
+        Pump.Run(() =>
+        {
+            using var timer = new MessageTimer();
+            timer.Interval = TimeSpan.FromMilliseconds(15);
+            timer.Tick += (_, _) => ticks++;
+            timer.Start();
+
+            Pump.WaitFor(() => ticks > 0, "the timer ticked");
+        });
+    }
+
+    [Fact]
+    public void A_timer_keeps_ticking_until_it_is_stopped()
+    {
+        var ticks = 0;
+
+        Pump.Run(() =>
+        {
+            using var timer = new MessageTimer();
+            timer.Interval = TimeSpan.FromMilliseconds(15);
+            timer.Tick += (_, _) => ticks++;
+            timer.Start();
+
+            Pump.WaitFor(() => ticks >= 3, "the timer ticked more than once");
+        });
+    }
+
+    // A stopped countdown does not go off, however long the loop runs afterwards
+    [Fact]
+    public void A_stopped_timer_does_not_tick()
+    {
+        var ticks = 0;
+
+        Pump.Run(() =>
+        {
+            using var timer = new MessageTimer();
+            timer.Interval = TimeSpan.FromMilliseconds(10);
+            timer.Tick += (_, _) => ticks++;
+            timer.Start();
+            timer.Stop();
+
+            Pump.Pause(TimeSpan.FromMilliseconds(80));
+        });
+
+        Assert.Equal(0, ticks);
+    }
+
+    // Restarting means from zero, so a countdown kept short by repeated restarts
+    // never reaches its end
+    [Fact]
+    public void Restarting_begins_the_countdown_again()
+    {
+        var ticks = 0;
+
+        Pump.Run(() =>
+        {
+            using var timer = new MessageTimer();
+            timer.Interval = TimeSpan.FromMilliseconds(60);
+            timer.Tick += (_, _) => ticks++;
+
+            for (var i = 0; i < 6; i++)
+            {
+                timer.Start();
+                Pump.Pause(TimeSpan.FromMilliseconds(20));
+            }
+
+            Assert.Equal(0, ticks);
+
+            Pump.WaitFor(() => ticks > 0, "left alone, the timer reached its end");
+        });
+    }
+
+    [Fact]
+    public void Closing_the_timer_ends_the_ticking()
+    {
+        var ticks = 0;
+
+        Pump.Run(() =>
+        {
+            var timer = new MessageTimer { Interval = TimeSpan.FromMilliseconds(15) };
+            timer.Tick += (_, _) => ticks++;
+            timer.Start();
+
+            Pump.WaitFor(() => ticks > 0, "the timer ticked");
+            timer.Dispose();
+
+            int seen = ticks;
+            Pump.Pause(TimeSpan.FromMilliseconds(80));
+
+            Assert.Equal(seen, ticks);
+        });
+    }
+
+    [Fact]
+    public void A_timer_that_was_never_started_says_so()
+    {
+        Pump.Run(() =>
+        {
+            using var timer = new MessageTimer();
+
+            Assert.False(timer.IsRunning);
+
+            timer.Start();
+            Assert.True(timer.IsRunning);
+
+            timer.Stop();
+            Assert.False(timer.IsRunning);
+        });
+    }
+}

CursorLang.Core/AssemblyInfo.cs

@@ -0,0 +1,10 @@
+using System.Runtime.CompilerServices;
+
+// Core keeps its interop and its arithmetic internal, as it always did. The two
+// processes built on it are not outside consumers but the other halves of the same
+// application, so they are let in rather than the surface being widened for them.
+[assembly: InternalsVisibleTo("CursorLang")]
+[assembly: InternalsVisibleTo("CursorLang.Settings")]
+[assembly: InternalsVisibleTo("CursorLang.Core.Tests")]
+[assembly: InternalsVisibleTo("CursorLang.Agent.Tests")]
+[assembly: InternalsVisibleTo("CursorLang.Settings.Tests")]

CursorLang.Core/CursorLang.Core.csproj

@@ -0,0 +1,40 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0-windows10.0.19041.0</TargetFramework>
+        <SupportedOSPlatformVersion>10.0.17763.0</SupportedOSPlatformVersion>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
+        <GenerateDocumentationFile>true</GenerateDocumentationFile>
+        <NoWarn>$(NoWarn);CS1591</NoWarn>
+        <RootNamespace>CursorLang.Core</RootNamespace>
+        <AssemblyName>CursorLang.Core</AssemblyName>
+        <PlatformTarget>AnyCPU</PlatformTarget>
+        <RuntimeIdentifiers>win-x64</RuntimeIdentifiers>
+        <Version>1.0.0</Version>
+        <AssemblyVersion>1.0.0.0</AssemblyVersion>
+        <FileVersion>1.0.0.0</FileVersion>
+        <Product>CursorLang</Product>
+        <Company>Aleksandr Neichev</Company>
+        <Description>Shared part of CursorLang: models, settings, layout tracking</Description>
+        <Copyright>Copyright (c) 2026</Copyright>
+    </PropertyGroup>
+
+    <PropertyGroup>
+        <CaretUiAutomation Condition="'$(CaretUiAutomation)' == ''">true</CaretUiAutomation>
+    </PropertyGroup>
+
+    <PropertyGroup Condition="'$(CaretUiAutomation)' == 'true'">
+        <DefineConstants>$(DefineConstants);CARET_UI_AUTOMATION</DefineConstants>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <FrameworkReference Include="Microsoft.WindowsDesktop.App.WPF" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <PackageReference Include="CommunityToolkit.Mvvm" Version="8.4.2" />
+    </ItemGroup>
+
+</Project>

CursorLang.Core/Interop/CaretNative.cs

@@ -0,0 +1,237 @@
+using System.Runtime.InteropServices;
+#if CARET_UI_AUTOMATION
+using System.Windows.Automation;
+using System.Windows.Automation.Text;
+#endif
+using Accessibility;
+
+namespace CursorLang.Core.Interop;
+
+/// <summary>
+/// Locates the caret in the active input field — including one in another application.
+/// </summary>
+/// <remarks>
+/// There is no single way to do it: classic Win32 applications create a system caret,
+/// while Chrome, Electron and others draw it themselves and report its position only
+/// through accessibility interfaces. So we ask the system first, then the application.
+///
+/// The UI Automation step is behind <c>CARET_UI_AUTOMATION</c>: it is the one part of
+/// the background process that reaches into the WPF half of the desktop runtime —
+/// TextPatternRange hands its rectangles back as System.Windows.Rect, which lives in
+/// WindowsBase — and it was measured at +3.9 MB private. It is also the last of the
+/// three steps and rarely reached. See the switch in CursorLang.Core.csproj.
+/// </remarks>
+internal static class CaretNative
+{
+    [DllImport("user32.dll")]
+    private static extern bool ClientToScreen(IntPtr hWnd, ref PopupWindowNative.Point lpPoint);
+
+    [DllImport("oleacc.dll")]
+    private static extern int AccessibleObjectFromWindow(IntPtr hwnd, uint dwObjectId,
+        ref Guid riid, out IAccessible ppvObject);
+
+    [DllImport("user32.dll")]
+    private static extern bool GetWindowRect(IntPtr hWnd, out PopupWindowNative.Rect lpRect);
+
+    [DllImport("user32.dll")]
+    private static extern uint GetDpiForWindow(IntPtr hWnd);
+
+    private const uint OBJID_CARET = 0xFFFFFFF8;
+    private const int CHILDID_SELF = 0;
+
+    /// <summary>
+    /// The caret rectangle in screen pixels, or <c>null</c> when the active
+    /// application does not report its position.
+    /// </summary>
+    internal static PopupWindowNative.Rect? TryGetCaretRect()
+    {
+        if (!ForegroundInputNative.TryGetInfo(out ForegroundInputNative.GuiThreadInfo info))
+        {
+            return null;
+        }
+
+        PopupWindowNative.Rect? caret = TryGetSystemCaret(info)
+            ?? TryGetAccessibleCaret(info.hwndFocus)
+            ?? TryGetAutomationCaret();
+
+        return caret is null ? null : Validate(caret.Value, info.hwndFocus);
+    }
+
+    /// <summary>
+    /// Filters out obviously wrong coordinates.
+    /// </summary>
+    /// <remarks>
+    /// Some applications report the caret position in their own coordinate system or
+    /// without accounting for display scaling, and the popup ends up far from the input
+    /// field. The caret must be inside the input window — that is what we check, and
+    /// before giving up we try to read the coordinates as unscaled ones.
+    /// </remarks>
+    private static PopupWindowNative.Rect? Validate(PopupWindowNative.Rect caret, IntPtr hwndFocus)
+    {
+        if (hwndFocus == IntPtr.Zero || !GetWindowRect(hwndFocus, out PopupWindowNative.Rect window))
+        {
+            return caret;
+        }
+
+        return Validate(caret, window, GetDpiForWindow(hwndFocus) / 96.0);
+    }
+
+    /// <summary>
+    /// The same check over plain numbers: the input window bounds and the scale of
+    /// its monitor are already known.
+    /// </summary>
+    internal static PopupWindowNative.Rect? Validate(
+        PopupWindowNative.Rect caret, PopupWindowNative.Rect window, double scale)
+    {
+        if (IsInside(caret, window))
+        {
+            return caret;
+        }
+
+        var scaled = new PopupWindowNative.Rect
+        {
+            Left = (int)(caret.Left * scale),
+            Top = (int)(caret.Top * scale),
+            Right = (int)(caret.Right * scale),
+            Bottom = (int)(caret.Bottom * scale),
+        };
+
+        return IsInside(scaled, window) ? scaled : null;
+    }
+
+    internal static bool IsInside(PopupWindowNative.Rect inner, PopupWindowNative.Rect outer) =>
+        inner.Left >= outer.Left && inner.Right <= outer.Right &&
+        inner.Top >= outer.Top && inner.Bottom <= outer.Bottom;
+
+#if CARET_UI_AUTOMATION
+    /// <summary>How long we wait for another application to answer over UI Automation.</summary>
+    private static readonly TimeSpan AutomationTimeout = TimeSpan.FromMilliseconds(150);
+
+    // Browsers and other applications with their own rendering engines draw the caret
+    // themselves and report its position only through UI Automation. The request goes
+    // into another process, so it is the slowest one and comes last
+    private static PopupWindowNative.Rect? TryGetAutomationCaret()
+    {
+        // A hung application must not hang the popup along with it: we wait for the
+        // answer for a limited time, otherwise we show the popup at the cursor
+        Task<PopupWindowNative.Rect?> query = Task.Run(QueryAutomationCaret);
+        return query.Wait(AutomationTimeout) ? query.Result : null;
+    }
+
+    private static PopupWindowNative.Rect? QueryAutomationCaret()
+    {
+        try
+        {
+            AutomationElement focused = AutomationElement.FocusedElement;
+            if (focused is null ||
+                !focused.TryGetCurrentPattern(TextPattern.Pattern, out object pattern))
+            {
+                return null;
+            }
+
+            TextPatternRange[] selection = ((TextPattern)pattern).GetSelection();
+            if (selection.Length == 0)
+            {
+                return null;
+            }
+
+            // The caret has an empty selection and therefore no rectangle,
+            // so we expand it to the nearest character
+            TextPatternRange range = selection[0].Clone();
+            range.ExpandToEnclosingUnit(TextUnit.Character);
+
+            System.Windows.Rect[] rectangles = range.GetBoundingRectangles();
+            if (rectangles.Length == 0 || rectangles[0].Height <= 0)
+            {
+                return null;
+            }
+
+            System.Windows.Rect caret = rectangles[0];
+            return new PopupWindowNative.Rect
+            {
+                Left = (int)caret.Left,
+                Top = (int)caret.Top,
+                Right = (int)caret.Right,
+                Bottom = (int)caret.Bottom,
+            };
+        }
+        catch (Exception e) when (e is ElementNotAvailableException
+                                   or InvalidOperationException
+                                   or COMException)
+        {
+            // The application closed or stopped responding — that must not take the popup down
+            return null;
+        }
+    }
+#else
+    // Built without UI Automation: Chromium and Electron keep the system caret and MSAA
+    // steps above, and where those stay silent the popup falls back to the cursor
+    private static PopupWindowNative.Rect? TryGetAutomationCaret() => null;
+#endif
+
+    // The system caret: its coordinates come relative to the window that owns it
+    private static PopupWindowNative.Rect? TryGetSystemCaret(ForegroundInputNative.GuiThreadInfo info)
+    {
+        if (info.hwndCaret == IntPtr.Zero || IsEmpty(info.rcCaret))
+        {
+            return null;
+        }
+
+        var topLeft = new PopupWindowNative.Point { X = info.rcCaret.Left, Y = info.rcCaret.Top };
+        var bottomRight = new PopupWindowNative.Point { X = info.rcCaret.Right, Y = info.rcCaret.Bottom };
+        if (!ClientToScreen(info.hwndCaret, ref topLeft) || !ClientToScreen(info.hwndCaret, ref bottomRight))
+        {
+            return null;
+        }
+
+        return new PopupWindowNative.Rect
+        {
+            Left = topLeft.X,
+            Top = topLeft.Y,
+            Right = bottomRight.X,
+            Bottom = bottomRight.Y,
+        };
+    }
+
+    // The caret through accessibility interfaces: this is where browsers and Electron land
+    private static PopupWindowNative.Rect? TryGetAccessibleCaret(IntPtr hwndFocus)
+    {
+        if (hwndFocus == IntPtr.Zero)
+        {
+            return null;
+        }
+
+        Guid iid = typeof(IAccessible).GUID;
+        if (AccessibleObjectFromWindow(hwndFocus, OBJID_CARET, ref iid, out IAccessible caret) != 0)
+        {
+            return null;
+        }
+
+        try
+        {
+            caret.accLocation(out int left, out int top, out int width, out int height, CHILDID_SELF);
+            var rect = new PopupWindowNative.Rect
+            {
+                Left = left,
+                Top = top,
+                Right = left + width,
+                Bottom = top + height,
+            };
+
+            return IsEmpty(rect) ? null : rect;
+        }
+        catch (COMException)
+        {
+            // The application declared support but did not report the position
+            return null;
+        }
+        finally
+        {
+            Marshal.ReleaseComObject(caret);
+        }
+    }
+
+    // When there is no caret, its rectangle comes back with zero height.
+    // We judge by height alone: zero coordinates are a normal start of an empty field
+    internal static bool IsEmpty(PopupWindowNative.Rect rect) => rect.Bottom - rect.Top <= 0;
+}

CursorLang.Core/Interop/ForegroundInputNative.cs

@@ -0,0 +1,41 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Interop;
+
+/// <summary>
+/// Input details of the active application: which window holds keyboard focus
+/// and where the caret is.
+/// </summary>
+internal static class ForegroundInputNative
+{
+    [StructLayout(LayoutKind.Sequential)]
+    internal struct GuiThreadInfo
+    {
+        public int cbSize;
+        public uint flags;
+        public IntPtr hwndActive;
+        public IntPtr hwndFocus;
+        public IntPtr hwndCapture;
+        public IntPtr hwndMenuOwner;
+        public IntPtr hwndMoveSize;
+        public IntPtr hwndCaret;
+        public PopupWindowNative.Rect rcCaret;
+    }
+
+    [DllImport("user32.dll")]
+    private static extern bool GetGUIThreadInfo(uint idThread, ref GuiThreadInfo lpgui);
+
+    /// <summary>
+    /// The input state of the foreground thread.
+    /// </summary>
+    /// <remarks>
+    /// The zero thread identifier is not accidental: in modern applications the
+    /// top-level window and the input window live in different threads, and the
+    /// question has to be about the foreground as a whole.
+    /// </remarks>
+    internal static bool TryGetInfo(out GuiThreadInfo info)
+    {
+        info = new GuiThreadInfo { cbSize = Marshal.SizeOf<GuiThreadInfo>() };
+        return GetGUIThreadInfo(0, ref info);
+    }
+}

CursorLang.Core/Interop/ForegroundPermissionNative.cs

@@ -0,0 +1,26 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Interop;
+
+/// <summary>
+/// The right to bring a window to the foreground.
+/// </summary>
+internal static class ForegroundPermissionNative
+{
+    /// <summary>ASFW_ANY — any process gets the right.</summary>
+    private const uint AnyProcess = 0xFFFFFFFF;
+
+    /// <summary>
+    /// Gives up our right to bring a window to the foreground in favour of other processes.
+    /// </summary>
+    /// <remarks>
+    /// Windows does not let just anyone change the foreground window: you have to be
+    /// the process the user interacted with last. An instance started long ago is not
+    /// one of those, and its window will come up only if the right is shared by the
+    /// process the user has just launched.
+    /// </remarks>
+    internal static void GrantToAnyProcess() => AllowSetForegroundWindow(AnyProcess);
+
+    [DllImport("user32.dll")]
+    private static extern bool AllowSetForegroundWindow(uint dwProcessId);
+}

CursorLang.Core/Interop/KeyboardLayoutNative.cs

@@ -0,0 +1,85 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Interop;
+
+/// <summary>
+/// Win32 API for reading the layout of the active application.
+/// </summary>
+internal static class KeyboardLayoutNative
+{
+    private const uint WmInputLangChangeRequest = 0x0050;
+
+    /// <summary>Take the next layout from the system list.</summary>
+    private static readonly IntPtr InputLangChangeForward = new(0x0002);
+
+    /// <summary>HKL_NEXT — the same request in the language of older Windows versions.</summary>
+    private static readonly IntPtr HklNext = new(1);
+
+    [DllImport("user32.dll")]
+    internal static extern IntPtr GetForegroundWindow();
+
+    [DllImport("user32.dll")]
+    private static extern uint GetWindowThreadProcessId(IntPtr hWnd, IntPtr lpdwProcessId);
+
+    [DllImport("user32.dll")]
+    private static extern IntPtr GetKeyboardLayout(uint idThread);
+
+    [DllImport("user32.dll")]
+    private static extern bool PostMessage(IntPtr hWnd, uint msg, IntPtr wParam, IntPtr lParam);
+
+    /// <summary>
+    /// The layout the user is currently typing with.
+    /// </summary>
+    internal static int GetActiveLocaleId() => GetLocaleIdOf(GetInputWindow());
+
+    /// <summary>
+    /// Asks the active application to switch to the next layout from the system list.
+    /// </summary>
+    /// <remarks>
+    /// Synthesizing a system shortcut such as Alt+Shift will not do: the user can
+    /// reassign it in the Windows settings or turn it off entirely. A request sent as
+    /// a message does not depend on those settings and works in another process.
+    /// </remarks>
+    internal static void RequestNextLayout()
+    {
+        IntPtr target = GetInputWindow();
+        if (target == IntPtr.Zero)
+        {
+            return;
+        }
+
+        // Both parameters mean the same thing: different Windows versions and different
+        // UI frameworks look either at the flag or at lParam
+        PostMessage(target, WmInputLangChangeRequest, InputLangChangeForward, HklNext);
+    }
+
+    /// <summary>
+    /// The window that owns keyboard input.
+    /// </summary>
+    /// <remarks>
+    /// We ask the window with keyboard focus rather than the foreground window: in
+    /// Windows 11 Notepad, the Start menu and other WinUI applications the input field
+    /// lives in a separate thread, and the layout changes only for that thread. For the
+    /// main window's thread it stays the same, and the switch goes unnoticed.
+    /// </remarks>
+    private static IntPtr GetInputWindow()
+    {
+        if (ForegroundInputNative.TryGetInfo(out ForegroundInputNative.GuiThreadInfo info) &&
+            info.hwndFocus != IntPtr.Zero)
+        {
+            return info.hwndFocus;
+        }
+
+        return GetForegroundWindow();
+    }
+
+    /// <summary>
+    /// The locale identifier for a window. In Windows the layout is bound to a thread,
+    /// so this reveals it for any application, not only for our own.
+    /// </summary>
+    internal static int GetLocaleIdOf(IntPtr hWnd)
+    {
+        uint threadId = GetWindowThreadProcessId(hWnd, IntPtr.Zero);
+        return (int)GetKeyboardLayout(threadId).ToInt64() & 0xFFFF;
+    }
+}

CursorLang.Core/Interop/LowLevelKeyboardHook.cs

@@ -0,0 +1,130 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Interop;
+
+/// <summary>
+/// A system keyboard hook (WH_KEYBOARD_LL): sees key presses in every application
+/// and can keep them from going any further.
+/// </summary>
+/// <remarks>
+/// Only the low-level hook is installed: a regular WH_KEYBOARD requires injecting a
+/// DLL into other processes, which is impossible for managed code. The callback
+/// arrives on the thread that installed the hook, and that thread must pump a message
+/// loop — hence the requirement to install the hook from the user interface thread.
+/// Returning control must not be delayed: once the system timeout expires, Windows
+/// silently removes the hook.
+/// </remarks>
+internal sealed class LowLevelKeyboardHook : IDisposable
+{
+    /// <summary>
+    /// A key event handler. Returns <c>true</c> when the event must be swallowed —
+    /// then the foreground application will not see it.
+    /// </summary>
+    internal delegate bool KeyFilter(int virtualKey, bool isKeyDown);
+
+    private const int WhKeyboardLowLevel = 13;
+    private const int HcAction = 0;
+
+    private const int WmKeyDown = 0x0100;
+    private const int WmKeyUp = 0x0101;
+    private const int WmSysKeyDown = 0x0104;
+    private const int WmSysKeyUp = 0x0105;
+
+    /// <summary>The event came from SendInput rather than from a real key press.</summary>
+    private const uint LowLevelKeyHookFlagInjected = 0x10;
+
+    private readonly KeyFilter _filter;
+
+    // The delegate lives in a field not for convenience: the only reference to it is
+    // held by Win32, which the garbage collector knows nothing about, and without the
+    // field the hook stops working after a random amount of time
+    private readonly HookProc _callback;
+
+    private IntPtr _handle;
+
+    internal LowLevelKeyboardHook(KeyFilter filter)
+    {
+        _filter = filter;
+        _callback = OnHookEvent;
+    }
+
+    private delegate IntPtr HookProc(int code, IntPtr wParam, IntPtr lParam);
+
+    internal bool IsInstalled => _handle != IntPtr.Zero;
+
+    /// <summary>
+    /// Installs the hook. Returns <c>false</c> when the system refuses.
+    /// </summary>
+    internal bool Install()
+    {
+        if (_handle != IntPtr.Zero)
+        {
+            return true;
+        }
+
+        _handle = SetWindowsHookEx(WhKeyboardLowLevel, _callback, GetModuleHandle(null), 0);
+        return _handle != IntPtr.Zero;
+    }
+
+    internal void Uninstall()
+    {
+        if (_handle == IntPtr.Zero)
+        {
+            return;
+        }
+
+        UnhookWindowsHookEx(_handle);
+        _handle = IntPtr.Zero;
+    }
+
+    public void Dispose() => Uninstall();
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern IntPtr SetWindowsHookEx(int idHook, HookProc lpfn, IntPtr hMod, uint dwThreadId);
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern bool UnhookWindowsHookEx(IntPtr hhk);
+
+    [DllImport("user32.dll")]
+    private static extern IntPtr CallNextHookEx(IntPtr hhk, int code, IntPtr wParam, IntPtr lParam);
+
+    [DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
+    private static extern IntPtr GetModuleHandle(string? lpModuleName);
+
+    private IntPtr OnHookEvent(int code, IntPtr wParam, IntPtr lParam)
+    {
+        if (code != HcAction)
+        {
+            return CallNextHookEx(_handle, code, wParam, lParam);
+        }
+
+        var message = (int)wParam;
+        bool isKeyDown = message is WmKeyDown or WmSysKeyDown;
+        bool isKeyUp = message is WmKeyUp or WmSysKeyUp;
+
+        var data = Marshal.PtrToStructure<KeyboardHookData>(lParam);
+
+        // Synthetic input comes from on-screen keyboards, text expanders and
+        // automation tools: overriding what they do is none of our business
+        bool injected = (data.flags & LowLevelKeyHookFlagInjected) != 0;
+
+        if ((isKeyDown || isKeyUp) && !injected && _filter((int)data.vkCode, isKeyDown))
+        {
+            // A non-zero result instead of CallNextHookEx breaks the chain: the event
+            // will reach neither the application nor the Windows caps-lock handler
+            return 1;
+        }
+
+        return CallNextHookEx(_handle, code, wParam, lParam);
+    }
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct KeyboardHookData
+    {
+        public uint vkCode;
+        public uint scanCode;
+        public uint flags;
+        public uint time;
+        public IntPtr dwExtraInfo;
+    }
+}

CursorLang.Core/Interop/PackageIdentityNative.cs

@@ -0,0 +1,38 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Interop;
+
+/// <summary>
+/// Answers whether the application runs from an MSIX package.
+/// </summary>
+/// <remarks>
+/// The same application runs both installed from the Store and simply unpacked
+/// into a folder. Some Windows features — startup through <c>StartupTask</c>, for
+/// instance — are available only to a package, and reaching for them without a
+/// check is not allowed: outside a package they throw.
+/// </remarks>
+internal static class PackageIdentityNative
+{
+    /// <summary>APPMODEL_ERROR_NO_PACKAGE — the process runs outside a package.</summary>
+    private const int NoPackage = 15700;
+
+    /// <summary>
+    /// The application runs from an MSIX package.
+    /// </summary>
+    /// <remarks>
+    /// The value is computed once: it cannot change during the lifetime of
+    /// the process.
+    /// </remarks>
+    internal static bool IsPackaged { get; } = DetectPackage();
+
+    private static bool DetectPackage()
+    {
+        // The answer comes from the return code rather than from the name itself, so
+        // no buffer is needed: with zero length a package replies complaining about space
+        uint length = 0;
+        return GetCurrentPackageFullName(ref length, IntPtr.Zero) != NoPackage;
+    }
+
+    [DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
+    private static extern int GetCurrentPackageFullName(ref uint packageFullNameLength, IntPtr packageFullName);
+}

CursorLang.Core/Interop/PopupWindowNative.cs

@@ -0,0 +1,112 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Interop;
+
+/// <summary>
+/// Win32 API for the popup window: styles, positioning near the cursor
+/// and the scale of the monitor the cursor is on.
+/// </summary>
+internal static class PopupWindowNative
+{
+    [StructLayout(LayoutKind.Sequential)]
+    internal struct Point
+    {
+        public int X;
+        public int Y;
+    }
+
+    [DllImport("user32.dll")]
+    private static extern bool GetCursorPos(out Point lpPoint);
+
+    [DllImport("user32.dll")]
+    private static extern IntPtr GetForegroundWindow();
+
+    [DllImport("user32.dll")]
+    private static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter,
+        int X, int Y, int cx, int cy, uint uFlags);
+
+    [DllImport("user32.dll")]
+    private static extern IntPtr MonitorFromPoint(Point pt, uint dwFlags);
+
+    [DllImport("shcore.dll")]
+    private static extern int GetDpiForMonitor(IntPtr hMonitor, int dpiType, out uint dpiX, out uint dpiY);
+
+    [DllImport("user32.dll")]
+    private static extern IntPtr MonitorFromWindow(IntPtr hWnd, uint dwFlags);
+
+    [DllImport("user32.dll")]
+    private static extern bool GetMonitorInfo(IntPtr hMonitor, ref MonitorInfo lpmi);
+
+    [StructLayout(LayoutKind.Sequential)]
+    internal struct Rect
+    {
+        public int Left;
+        public int Top;
+        public int Right;
+        public int Bottom;
+    }
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct MonitorInfo
+    {
+        public int cbSize;
+        public Rect rcMonitor;
+        public Rect rcWork;
+        public uint dwFlags;
+    }
+
+    private const uint SWP_NOSIZE = 0x0001;
+    private const uint SWP_NOZORDER = 0x0004;
+    private const uint SWP_NOACTIVATE = 0x0010;
+
+    private const uint MONITOR_DEFAULTTONEAREST = 2;
+    private const int MDT_EFFECTIVE_DPI = 0;
+
+    internal static Point GetCursorPosition()
+    {
+        GetCursorPos(out Point cursor);
+        return cursor;
+    }
+
+    /// <summary>
+    /// Moves the window to a screen point without changing its size or z-order.
+    /// The coordinates are physical pixels: monitors have different scaling, while
+    /// Window.Left/Top are converted using the DPI of the monitor the window is on
+    /// right now, which misses the target on a neighbouring monitor.
+    /// </summary>
+    internal static void MoveTo(IntPtr hWnd, int x, int y)
+    {
+        SetWindowPos(hWnd, IntPtr.Zero, x, y, 0, 0, SWP_NOSIZE | SWP_NOZORDER | SWP_NOACTIVATE);
+    }
+
+    /// <summary>The scale of the monitor the point is on (1.0 at 96 DPI).</summary>
+    internal static double GetScaleAt(Point point) =>
+        GetScaleOf(MonitorFromPoint(point, MONITOR_DEFAULTTONEAREST));
+
+    /// <summary>
+    /// The work area of the monitor holding the active window — without the taskbar —
+    /// and its scale. That is the monitor the user is working on right now.
+    /// </summary>
+    internal static (Rect WorkArea, double Scale) GetActiveMonitorWorkArea()
+    {
+        IntPtr monitor = MonitorFromWindow(GetForegroundWindow(), MONITOR_DEFAULTTONEAREST);
+
+        var info = new MonitorInfo { cbSize = Marshal.SizeOf<MonitorInfo>() };
+        if (!GetMonitorInfo(monitor, ref info))
+        {
+            return (new Rect(), 1.0);
+        }
+
+        return (info.rcWork, GetScaleOf(monitor));
+    }
+
+    private static double GetScaleOf(IntPtr monitor)
+    {
+        if (GetDpiForMonitor(monitor, MDT_EFFECTIVE_DPI, out uint dpiX, out _) < 0)
+        {
+            return 1.0;
+        }
+
+        return dpiX / 96.0;
+    }
+}

CursorLang.Core/Models/AppSettings.cs

@@ -0,0 +1,145 @@
+using System.ComponentModel;
+using System.Text.Json.Serialization;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace CursorLang.Core.Models;
+
+/// <summary>
+/// The application settings, as the settings window writes them to settings.json.
+/// </summary>
+/// <remarks>
+/// Everything about the popup — the side, the offset, the font size, the opacity and
+/// both colours — belongs to a placement mode rather than to the application, and is
+/// kept in a <see cref="PopupModeSettings"/> of its own for each of them. So the
+/// settings of a mode are still there after a trip through the other two.
+///
+/// Both processes hold an instance of this, but only the settings window writes: the
+/// agent re-reads the file and pours the fresh values into the instance it already has,
+/// so everything subscribed to it stays subscribed. See <see cref="CopyFrom"/>.
+/// </remarks>
+public sealed partial class AppSettings : ObservableObject
+{
+    /// <summary>The interface language as a culture code: "ru", "en".</summary>
+    [ObservableProperty]
+    private string _language = "en";
+
+    /// <summary>The look of the settings window.</summary>
+    [ObservableProperty]
+    private AppTheme _theme = AppTheme.System;
+
+    /// <summary>Where the popup is shown: at the cursor, at the caret or at a fixed point.</summary>
+    [ObservableProperty]
+    private PopupPlacementMode _placementMode = PopupPlacementMode.AtCursor;
+
+    /// <summary>How long the popup stays on screen, in milliseconds.</summary>
+    [ObservableProperty]
+    private double _durationMilliseconds = 500;
+
+    /// <summary>
+    /// Intercept Caps Lock and switch the layout with it instead of changing the case.
+    /// Off by default: the application must not change the behaviour of the system
+    /// until it is asked to.
+    /// </summary>
+    [ObservableProperty]
+    private bool _useCapsLockHotkey;
+
+    /// <summary>
+    /// After how long a Caps Lock hold cancels the switch, in milliseconds.
+    /// </summary>
+    [ObservableProperty]
+    private double _capsLockHoldMilliseconds = 300;
+
+    public AppSettings()
+    {
+        AtCursor.PropertyChanged += OnModeChanged;
+        AtCaret.PropertyChanged += OnModeChanged;
+        FixedPoint.PropertyChanged += OnModeChanged;
+    }
+
+    /// <summary>The <see cref="PopupPlacementMode.AtCursor"/> mode.</summary>
+    /// <remarks>
+    /// The modes are handed out rather than replaced — the instance is the same for the
+    /// life of the settings, so a binding and a subscription to it hold. That is what
+    /// the creation handling is for: without it the deserializer would want a setter,
+    /// and reading the file would swap the object everything is bound to.
+    /// </remarks>
+    [JsonObjectCreationHandling(JsonObjectCreationHandling.Populate)]
+    public CursorModeSettings AtCursor { get; } = new();
+
+    /// <summary>The <see cref="PopupPlacementMode.AtCaret"/> mode.</summary>
+    [JsonObjectCreationHandling(JsonObjectCreationHandling.Populate)]
+    public CaretModeSettings AtCaret { get; } = new();
+
+    /// <summary>The <see cref="PopupPlacementMode.FixedPoint"/> mode.</summary>
+    [JsonObjectCreationHandling(JsonObjectCreationHandling.Populate)]
+    public FixedPointModeSettings FixedPoint { get; } = new();
+
+    /// <summary>The settings of the mode currently chosen.</summary>
+    /// <remarks>
+    /// The settings window binds the look through here, and the popup asks for it here
+    /// too: what is on screen is always the settings of the mode in force. A change of
+    /// <see cref="PlacementMode"/> announces this as changed, so the bindings follow.
+    /// </remarks>
+    [JsonIgnore]
+    public PopupModeSettings Current => PlacementMode switch
+    {
+        PopupPlacementMode.AtCaret => AtCaret,
+        PopupPlacementMode.FixedPoint => FixedPoint,
+        _ => AtCursor,
+    };
+
+    /// <summary><see cref="DurationMilliseconds"/> as a <see cref="TimeSpan"/>.</summary>
+    [JsonIgnore]
+    public TimeSpan Duration => TimeSpan.FromMilliseconds(DurationMilliseconds);
+
+    /// <summary><see cref="CapsLockHoldMilliseconds"/> as a <see cref="TimeSpan"/>.</summary>
+    [JsonIgnore]
+    public TimeSpan CapsLockHoldDelay => TimeSpan.FromMilliseconds(CapsLockHoldMilliseconds);
+
+    /// <summary>
+    /// Takes the values of another instance over, raising a change notification for
+    /// every property that has actually moved.
+    /// </summary>
+    /// <remarks>
+    /// This is how the agent learns about an edit: the settings window is a separate
+    /// process, so the fresh values arrive as a freshly parsed instance and are poured
+    /// into the one everything is already bound to, rather than replacing it. The modes
+    /// are filled the same way, and for the same reason.
+    /// </remarks>
+    public void CopyFrom(AppSettings other)
+    {
+        Language = other.Language;
+        Theme = other.Theme;
+        PlacementMode = other.PlacementMode;
+        DurationMilliseconds = other.DurationMilliseconds;
+        UseCapsLockHotkey = other.UseCapsLockHotkey;
+        CapsLockHoldMilliseconds = other.CapsLockHoldMilliseconds;
+
+        AtCursor.CopyFrom(other.AtCursor);
+        AtCaret.CopyFrom(other.AtCaret);
+        FixedPoint.CopyFrom(other.FixedPoint);
+    }
+
+    // The mode in force decides what the look means, so a switch of the mode is a
+    // change of everything bound through Current
+    partial void OnPlacementModeChanged(PopupPlacementMode value) => OnPropertyChanged(nameof(Current));
+
+    /// <summary>
+    /// Passes a change made inside a mode on as a change of the settings.
+    /// </summary>
+    /// <remarks>
+    /// A mode is an object of its own, so an edit in it is not an edit of this one as
+    /// far as <see cref="INotifyPropertyChanged"/> goes. The settings window binds
+    /// through the path and hears the mode itself, but the service that writes the file
+    /// listens to the settings alone — and without this a font size dragged in the
+    /// window would never reach the disk. The name says where the change happened:
+    /// "AtCaret.FontSize".
+    /// </remarks>
+    private void OnModeChanged(object? sender, PropertyChangedEventArgs e) =>
+        OnPropertyChanged($"{NameOfMode(sender)}.{e.PropertyName}");
+
+    private string NameOfMode(object? mode) =>
+        ReferenceEquals(mode, AtCaret) ? nameof(AtCaret)
+            : ReferenceEquals(mode, FixedPoint) ? nameof(FixedPoint)
+            : nameof(AtCursor);
+}

CursorLang.Core/Models/AppTheme.cs

@@ -0,0 +1,12 @@
+namespace CursorLang.Core.Models;
+
+/// <summary>
+/// The look of the settings window. By default the application follows the Windows
+/// theme, but the user can pin the light or the dark one.
+/// </summary>
+public enum AppTheme
+{
+    System,
+    Light,
+    Dark
+}

CursorLang.Core/Models/KeyboardLayout.cs

@@ -0,0 +1,41 @@
+using System.Globalization;
+
+namespace CursorLang.Core.Models;
+
+/// <summary>
+/// A keyboard layout in a form convenient for display.
+/// </summary>
+/// <param name="LocaleId">The locale identifier (the low word of HKL).</param>
+/// <param name="ShortName">A short name for the popup at the cursor, "RU" for instance.</param>
+/// <param name="DisplayName">The full name, "RU — русский (Россия)" for instance.</param>
+public sealed record KeyboardLayout(int LocaleId, string ShortName, string DisplayName)
+{
+    /// <summary>
+    /// Builds the model from a locale identifier. Unknown locales are not an
+    /// error: for them we show the identifier itself.
+    /// </summary>
+    public static KeyboardLayout FromLocaleId(int localeId)
+    {
+        CultureInfo? culture = TryGetCulture(localeId);
+        if (culture is null)
+        {
+            string fallback = $"0x{localeId:X4}";
+            return new KeyboardLayout(localeId, fallback, fallback);
+        }
+
+        string shortName = culture.TwoLetterISOLanguageName.ToUpperInvariant();
+        return new KeyboardLayout(localeId, shortName, $"{shortName} — {culture.NativeName}");
+    }
+
+    private static CultureInfo? TryGetCulture(int localeId)
+    {
+        try
+        {
+            return new CultureInfo(localeId);
+        }
+        catch (CultureNotFoundException)
+        {
+            return null;
+        }
+    }
+}

CursorLang.Core/Models/LayoutChangedEventArgs.cs

@@ -0,0 +1,23 @@
+namespace CursorLang.Core.Models;
+
+/// <summary>
+/// Why the current layout has changed.
+/// </summary>
+public enum LayoutChangeReason
+{
+    /// <summary>The user switched the layout in the active application.</summary>
+    UserSwitched,
+
+    /// <summary>The user moved to another application that has a layout of its own.</summary>
+    ApplicationSwitched,
+}
+
+/// <summary>
+/// The data of a layout change event.
+/// </summary>
+public sealed class LayoutChangedEventArgs(KeyboardLayout layout, LayoutChangeReason reason) : EventArgs
+{
+    public KeyboardLayout Layout { get; } = layout;
+
+    public LayoutChangeReason Reason { get; } = reason;
+}

CursorLang.Core/Models/PopupModeSettings.cs

@@ -0,0 +1,141 @@
+using System.Drawing;
+using System.Text.Json.Serialization;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace CursorLang.Core.Models;
+
+/// <summary>
+/// The settings of one placement mode: where the popup goes and how it looks there.
+/// </summary>
+/// <remarks>
+/// Every mode keeps a set of its own. The popup next to the caret sits inside a text
+/// being read and is wanted small and quiet; the one in the corner of the monitor is
+/// looked for on purpose and is wanted large. A look shared by the modes meant setting
+/// it up again after every switch — and switching modes to see what they do undid what
+/// had just been set up.
+///
+/// The colours are <see cref="System.Drawing.Color"/> for the same reason as in
+/// <see cref="AppSettings"/>: the agent reads them and must stay clear of WPF.
+/// </remarks>
+public abstract partial class PopupModeSettings : ObservableObject
+{
+    /// <summary>The distance from whatever the popup is placed by, in WPF units.</summary>
+    [ObservableProperty]
+    private double _offset = 16;
+
+    /// <summary>The size of the layout name in the popup, in WPF units.</summary>
+    [ObservableProperty]
+    private double _fontSize = 20;
+
+    /// <summary>The popup opacity: 1.0 is fully opaque.</summary>
+    [ObservableProperty]
+    private double _opacity = 0.9;
+
+    /// <summary>The fill of the popup. The opacity is set by <see cref="Opacity"/>.</summary>
+    [ObservableProperty]
+    private Color _backgroundColor = Color.FromArgb(0xFF, 0x20, 0x20, 0x20);
+
+    /// <summary>The colour of the layout name in the popup.</summary>
+    [ObservableProperty]
+    private Color _foregroundColor = Color.FromArgb(0xFF, 0xFF, 0xFF, 0xFF);
+
+    /// <summary>
+    /// Takes the offset and the look of another mode over. What tells the modes apart —
+    /// the side, the place on the monitor — is copied by the mode itself.
+    /// </summary>
+    protected void CopyLookFrom(PopupModeSettings other)
+    {
+        Offset = other.Offset;
+        FontSize = other.FontSize;
+        Opacity = other.Opacity;
+        BackgroundColor = other.BackgroundColor;
+        ForegroundColor = other.ForegroundColor;
+    }
+}
+
+/// <summary>The mode that puts the popup next to the mouse cursor.</summary>
+public sealed partial class CursorModeSettings : PopupModeSettings
+{
+    /// <summary>The corner or the side of the cursor the popup is put on.</summary>
+    [ObservableProperty]
+    private AnchorSide _side = AnchorSide.BottomRight;
+
+    /// <summary>Takes the values of another mode of the same kind over.</summary>
+    public void CopyFrom(CursorModeSettings other)
+    {
+        CopyLookFrom(other);
+        Side = other.Side;
+    }
+}
+
+/// <summary>
+/// The mode that puts the popup next to the caret of the active input field.
+/// </summary>
+public sealed partial class CaretModeSettings : PopupModeSettings
+{
+    /// <summary>The side of the caret the popup is put on: only left or right.</summary>
+    [ObservableProperty]
+    private CaretSide _side = CaretSide.Right;
+
+    /// <summary>
+    /// The side as the layout arithmetic wants it. Both of them line the popup up with
+    /// the caret rather than putting it above or below.
+    /// </summary>
+    [JsonIgnore]
+    public AnchorSide Anchor => Side == CaretSide.Left ? AnchorSide.Left : AnchorSide.Right;
+
+    /// <summary>Takes the values of another mode of the same kind over.</summary>
+    public void CopyFrom(CaretModeSettings other)
+    {
+        CopyLookFrom(other);
+        Side = other.Side;
+    }
+
+    // The side decides what the popup is lined up against, so it is a change of that too
+    partial void OnSideChanged(CaretSide value) => OnPropertyChanged(nameof(Anchor));
+}
+
+/// <summary>
+/// The mode that puts the popup at a fixed place of the monitor holding the active
+/// window. Here <see cref="PopupModeSettings.Offset"/> is the distance from the edge
+/// of the monitor.
+/// </summary>
+/// <remarks>
+/// In the middle of the monitor there is no edge to keep a distance from, so the offset
+/// is zero there and the settings window does not offer it. Choosing the middle puts it
+/// back to zero rather than remembering it for later: a setting that is not shown must
+/// not be one that still applies.
+/// </remarks>
+public sealed partial class FixedPointModeSettings : PopupModeSettings
+{
+    /// <summary>The place on the monitor the popup is put at.</summary>
+    [ObservableProperty]
+    private ScreenPosition _position = ScreenPosition.Center;
+
+    public FixedPointModeSettings()
+    {
+        // The middle is the default, and it has no edge to stand off from
+        Offset = 0;
+    }
+
+    /// <summary>Whether the popup is put at an edge of the monitor rather than in its middle.</summary>
+    [JsonIgnore]
+    public bool IsAtAnEdge => Position != ScreenPosition.Center;
+
+    /// <summary>Takes the values of another mode of the same kind over.</summary>
+    public void CopyFrom(FixedPointModeSettings other)
+    {
+        CopyLookFrom(other);
+        Position = other.Position;
+    }
+
+    partial void OnPositionChanged(ScreenPosition value)
+    {
+        OnPropertyChanged(nameof(IsAtAnEdge));
+
+        if (value == ScreenPosition.Center)
+        {
+            Offset = 0;
+        }
+    }
+}

CursorLang.Core/Models/PopupPlacement.cs

@@ -0,0 +1,61 @@
+namespace CursorLang.Core.Models;
+
+/// <summary>
+/// How the place for the popup is chosen.
+/// </summary>
+public enum PopupPlacementMode
+{
+    /// <summary>Next to the mouse cursor.</summary>
+    AtCursor,
+
+    /// <summary>
+    /// Next to the caret in the active input field. When the application does not
+    /// report its position, the popup is shown at the mouse cursor.
+    /// </summary>
+    AtCaret,
+
+    /// <summary>At a fixed point of the monitor holding the active window.</summary>
+    FixedPoint,
+}
+
+/// <summary>
+/// Which side of the cursor to show the popup on.
+/// </summary>
+public enum AnchorSide
+{
+    TopLeft,
+    TopRight,
+    Left,
+    Right,
+    BottomLeft,
+    BottomRight,
+}
+
+/// <summary>
+/// Which side of the caret to show the popup on.
+/// </summary>
+/// <remarks>
+/// Only the two sides, unlike <see cref="AnchorSide"/>. The caret stands in a line of
+/// text being written, and above or below it is exactly where the next line is: the
+/// popup would cover what is being read. To the side it covers nothing, and the line
+/// it lines up with is the one the caret is in.
+/// </remarks>
+public enum CaretSide
+{
+    Left,
+    Right,
+}
+
+/// <summary>
+/// The place on the monitor for the <see cref="PopupPlacementMode.FixedPoint"/> mode.
+/// </summary>
+public enum ScreenPosition
+{
+    TopLeft,
+    Top,
+    TopRight,
+    Center,
+    BottomLeft,
+    Bottom,
+    BottomRight,
+}

CursorLang.Core/Models/StartupState.cs

@@ -0,0 +1,30 @@
+namespace CursorLang.Core.Models;
+
+/// <summary>
+/// The state of startup. Follows <c>StartupTaskState</c> of Windows: the user and
+/// the administrator each have their own way of forbidding startup, and the app has
+/// to tell them apart so as not to promise what it cannot do.
+/// </summary>
+public enum StartupState
+{
+    /// <summary>Windows will not answer about startup — the setting is not shown.</summary>
+    Unavailable,
+
+    /// <summary>Off, and the app can switch it on.</summary>
+    Disabled,
+
+    /// <summary>On.</summary>
+    Enabled,
+
+    /// <summary>
+    /// Switched off by the user in the settings of Windows. It goes back on only
+    /// there: a ban by the user is not for the app to overrule.
+    /// </summary>
+    DisabledByUser,
+
+    /// <summary>Forbidden by the policy of the organisation.</summary>
+    DisabledByPolicy,
+
+    /// <summary>Switched on by the policy of the organisation and not to be switched off.</summary>
+    EnabledByPolicy,
+}

CursorLang.Core/Models/UpdateStatus.cs

@@ -0,0 +1,29 @@
+namespace CursorLang.Core.Models;
+
+/// <summary>
+/// Which step the update is at. One value — one state of the interface:
+/// the caption, the button and the progress bar are shown from it.
+/// </summary>
+public enum UpdateStatus
+{
+    /// <summary>There have been no checks yet.</summary>
+    Idle,
+
+    /// <summary>A request to the repository is in flight.</summary>
+    Checking,
+
+    /// <summary>The latest version is installed.</summary>
+    UpToDate,
+
+    /// <summary>There is a newer version — it can be downloaded.</summary>
+    Available,
+
+    /// <summary>The package is downloading.</summary>
+    Downloading,
+
+    /// <summary>The package is downloaded and ready to install.</summary>
+    Ready,
+
+    /// <summary>The check or the download failed.</summary>
+    Failed,
+}

CursorLang.Core/Resources/Strings.resx

@@ -0,0 +1,214 @@
+<?xml version="1.0" encoding="utf-8"?>
+<root>
+  <xsd:schema id="root" xmlns="" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
+    <xsd:import namespace="http://www.w3.org/XML/1998/namespace" />
+    <xsd:element name="root" msdata:IsDataSet="true">
+      <xsd:complexType>
+        <xsd:choice maxOccurs="unbounded">
+          <xsd:element name="metadata">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" />
+              </xsd:sequence>
+              <xsd:attribute name="name" use="required" type="xsd:string" />
+              <xsd:attribute name="type" type="xsd:string" />
+              <xsd:attribute name="mimetype" type="xsd:string" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="assembly">
+            <xsd:complexType>
+              <xsd:attribute name="alias" type="xsd:string" />
+              <xsd:attribute name="name" type="xsd:string" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="data">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+                <xsd:element name="comment" type="xsd:string" minOccurs="0" msdata:Ordinal="2" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" msdata:Ordinal="1" />
+              <xsd:attribute name="type" type="xsd:string" msdata:Ordinal="3" />
+              <xsd:attribute name="mimetype" type="xsd:string" msdata:Ordinal="4" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="resheader">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" />
+            </xsd:complexType>
+          </xsd:element>
+        </xsd:choice>
+      </xsd:complexType>
+    </xsd:element>
+  </xsd:schema>
+  <resheader name="resmimetype">
+    <value>text/microsoft-resx</value>
+  </resheader>
+  <resheader name="version">
+    <value>2.0</value>
+  </resheader>
+  <resheader name="reader">
+    <value>System.Resources.ResXResourceReader, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+  <resheader name="writer">
+    <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+  <data name="SettingsTitle" xml:space="preserve">
+    <value>CursorLang {0} — Settings</value>
+  </data>
+  <data name="TrayMenuSettings" xml:space="preserve">
+    <value>Settings</value>
+  </data>
+  <data name="TrayMenuExit" xml:space="preserve">
+    <value>Exit</value>
+  </data>
+  <data name="SectionInterface" xml:space="preserve">
+    <value>Interface</value>
+  </data>
+  <data name="LanguageLabel" xml:space="preserve">
+    <value>Interface language</value>
+  </data>
+  <data name="ThemeLabel" xml:space="preserve">
+    <value>Theme</value>
+  </data>
+  <data name="AppTheme_System" xml:space="preserve">
+    <value>Same as Windows</value>
+  </data>
+  <data name="AppTheme_Light" xml:space="preserve">
+    <value>Light</value>
+  </data>
+  <data name="AppTheme_Dark" xml:space="preserve">
+    <value>Dark</value>
+  </data>
+  <data name="SectionPopup" xml:space="preserve">
+    <value>Layout popup</value>
+  </data>
+  <data name="PlacementModeLabel" xml:space="preserve">
+    <value>Mode</value>
+  </data>
+  <data name="PopupPlacementMode_AtCursor" xml:space="preserve">
+    <value>Near the cursor</value>
+  </data>
+  <data name="PopupPlacementMode_AtCaret" xml:space="preserve">
+    <value>Near the text caret</value>
+  </data>
+  <data name="PopupPlacementMode_FixedPoint" xml:space="preserve">
+    <value>Fixed point on screen</value>
+  </data>
+  <data name="CursorCornerLabel" xml:space="preserve">
+    <value>Side</value>
+  </data>
+  <data name="AnchorSide_BottomRight" xml:space="preserve">
+    <value>Bottom right</value>
+  </data>
+  <data name="AnchorSide_BottomLeft" xml:space="preserve">
+    <value>Bottom left</value>
+  </data>
+  <data name="AnchorSide_TopRight" xml:space="preserve">
+    <value>Top right</value>
+  </data>
+  <data name="AnchorSide_TopLeft" xml:space="preserve">
+    <value>Top left</value>
+  </data>
+  <data name="AnchorSide_Left" xml:space="preserve">
+    <value>Left</value>
+  </data>
+  <data name="AnchorSide_Right" xml:space="preserve">
+    <value>Right</value>
+  </data>
+  <data name="CaretSide_Left" xml:space="preserve">
+    <value>Left</value>
+  </data>
+  <data name="CaretSide_Right" xml:space="preserve">
+    <value>Right</value>
+  </data>
+  <data name="CursorOffsetLabel" xml:space="preserve">
+    <value>Offset</value>
+  </data>
+  <data name="ScreenPositionLabel" xml:space="preserve">
+    <value>Position on screen</value>
+  </data>
+  <data name="ScreenPosition_TopLeft" xml:space="preserve">
+    <value>Top left</value>
+  </data>
+  <data name="ScreenPosition_Top" xml:space="preserve">
+    <value>Top center</value>
+  </data>
+  <data name="ScreenPosition_Bottom" xml:space="preserve">
+    <value>Bottom center</value>
+  </data>
+  <data name="ScreenPosition_TopRight" xml:space="preserve">
+    <value>Top right</value>
+  </data>
+  <data name="ScreenPosition_BottomLeft" xml:space="preserve">
+    <value>Bottom left</value>
+  </data>
+  <data name="ScreenPosition_BottomRight" xml:space="preserve">
+    <value>Bottom right</value>
+  </data>
+  <data name="ScreenPosition_Center" xml:space="preserve">
+    <value>Center</value>
+  </data>
+  <data name="ScreenMarginLabel" xml:space="preserve">
+    <value>Margin from screen edge</value>
+  </data>
+  <data name="PopupLookPerModeHint" xml:space="preserve">
+    <value>The look below is remembered for the chosen placement mode alone.</value>
+  </data>
+  <data name="FontSizeLabel" xml:space="preserve">
+    <value>Font size</value>
+  </data>
+  <data name="OpacityLabel" xml:space="preserve">
+    <value>Opacity</value>
+  </data>
+  <data name="BackgroundColorLabel" xml:space="preserve">
+    <value>Background color</value>
+  </data>
+  <data name="TextColorLabel" xml:space="preserve">
+    <value>Text color</value>
+  </data>
+  <data name="SectionBehavior" xml:space="preserve">
+    <value>Behavior</value>
+  </data>
+  <data name="DurationLabel" xml:space="preserve">
+    <value>Display time</value>
+  </data>
+  <data name="PreviewLabel" xml:space="preserve">
+    <value>Preview</value>
+  </data>
+  <data name="MillisecondsSuffix" xml:space="preserve">
+    <value>ms</value>
+  </data>
+  <data name="CapsLockLabel" xml:space="preserve">
+    <value>Caps Lock</value>
+  </data>
+  <data name="CapsLockHotkeyCheck" xml:space="preserve">
+    <value>Switch the layout instead of changing case</value>
+  </data>
+  <data name="CapsLockHoldLabel" xml:space="preserve">
+    <value>Hold threshold</value>
+  </data>
+  <data name="CapsLockHoldHint" xml:space="preserve">
+    <value>Holding the key longer shows the tooltip without switching the layout.</value>
+  </data>
+  <data name="MoreInfoLink" xml:space="preserve">
+    <value>More info</value>
+  </data>
+  <data name="CapsLockElevationHint" xml:space="preserve">
+    <value>The shortcut has no effect while a window running as administrator is in focus — Task Manager, Registry Editor, UAC prompts. Windows does not deliver keystrokes there to ordinary applications. The tooltip itself keeps working everywhere.</value>
+  </data>
+  <data name="StartupLabel" xml:space="preserve">
+    <value>Startup</value>
+  </data>
+  <data name="StartupCheck" xml:space="preserve">
+    <value>Start with Windows</value>
+  </data>
+  <data name="StartupLockedHint" xml:space="preserve">
+    <value>Startup for this app is now controlled by Windows: Settings — Apps — Startup.</value>
+  </data>
+</root>

CursorLang.Core/Resources/Strings.ru.resx

@@ -0,0 +1,214 @@
+<?xml version="1.0" encoding="utf-8"?>
+<root>
+  <xsd:schema id="root" xmlns="" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
+    <xsd:import namespace="http://www.w3.org/XML/1998/namespace" />
+    <xsd:element name="root" msdata:IsDataSet="true">
+      <xsd:complexType>
+        <xsd:choice maxOccurs="unbounded">
+          <xsd:element name="metadata">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" />
+              </xsd:sequence>
+              <xsd:attribute name="name" use="required" type="xsd:string" />
+              <xsd:attribute name="type" type="xsd:string" />
+              <xsd:attribute name="mimetype" type="xsd:string" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="assembly">
+            <xsd:complexType>
+              <xsd:attribute name="alias" type="xsd:string" />
+              <xsd:attribute name="name" type="xsd:string" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="data">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+                <xsd:element name="comment" type="xsd:string" minOccurs="0" msdata:Ordinal="2" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" msdata:Ordinal="1" />
+              <xsd:attribute name="type" type="xsd:string" msdata:Ordinal="3" />
+              <xsd:attribute name="mimetype" type="xsd:string" msdata:Ordinal="4" />
+              <xsd:attribute ref="xml:space" />
+            </xsd:complexType>
+          </xsd:element>
+          <xsd:element name="resheader">
+            <xsd:complexType>
+              <xsd:sequence>
+                <xsd:element name="value" type="xsd:string" minOccurs="0" msdata:Ordinal="1" />
+              </xsd:sequence>
+              <xsd:attribute name="name" type="xsd:string" use="required" />
+            </xsd:complexType>
+          </xsd:element>
+        </xsd:choice>
+      </xsd:complexType>
+    </xsd:element>
+  </xsd:schema>
+  <resheader name="resmimetype">
+    <value>text/microsoft-resx</value>
+  </resheader>
+  <resheader name="version">
+    <value>2.0</value>
+  </resheader>
+  <resheader name="reader">
+    <value>System.Resources.ResXResourceReader, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+  <resheader name="writer">
+    <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
+  </resheader>
+  <data name="SettingsTitle" xml:space="preserve">
+    <value>CursorLang {0} — Настройки</value>
+  </data>
+  <data name="TrayMenuSettings" xml:space="preserve">
+    <value>Настройки</value>
+  </data>
+  <data name="TrayMenuExit" xml:space="preserve">
+    <value>Выход</value>
+  </data>
+  <data name="SectionInterface" xml:space="preserve">
+    <value>Интерфейс</value>
+  </data>
+  <data name="LanguageLabel" xml:space="preserve">
+    <value>Язык интерфейса</value>
+  </data>
+  <data name="ThemeLabel" xml:space="preserve">
+    <value>Тема</value>
+  </data>
+  <data name="AppTheme_System" xml:space="preserve">
+    <value>Как в Windows</value>
+  </data>
+  <data name="AppTheme_Light" xml:space="preserve">
+    <value>Светлая</value>
+  </data>
+  <data name="AppTheme_Dark" xml:space="preserve">
+    <value>Тёмная</value>
+  </data>
+  <data name="SectionPopup" xml:space="preserve">
+    <value>Подсказка о раскладке</value>
+  </data>
+  <data name="PlacementModeLabel" xml:space="preserve">
+    <value>Режим</value>
+  </data>
+  <data name="PopupPlacementMode_AtCursor" xml:space="preserve">
+    <value>Рядом с курсором</value>
+  </data>
+  <data name="PopupPlacementMode_AtCaret" xml:space="preserve">
+    <value>Рядом с кареткой ввода</value>
+  </data>
+  <data name="PopupPlacementMode_FixedPoint" xml:space="preserve">
+    <value>В заданной точке экрана</value>
+  </data>
+  <data name="CursorCornerLabel" xml:space="preserve">
+    <value>Сторона</value>
+  </data>
+  <data name="AnchorSide_BottomRight" xml:space="preserve">
+    <value>Справа снизу</value>
+  </data>
+  <data name="AnchorSide_BottomLeft" xml:space="preserve">
+    <value>Слева снизу</value>
+  </data>
+  <data name="AnchorSide_TopRight" xml:space="preserve">
+    <value>Справа сверху</value>
+  </data>
+  <data name="AnchorSide_TopLeft" xml:space="preserve">
+    <value>Слева сверху</value>
+  </data>
+  <data name="AnchorSide_Left" xml:space="preserve">
+    <value>Слева</value>
+  </data>
+  <data name="AnchorSide_Right" xml:space="preserve">
+    <value>Справа</value>
+  </data>
+  <data name="CaretSide_Left" xml:space="preserve">
+    <value>Слева</value>
+  </data>
+  <data name="CaretSide_Right" xml:space="preserve">
+    <value>Справа</value>
+  </data>
+  <data name="CursorOffsetLabel" xml:space="preserve">
+    <value>Отступ</value>
+  </data>
+  <data name="ScreenPositionLabel" xml:space="preserve">
+    <value>Позиция на экране</value>
+  </data>
+  <data name="ScreenPosition_TopLeft" xml:space="preserve">
+    <value>Слева сверху</value>
+  </data>
+  <data name="ScreenPosition_Top" xml:space="preserve">
+    <value>Сверху по центру</value>
+  </data>
+  <data name="ScreenPosition_Bottom" xml:space="preserve">
+    <value>Снизу по центру</value>
+  </data>
+  <data name="ScreenPosition_TopRight" xml:space="preserve">
+    <value>Справа сверху</value>
+  </data>
+  <data name="ScreenPosition_BottomLeft" xml:space="preserve">
+    <value>Слева снизу</value>
+  </data>
+  <data name="ScreenPosition_BottomRight" xml:space="preserve">
+    <value>Справа снизу</value>
+  </data>
+  <data name="ScreenPosition_Center" xml:space="preserve">
+    <value>По центру</value>
+  </data>
+  <data name="ScreenMarginLabel" xml:space="preserve">
+    <value>Отступ от края экрана</value>
+  </data>
+  <data name="PopupLookPerModeHint" xml:space="preserve">
+    <value>Оформление ниже запоминается только для выбранного режима.</value>
+  </data>
+  <data name="FontSizeLabel" xml:space="preserve">
+    <value>Размер шрифта</value>
+  </data>
+  <data name="OpacityLabel" xml:space="preserve">
+    <value>Прозрачность</value>
+  </data>
+  <data name="BackgroundColorLabel" xml:space="preserve">
+    <value>Цвет фона</value>
+  </data>
+  <data name="TextColorLabel" xml:space="preserve">
+    <value>Цвет текста</value>
+  </data>
+  <data name="SectionBehavior" xml:space="preserve">
+    <value>Поведение</value>
+  </data>
+  <data name="DurationLabel" xml:space="preserve">
+    <value>Время отображения</value>
+  </data>
+  <data name="PreviewLabel" xml:space="preserve">
+    <value>Предпросмотр</value>
+  </data>
+  <data name="MillisecondsSuffix" xml:space="preserve">
+    <value>мс</value>
+  </data>
+  <data name="CapsLockLabel" xml:space="preserve">
+    <value>Caps Lock</value>
+  </data>
+  <data name="CapsLockHotkeyCheck" xml:space="preserve">
+    <value>Переключать раскладку вместо смены регистра</value>
+  </data>
+  <data name="CapsLockHoldLabel" xml:space="preserve">
+    <value>Порог удержания</value>
+  </data>
+  <data name="CapsLockHoldHint" xml:space="preserve">
+    <value>Более долгое нажатие показывает подсказку и не переключает раскладку.</value>
+  </data>
+  <data name="MoreInfoLink" xml:space="preserve">
+    <value>Подробнее</value>
+  </data>
+  <data name="CapsLockElevationHint" xml:space="preserve">
+    <value>Пока в фокусе окно, запущенное от имени администратора, — диспетчер задач, редактор реестра, запрос UAC — сочетание не сработает: там Windows не отдаёт нажатия обычным приложениям. Сама подсказка показывается везде.</value>
+  </data>
+  <data name="StartupLabel" xml:space="preserve">
+    <value>Автозапуск</value>
+  </data>
+  <data name="StartupCheck" xml:space="preserve">
+    <value>Запускать вместе с Windows</value>
+  </data>
+  <data name="StartupLockedHint" xml:space="preserve">
+    <value>Автозапуском этого приложения теперь распоряжается Windows: «Параметры» — «Приложения» — «Автозагрузка».</value>
+  </data>
+</root>

CursorLang.Core/Services/AgentExecutable.cs

@@ -0,0 +1,35 @@
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Where the background half of the application lives on disk.
+/// </summary>
+/// <remarks>
+/// Two processes now share one folder, and each of them at some point needs the path
+/// of the other: the settings window registers the agent for startup and must not
+/// register itself, and the agent starts the settings window from the tray menu.
+/// <c>Environment.ProcessPath</c> answers the wrong question for both, so the paths
+/// are worked out from the folder the assemblies were loaded from.
+/// </remarks>
+internal static class AgentExecutable
+{
+    /// <summary>The background process — the one Windows starts at sign-in.</summary>
+    internal const string AgentFileName = "CursorLang.exe";
+
+    /// <summary>The settings window, started on demand and gone when closed.</summary>
+    internal const string SettingsFileName = "CursorLang.Settings.exe";
+
+    /// <summary>
+    /// The full path of the agent, or <c>null</c> when it is not next to us — which
+    /// happens in the tests and would happen to a half-copied installation.
+    /// </summary>
+    internal static string? AgentPath => Beside(AgentFileName);
+
+    /// <summary>The full path of the settings window, on the same terms.</summary>
+    internal static string? SettingsPath => Beside(SettingsFileName);
+
+    private static string? Beside(string fileName)
+    {
+        string path = Path.Combine(AppContext.BaseDirectory, fileName);
+        return File.Exists(path) ? path : null;
+    }
+}

CursorLang.Core/Services/AppVersion.cs

@@ -0,0 +1,37 @@
+using System.Reflection;
+using System.Runtime.InteropServices;
+using CursorLang.Core.Interop;
+using Windows.ApplicationModel;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// The version of the running application.
+/// </summary>
+/// <remarks>
+/// The same application runs from a package and from a folder, and the two keep
+/// their version in different places. The answer is computed once: it cannot
+/// change while the process lives.
+/// </remarks>
+public static class AppVersion
+{
+    public static Version Current { get; } = Detect();
+
+    private static Version Detect()
+    {
+        if (PackageIdentityNative.IsPackaged)
+        {
+            try
+            {
+                PackageVersion version = Package.Current.Id.Version;
+                return new Version(version.Major, version.Minor, version.Build, version.Revision);
+            }
+            catch (Exception e) when (e is COMException or InvalidOperationException)
+            {
+                // The package was built without a version in the manifest — the assembly version is left
+            }
+        }
+
+        return Assembly.GetEntryAssembly()?.GetName().Version ?? new Version(0, 0, 0, 0);
+    }
+}

CursorLang.Core/Services/CapsLockSwitchCoordinator.cs

@@ -0,0 +1,77 @@
+using System.ComponentModel;
+using CursorLang.Core.Models;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Turns Caps Lock presses into actions: a short one switches the layout, a long one
+/// only shows the popup. It also turns the hook on and off following the checkbox in
+/// the settings.
+/// </summary>
+public sealed class CapsLockSwitchCoordinator : IDisposable
+{
+    private readonly ICapsLockHotkeyService _hotkeyService;
+    private readonly IKeyboardLayoutService _layoutService;
+    private readonly ILayoutPopupService _popupService;
+    private readonly AppSettings _settings;
+
+    public CapsLockSwitchCoordinator(
+        ICapsLockHotkeyService hotkeyService,
+        IKeyboardLayoutService layoutService,
+        ILayoutPopupService popupService,
+        AppSettings settings)
+    {
+        _hotkeyService = hotkeyService;
+        _layoutService = layoutService;
+        _popupService = popupService;
+        _settings = settings;
+    }
+
+    public void Start()
+    {
+        _hotkeyService.Tapped += OnTapped;
+        _hotkeyService.HoldStarted += OnHoldStarted;
+        _hotkeyService.HoldEnded += OnHoldEnded;
+        _settings.PropertyChanged += OnSettingsChanged;
+
+        ApplySetting();
+    }
+
+    public void Dispose()
+    {
+        _settings.PropertyChanged -= OnSettingsChanged;
+        _hotkeyService.Tapped -= OnTapped;
+        _hotkeyService.HoldStarted -= OnHoldStarted;
+        _hotkeyService.HoldEnded -= OnHoldEnded;
+        _hotkeyService.Stop();
+    }
+
+    private void OnSettingsChanged(object? sender, PropertyChangedEventArgs e)
+    {
+        if (e.PropertyName == nameof(AppSettings.UseCapsLockHotkey))
+        {
+            ApplySetting();
+        }
+    }
+
+    private void ApplySetting()
+    {
+        if (_settings.UseCapsLockHotkey)
+        {
+            _hotkeyService.Start();
+        }
+        else
+        {
+            _hotkeyService.Stop();
+        }
+    }
+
+    private void OnTapped(object? sender, EventArgs e) => _layoutService.SwitchToNext();
+
+    // We do not change the layout, but staying silent will not do either: without the
+    // popup a long press looks as if the key simply did not work
+    private void OnHoldStarted(object? sender, EventArgs e) =>
+        _popupService.ShowUntilHidden(_layoutService.Current);
+
+    private void OnHoldEnded(object? sender, EventArgs e) => _popupService.Hide();
+}

CursorLang.Core/Services/ColorJsonConverter.cs

@@ -0,0 +1,56 @@
+using System.Drawing;
+using System.Globalization;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Reads and writes a colour as "#AARRGGBB".
+/// </summary>
+/// <remarks>
+/// That is the form earlier versions wrote, when the colours were WPF ones and
+/// <c>Color.ToString()</c> produced it, so files already on disk keep working. The
+/// parsing is done here rather than by <c>ColorConverter</c> because that one lives in
+/// PresentationCore, and Core is read by the agent. Named colours are accepted too:
+/// nothing writes them, but the file is plain text and people edit it by hand.
+/// </remarks>
+internal sealed class ColorJsonConverter : JsonConverter<Color>
+{
+    public override Color Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        string? value = reader.GetString();
+        if (string.IsNullOrWhiteSpace(value))
+        {
+            return Color.Black;
+        }
+
+        value = value.Trim();
+
+        if (!value.StartsWith('#'))
+        {
+            Color named = Color.FromName(value);
+
+            // Unpacked back into a plain colour on purpose: a known colour carries its
+            // name with it and does not compare equal to the same bytes written in hex,
+            // which would make "Red" and "#FFFF0000" two different settings
+            return named.IsKnownColor ? Color.FromArgb(named.ToArgb()) : Color.Black;
+        }
+
+        ReadOnlySpan<char> digits = value.AsSpan(1);
+        if (!uint.TryParse(digits, NumberStyles.HexNumber, CultureInfo.InvariantCulture, out uint packed))
+        {
+            return Color.Black;
+        }
+
+        return digits.Length switch
+        {
+            6 => Color.FromArgb((int)(packed | 0xFF000000)),
+            8 => Color.FromArgb((int)packed),
+            _ => Color.Black,
+        };
+    }
+
+    public override void Write(Utf8JsonWriter writer, Color value, JsonSerializerOptions options) =>
+        writer.WriteStringValue($"#{value.A:X2}{value.R:X2}{value.G:X2}{value.B:X2}");
+}

CursorLang.Core/Services/ICapsLockHotkeyService.cs

@@ -0,0 +1,29 @@
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Intercepts Caps Lock at the system level and splits the presses into short and
+/// long ones. What to do with them is up to the subscribers.
+/// </summary>
+public interface ICapsLockHotkeyService
+{
+    /// <summary>A short press: the key was released before the hold threshold.</summary>
+    event EventHandler? Tapped;
+
+    /// <summary>The hold threshold has passed, the key is still held.</summary>
+    event EventHandler? HoldStarted;
+
+    /// <summary>The hold is over: the key was released.</summary>
+    event EventHandler? HoldEnded;
+
+    /// <summary>Whether the hook is installed right now.</summary>
+    bool IsRunning { get; }
+
+    /// <summary>
+    /// Starts intercepting. Must be called from the user interface thread:
+    /// a system keyboard hook works only on a thread with a message loop.
+    /// </summary>
+    void Start();
+
+    /// <summary>Removes the hook, giving the key its usual behaviour back.</summary>
+    void Stop();
+}

CursorLang.Core/Services/IKeyboardLayoutService.cs

@@ -0,0 +1,21 @@
+using CursorLang.Core.Models;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Tracks the layout of the active window — including in other applications —
+/// and can switch it.
+/// </summary>
+public interface IKeyboardLayoutService
+{
+    /// <summary>The layout of the active window at the moment.</summary>
+    KeyboardLayout Current { get; }
+
+    event EventHandler<LayoutChangedEventArgs>? LayoutChanged;
+
+    void Start();
+
+    void Stop();
+
+    void SwitchToNext();
+}

CursorLang.Core/Services/ILayoutPopupService.cs

@@ -0,0 +1,20 @@
+using CursorLang.Core.Models;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Shows the layout popup at the cursor.
+/// </summary>
+public interface ILayoutPopupService
+{
+    /// <summary>Shows the popup and takes it down after the time set in the settings.</summary>
+    void Show(KeyboardLayout layout);
+
+    /// <summary>
+    /// Shows the popup until <see cref="Hide"/> is called explicitly: needed where the
+    /// show time is set by a user action rather than by a timer.
+    /// </summary>
+    void ShowUntilHidden(KeyboardLayout layout);
+
+    void Hide();
+}

CursorLang.Core/Services/ILayoutPopupWindow.cs

@@ -0,0 +1,28 @@
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// The popup window as seen by whoever decides when it is shown.
+/// </summary>
+/// <remarks>
+/// The window picks its place and size itself, and only three actions are needed from
+/// it on the outside. The tests check the work of the popup service through the same
+/// interface: there is no point bringing up a real window to check a timer.
+/// </remarks>
+public interface ILayoutPopupWindow
+{
+    /// <summary>
+    /// Shows the window with the given text at the place set by the settings.
+    /// </summary>
+    /// <remarks>
+    /// The text is passed in rather than bound: there is no view model behind the
+    /// window any more, and no data binding either — it is a Win32 window that paints
+    /// one line of text itself.
+    /// </remarks>
+    void ShowPopup(string shortName);
+
+    /// <summary>Takes the window off the screen without destroying it.</summary>
+    void Hide();
+
+    /// <summary>Closes the window for good.</summary>
+    void Close();
+}

CursorLang.Core/Services/ILocalizationService.cs

@@ -0,0 +1,25 @@
+using System.ComponentModel;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>An interface language to choose from in the settings.</summary>
+/// <param name="Code">The culture code: "ru", "en".</param>
+/// <param name="DisplayName">The name in that very language.</param>
+public sealed record LanguageOption(string Code, string DisplayName)
+{
+    // Accessibility tools take the name of the list item from here
+    public override string ToString() => DisplayName;
+}
+
+/// <summary>
+/// Provides the interface strings and can change the language without a restart.
+/// </summary>
+public interface ILocalizationService : INotifyPropertyChanged
+{
+    /// <summary>The string for a resource key. Bindings update when the language changes.</summary>
+    string this[string key] { get; }
+
+    IReadOnlyList<LanguageOption> AvailableLanguages { get; }
+
+    string CurrentLanguage { get; set; }
+}

CursorLang.Core/Services/IStartupService.cs

@@ -0,0 +1,18 @@
+using CursorLang.Core.Models;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Starting the app together with Windows.
+/// </summary>
+public interface IStartupService
+{
+    /// <summary>Finds out the current state of startup.</summary>
+    Task<StartupState> GetStateAsync();
+
+    /// <summary>
+    /// Asks for startup to be switched on or off and answers with the state that
+    /// came of it: the request to switch it on may well be turned down.
+    /// </summary>
+    Task<StartupState> SetEnabledAsync(bool enabled);
+}

CursorLang.Core/Services/KeyboardLayoutService.cs

@@ -0,0 +1,121 @@
+using CursorLang.Core.Interop;
+using CursorLang.Core.Models;
+using CursorLang.Core.Threading;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// The settings of layout tracking.
+/// </summary>
+public sealed class KeyboardLayoutOptions
+{
+    /// <summary>How often to check the layout of the active window.</summary>
+    public TimeSpan PollInterval { get; init; } = TimeSpan.FromMilliseconds(150);
+}
+
+/// <summary>
+/// Polls the active window on a timer.
+/// </summary>
+/// <remarks>
+/// Polling was chosen not for simplicity: there is no event-based way to learn about
+/// a layout change in another process from managed code. HSHELL_LANGUAGE from
+/// RegisterShellHookWindow does not arrive on Windows 10/11, and TSF notifications
+/// (ITfLanguageProfileNotifySink) report only language changes inside our own process
+/// — both options were tried and did not work. One tick is three Win32 calls reading
+/// data from kernel memory.
+///
+/// The timer ticks on the message loop of whatever thread starts it, the same as a
+/// dispatcher timer did: the agent has a plain Win32 loop and no dispatcher to offer.
+/// </remarks>
+public sealed class KeyboardLayoutService : IKeyboardLayoutService, IDisposable
+{
+    private readonly MessageTimer _pollTimer;
+    private readonly Func<IntPtr> _getForegroundWindow;
+    private readonly Func<int> _getActiveLocaleId;
+    private readonly Action _requestNextLayout;
+
+    private int _lastLocaleId = -1;
+    private IntPtr _lastForegroundWindow;
+
+    public KeyboardLayoutService(KeyboardLayoutOptions options)
+        : this(
+            options,
+            KeyboardLayoutNative.GetForegroundWindow,
+            KeyboardLayoutNative.GetActiveLocaleId,
+            KeyboardLayoutNative.RequestNextLayout)
+    {
+    }
+
+    /// <summary>
+    /// Takes the sources of system information explicitly: in tests the layout and the
+    /// active window are not provided by Windows.
+    /// </summary>
+    internal KeyboardLayoutService(
+        KeyboardLayoutOptions options,
+        Func<IntPtr> getForegroundWindow,
+        Func<int> getActiveLocaleId,
+        Action requestNextLayout)
+    {
+        _getForegroundWindow = getForegroundWindow;
+        _getActiveLocaleId = getActiveLocaleId;
+        _requestNextLayout = requestNextLayout;
+
+        _pollTimer = new MessageTimer { Interval = options.PollInterval };
+        _pollTimer.Tick += OnTick;
+    }
+
+    public event EventHandler<LayoutChangedEventArgs>? LayoutChanged;
+
+    public KeyboardLayout Current => KeyboardLayout.FromLocaleId(_getActiveLocaleId());
+
+    public void Start()
+    {
+        _lastForegroundWindow = _getForegroundWindow();
+        _lastLocaleId = _getActiveLocaleId();
+        _pollTimer.Start();
+    }
+
+    public void Stop() => _pollTimer.Stop();
+
+    public void SwitchToNext() => _requestNextLayout();
+
+    public void Dispose()
+    {
+        _pollTimer.Tick -= OnTick;
+        _pollTimer.Dispose();
+    }
+
+    private void OnTick(object? sender, EventArgs e) => Poll();
+
+    // A single poll step. Called by the timer, and in tests — directly:
+    // there is no point waiting for a tick to check how the reason for a layout
+    // change is decided
+    internal void Poll()
+    {
+        IntPtr foreground = _getForegroundWindow();
+        if (foreground == IntPtr.Zero)
+        {
+            return;
+        }
+
+        bool appSwitched = foreground != _lastForegroundWindow;
+        _lastForegroundWindow = foreground;
+
+        int localeId = _getActiveLocaleId();
+        if (localeId == _lastLocaleId)
+        {
+            return;
+        }
+
+        _lastLocaleId = localeId;
+
+        // Moving to another application with a layout of its own is not the same as
+        // the user switching the layout, and the subscribers are free to react to
+        // these cases differently
+        LayoutChangeReason reason = appSwitched
+            ? LayoutChangeReason.ApplicationSwitched
+            : LayoutChangeReason.UserSwitched;
+
+        LayoutChanged?.Invoke(this, new LayoutChangedEventArgs(KeyboardLayout.FromLocaleId(localeId), reason));
+    }
+}

CursorLang.Core/Services/LayoutNotificationCoordinator.cs

@@ -0,0 +1,41 @@
+using CursorLang.Core.Models;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Ties layout tracking to showing the popup.
+/// Lives for as long as the application runs, regardless of the open windows.
+/// </summary>
+public sealed class LayoutNotificationCoordinator : IDisposable
+{
+    private readonly IKeyboardLayoutService _layoutService;
+    private readonly ILayoutPopupService _popupService;
+
+    public LayoutNotificationCoordinator(IKeyboardLayoutService layoutService, ILayoutPopupService popupService)
+    {
+        _layoutService = layoutService;
+        _popupService = popupService;
+    }
+
+    public void Start()
+    {
+        _layoutService.LayoutChanged += OnLayoutChanged;
+        _layoutService.Start();
+    }
+
+    public void Dispose()
+    {
+        _layoutService.LayoutChanged -= OnLayoutChanged;
+        _layoutService.Stop();
+    }
+
+    private void OnLayoutChanged(object? sender, LayoutChangedEventArgs e)
+    {
+        // When moving to another application the layout changes without the user
+        // taking part, and a popup would be intrusive
+        if (e.Reason == LayoutChangeReason.UserSwitched)
+        {
+            _popupService.Show(e.Layout);
+        }
+    }
+}

CursorLang.Core/Services/LocalizationService.cs

@@ -0,0 +1,58 @@
+using System.Globalization;
+using System.Resources;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Takes the strings from the resources and, when the language changes, asks WPF to
+/// re-read every binding.
+/// </summary>
+/// <remarks>
+/// Both processes use it: the settings window for its whole interface, the agent for
+/// the three captions of the tray menu. The theme no longer reaches that menu — Windows
+/// draws it — but the language still does.
+/// </remarks>
+public sealed class LocalizationService : ObservableObject, ILocalizationService
+{
+    /// <summary>
+    /// The name WPF reports when an indexer changes. Spelt out rather than taken from
+    /// <c>Binding.IndexerName</c>: that constant lives in PresentationFramework, and
+    /// Core is read by the agent, which does not load WPF.
+    /// </summary>
+    public const string IndexerName = "Item[]";
+
+    private static readonly ResourceManager Resources =
+        new("CursorLang.Core.Resources.Strings", typeof(LocalizationService).Assembly);
+
+    private CultureInfo _culture = CultureInfo.GetCultureInfo("en");
+
+    public string this[string key] => Resources.GetString(key, _culture) ?? key;
+
+    public IReadOnlyList<LanguageOption> AvailableLanguages { get; } =
+    [
+        new LanguageOption("en", "English"),
+        new LanguageOption("ru", "Русский"),
+    ];
+
+    public string CurrentLanguage
+    {
+        get => _culture.TwoLetterISOLanguageName;
+        set
+        {
+            if (string.IsNullOrWhiteSpace(value) || value == CurrentLanguage)
+            {
+                return;
+            }
+
+            _culture = CultureInfo.GetCultureInfo(value);
+            CultureInfo.CurrentUICulture = _culture;
+
+            OnPropertyChanged(nameof(CurrentLanguage));
+
+            // We report a change of the indexer: that is how every binding of the
+            // {Binding Localization[Key]} kind updates, that is, all the interface text
+            OnPropertyChanged(IndexerName);
+        }
+    }
+}

CursorLang.Core/Services/PopupLayout.cs

@@ -0,0 +1,95 @@
+using CursorLang.Core.Interop;
+using CursorLang.Core.Models;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Computes the screen point to show the popup at.
+/// </summary>
+/// <remarks>
+/// There is nothing but arithmetic here: where the cursor is, where the caret is and
+/// what the monitor bounds are is figured out by the window itself — it has a handle
+/// of its own for that. The computation is kept apart because it is exactly the place
+/// where a sign or half a size is easy to get wrong, and this way it can be checked
+/// without a single window on screen.
+///
+/// All the values are in physical pixels: monitors have different scaling, and
+/// converting to WPF units halfway would mean rounding twice.
+/// </remarks>
+internal static class PopupLayout
+{
+    /// <summary>
+    /// The popup position next to the anchor point — the cursor or the caret.
+    /// The cursor arrives here as a rectangle of zero size.
+    /// </summary>
+    internal static PopupWindowNative.Point NearAnchor(
+        PopupWindowNative.Rect anchor,
+        AnchorSide side,
+        int offset,
+        int width,
+        int height)
+    {
+        int toLeftOf = anchor.Left - offset - width;
+        int toRightOf = anchor.Right + offset;
+        int above = anchor.Top - offset - height;
+        int below = anchor.Bottom + offset;
+
+        // For the "left" and "right" sides the popup lines up with the anchor point
+        int middle = anchor.Top + (((anchor.Bottom - anchor.Top) - height) / 2);
+
+        (int x, int y) = side switch
+        {
+            AnchorSide.TopLeft => (toLeftOf, above),
+            AnchorSide.TopRight => (toRightOf, above),
+            AnchorSide.Left => (toLeftOf, middle),
+            AnchorSide.Right => (toRightOf, middle),
+            AnchorSide.BottomLeft => (toLeftOf, below),
+            _ => (toRightOf, below),
+        };
+
+        return new PopupWindowNative.Point { X = x, Y = y };
+    }
+
+    /// <summary>
+    /// The popup position in the given corner of the monitor work area.
+    /// </summary>
+    internal static PopupWindowNative.Point OnScreen(
+        PopupWindowNative.Rect work,
+        ScreenPosition position,
+        int margin,
+        int width,
+        int height)
+    {
+        int left = work.Left + margin;
+        int right = work.Right - margin - width;
+        int top = work.Top + margin;
+        int bottom = work.Bottom - margin - height;
+        int centerX = work.Left + ((work.Right - work.Left - width) / 2);
+        int centerY = work.Top + ((work.Bottom - work.Top - height) / 2);
+
+        (int x, int y) = position switch
+        {
+            ScreenPosition.TopLeft => (left, top),
+            ScreenPosition.Top => (centerX, top),
+            ScreenPosition.TopRight => (right, top),
+            ScreenPosition.BottomLeft => (left, bottom),
+            ScreenPosition.Bottom => (centerX, bottom),
+            ScreenPosition.BottomRight => (right, bottom),
+            _ => (centerX, centerY),
+        };
+
+        return new PopupWindowNative.Point { X = x, Y = y };
+    }
+
+    /// <summary>A rectangle of zero size at a point — the mouse cursor as an anchor.</summary>
+    internal static PopupWindowNative.Rect AsAnchor(PopupWindowNative.Point point) => new()
+    {
+        Left = point.X,
+        Top = point.Y,
+        Right = point.X,
+        Bottom = point.Y,
+    };
+
+    /// <summary>WPF units into physical pixels of a monitor with the given scale.</summary>
+    internal static int ToPixels(double wpfUnits, double scale) => (int)Math.Round(wpfUnits * scale);
+}

CursorLang.Core/Services/RegistryStartup.cs

@@ -0,0 +1,130 @@
+using System.Security;
+using CursorLang.Core.Models;
+using Microsoft.Win32;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Startup for a build that is not a package: a value under the Run key.
+/// </summary>
+/// <remarks>
+/// A package declares its startup task in the manifest and asks Windows to switch
+/// it on. A build unpacked into a folder has no manifest, so it registers itself
+/// the way desktop programs always have — under the Run key of the current user.
+/// Administrator rights are not needed for that: the key belongs to the user.
+///
+/// Windows keeps the user's own verdict apart from the entry itself. Turning the
+/// app off in Settings — Apps — Startup leaves the Run value where it is and marks
+/// it disabled under StartupApproved. The mark is obeyed here the same way a
+/// package obeys DisabledByUser: the app does not argue with the user.
+/// </remarks>
+internal sealed class RegistryStartup
+{
+    private const string RunPath = @"Software\Microsoft\Windows\CurrentVersion\Run";
+
+    private const string ApprovedPath =
+        @"Software\Microsoft\Windows\CurrentVersion\Explorer\StartupApproved\Run";
+
+    /// <summary>The name of the value — Windows shows it in the startup list.</summary>
+    private const string ValueName = "CursorLang";
+
+    private readonly RegistryKey _root;
+    private readonly string? _command;
+
+    internal RegistryStartup()
+        : this(Registry.CurrentUser, GetCommand())
+    {
+    }
+
+    /// <summary>A root of the test's own, so that the real startup list is left alone.</summary>
+    internal RegistryStartup(RegistryKey root, string? command)
+    {
+        _root = root;
+        _command = command;
+    }
+
+    internal StartupState GetState()
+    {
+        if (_command is null)
+        {
+            return StartupState.Unavailable;
+        }
+
+        try
+        {
+            using RegistryKey? run = _root.OpenSubKey(RunPath);
+
+            if (run?.GetValue(ValueName) is null)
+            {
+                return StartupState.Disabled;
+            }
+
+            return IsApprovedByUser() ? StartupState.Enabled : StartupState.DisabledByUser;
+        }
+        catch (Exception e) when (e is SecurityException or UnauthorizedAccessException or IOException)
+        {
+            return StartupState.Unavailable;
+        }
+    }
+
+    internal StartupState SetEnabled(bool enabled)
+    {
+        if (_command is null)
+        {
+            return StartupState.Unavailable;
+        }
+
+        try
+        {
+            using RegistryKey run = _root.CreateSubKey(RunPath);
+
+            if (enabled)
+            {
+                // The path is written afresh every time: the app may have been moved
+                run.SetValue(ValueName, _command, RegistryValueKind.String);
+            }
+            else
+            {
+                run.DeleteValue(ValueName, throwOnMissingValue: false);
+            }
+        }
+        catch (Exception e) when (e is SecurityException or UnauthorizedAccessException or IOException)
+        {
+            return StartupState.Unavailable;
+        }
+
+        // The answer is read back rather than assumed: an entry the user has
+        // banned stays banned no matter what was just written next to it
+        return GetState();
+    }
+
+    /// <summary>
+    /// Whether the user has left the entry alone. The verdict is a blob whose
+    /// lowest bit of the first byte stands for the ban; no value means untouched.
+    /// </summary>
+    private bool IsApprovedByUser()
+    {
+        using RegistryKey? approved = _root.OpenSubKey(ApprovedPath);
+
+        return approved?.GetValue(ValueName) is not byte[] { Length: > 0 } verdict
+            || (verdict[0] & 1) == 0;
+    }
+
+    /// <summary>
+    /// What Windows is to run. <c>null</c> — the agent is not where it should be, and
+    /// there is nothing to write down.
+    /// </summary>
+    /// <remarks>
+    /// The agent by name rather than <c>Environment.ProcessPath</c>: this setting is
+    /// switched from the settings window, and its own path would put the wrong process
+    /// into the startup list — one that shows a window and exits.
+    ///
+    /// The argument is how the agent recognises a launch of this kind and goes straight
+    /// to the tray without the settings window: see <see cref="StartupLaunch"/>. The
+    /// user starting the application themselves passes no such thing and gets the window.
+    /// </remarks>
+    internal static string? GetCommand() =>
+        AgentExecutable.AgentPath is { Length: > 0 } path
+            ? $"\"{path}\" {StartupLaunch.Argument}"
+            : null;
+}

CursorLang.Core/Services/SettingsCloseSignal.cs

@@ -0,0 +1,98 @@
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Tells the settings window that the agent is quitting and it is to close with it.
+/// </summary>
+/// <remarks>
+/// The one thing the agent says to the settings window, and the mirror image of
+/// <see cref="SettingsSignal"/>. "Exit" in the tray menu means the application is done
+/// with, and a settings window left alone on the screen after it is a window belonging
+/// to nothing: the tray icon it was opened from is gone, and closing it would be the
+/// user's only remaining move.
+///
+/// A named event rather than a window message, because the agent has no handle to send
+/// one to: the settings window lives in a process the agent starts and deliberately
+/// does not keep hold of. The name has no <c>Global</c> prefix, so it lives in the
+/// session namespace — same reasoning as <see cref="SingleInstanceGate"/>, and the same
+/// consequence: with fast user switching each user's halves talk to their own.
+///
+/// Only the settings window creates the object; the agent opens what is already there
+/// and stays silent when there is nothing. Were it the other way round, the request
+/// would sit in an auto-reset event waiting for the next settings window to open and
+/// close it the moment it did.
+/// </remarks>
+internal sealed class SettingsCloseSignal : IDisposable
+{
+    private const string EventName = "CursorLang.CloseSettings";
+
+    private readonly string _eventName;
+
+    private EventWaitHandle? _request;
+    private RegisteredWaitHandle? _wait;
+
+    /// <summary>
+    /// Listens on the name the two halves agree on.
+    /// </summary>
+    /// <param name="nameSuffix">
+    /// A namespace of its own. Empty for the application; the tests pass one so that
+    /// they do not answer for — or worse, close — a settings window someone is using.
+    /// </param>
+    internal SettingsCloseSignal(string nameSuffix = "") => _eventName = EventName + nameSuffix;
+
+    /// <summary>The agent asks for the window to be closed.</summary>
+    /// <remarks>
+    /// Raised on a thread pool thread, wherever the wait happened to be answered — a
+    /// window obeys only its own, so the handler has to get back to it.
+    /// </remarks>
+    internal event EventHandler? CloseRequested;
+
+    /// <summary>
+    /// Asks the settings window of this session, if one is open, to close. Silence is
+    /// a normal answer: most of the time the user quits with no window on the screen.
+    /// </summary>
+    /// <returns>Whether there was anybody to hear it.</returns>
+    internal static bool RequestClose(string nameSuffix = "")
+    {
+        if (!EventWaitHandle.TryOpenExisting(EventName + nameSuffix, out EventWaitHandle? request))
+        {
+            return false;
+        }
+
+        using (request)
+        {
+            return request.Set();
+        }
+    }
+
+    /// <summary>Starts waiting for the request. Called once, by the settings window.</summary>
+    internal void Listen()
+    {
+        if (_request is not null)
+        {
+            return;
+        }
+
+        _request = new EventWaitHandle(false, EventResetMode.AutoReset, _eventName);
+
+        // As in the gate: the thread pool holds the wait, there is no reason to keep a
+        // thread of our own for a request that may never come
+        _wait = ThreadPool.RegisterWaitForSingleObject(
+            _request,
+            OnCloseSignalled,
+            state: null,
+            Timeout.Infinite,
+            executeOnlyOnce: false);
+    }
+
+    public void Dispose()
+    {
+        _wait?.Unregister(null);
+        _wait = null;
+
+        _request?.Dispose();
+        _request = null;
+    }
+
+    private void OnCloseSignalled(object? state, bool timedOut) =>
+        CloseRequested?.Invoke(this, EventArgs.Empty);
+}

CursorLang.Core/Services/SettingsService.cs

@@ -0,0 +1,249 @@
+using System.ComponentModel;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using CursorLang.Core.Interop;
+using CursorLang.Core.Models;
+using CursorLang.Core.Threading;
+using Windows.Storage;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Keeps the settings in the settings.json file.
+/// </summary>
+/// <remarks>
+/// The file is the whole of the connection between the two processes, and they use it
+/// from opposite ends. The settings window calls <see cref="TrackChanges"/> and is the
+/// only writer; the agent only ever reads, and re-reads when the window tells it to.
+/// A second writer would mean two processes racing for one file and an edit going missing.
+///
+/// The location depends on how the application is installed. A package from the Store
+/// keeps its settings in a folder of its own: Windows removes it together with the
+/// application, and after the removal nothing superfluous is left in the system — that
+/// is what Store applications are expected to do. A separately installed application
+/// keeps its settings in %APPDATA%, as before.
+///
+/// Settings left over from a separately installed application are picked up by the
+/// package on the first launch and moved over. The original file stays where it is:
+/// both versions can be installed side by side, and the application has no right to
+/// delete settings that are not its own.
+/// </remarks>
+public sealed class SettingsService : IDisposable
+{
+    private static readonly JsonSerializerOptions SerializerOptions = new()
+    {
+        WriteIndented = true,
+        Converters = { new ColorJsonConverter(), new JsonStringEnumConverter() },
+    };
+
+    private const string FileName = "settings.json";
+
+    // Sliders change their values continuously, so writing to disk
+    // is postponed until there is a pause in the changes
+    private static readonly TimeSpan SaveDelay = TimeSpan.FromMilliseconds(500);
+
+    private readonly string _filePath;
+    private readonly string _inheritedFilePath;
+    private readonly MessageTimer _saveTimer;
+
+    private AppSettings? _settings;
+    private bool _isTrackingChanges;
+
+    public SettingsService()
+        : this(
+            Path.Combine(GetSettingsFolder(), FileName),
+            Path.Combine(GetSeparateInstallFolder(), FileName),
+            SaveDelay)
+    {
+    }
+
+    /// <summary>
+    /// Sets the storage locations and the save delay explicitly — thereby making it
+    /// possible to check the work with the file without touching the settings of the
+    /// user themselves.
+    /// </summary>
+    internal SettingsService(string filePath, string inheritedFilePath, TimeSpan saveDelay)
+    {
+        _filePath = filePath;
+        _inheritedFilePath = inheritedFilePath;
+
+        _saveTimer = new MessageTimer { Interval = saveDelay };
+        _saveTimer.Tick += OnSaveTimerTick;
+    }
+
+    /// <summary>
+    /// Reads the settings from disk or returns the default values.
+    /// </summary>
+    /// <remarks>
+    /// Asking twice hands out the same instance rather than reading again. Everything
+    /// binds to what this returns — the window, the popup, the hook — and a second
+    /// instance would mean one of them editing settings nobody else can see.
+    /// </remarks>
+    public AppSettings Load() => _settings ??= ReadOrInherit();
+
+    private AppSettings ReadOrInherit()
+    {
+        AppSettings? stored = ReadFile(_filePath);
+
+        // There is no file of our own — the application may well have been configured
+        // before the move to a package. Taking the settings from there beats starting
+        // from a blank slate
+        bool inherited = stored is null && _filePath != _inheritedFilePath;
+        if (inherited)
+        {
+            stored = ReadFile(_inheritedFilePath);
+            inherited = stored is not null;
+        }
+
+        _settings = stored ?? CreateDefault();
+
+        // Moved settings are fixed in the new place right away rather than on the
+        // first edit: otherwise the application would read someone else's file every
+        // time until then
+        if (inherited)
+        {
+            Save();
+        }
+
+        return _settings;
+    }
+
+    /// <summary>
+    /// Starts saving every change, after a pause. For the settings window: it is the
+    /// only process allowed to write.
+    /// </summary>
+    /// <remarks>
+    /// Reads the file if that has not happened yet. The settings window asks in exactly
+    /// that order — its container hands out this service first and the settings only
+    /// when something needs them — and a version of this that quietly did nothing
+    /// before the first read left the window saving nothing at all.
+    /// </remarks>
+    public void TrackChanges()
+    {
+        if (_isTrackingChanges)
+        {
+            return;
+        }
+
+        Load().PropertyChanged += OnSettingsChanged;
+        _isTrackingChanges = true;
+    }
+
+    /// <summary>
+    /// Re-reads the file. For the agent, when the settings window says it has written.
+    /// </summary>
+    /// <remarks>
+    /// There is nothing to wait for and nothing to debounce: the window writes the file
+    /// whole and moves it into place in one step, and only then says so. Nobody else
+    /// writes it — the agent does not watch the file, and an edit made behind the
+    /// application's back is not a case it is built for.
+    /// </remarks>
+    public void Reload()
+    {
+        if (_settings is not null && ReadFile(_filePath) is { } fresh)
+        {
+            _settings.CopyFrom(fresh);
+        }
+    }
+
+    /// <summary>
+    /// Writes the settings and tells the agent to pick them up.
+    /// </summary>
+    /// <remarks>
+    /// The file is written beside its destination and moved onto it, which on one
+    /// volume is a single step. That way the agent, which is told to re-read the moment
+    /// this returns, never meets a half-written file.
+    /// </remarks>
+    public void Save()
+    {
+        if (_settings is null)
+        {
+            return;
+        }
+
+        _saveTimer.Stop();
+
+        try
+        {
+            Directory.CreateDirectory(Path.GetDirectoryName(_filePath)!);
+
+            string temporary = _filePath + ".tmp";
+            File.WriteAllText(temporary, JsonSerializer.Serialize(_settings, SerializerOptions));
+            File.Move(temporary, _filePath, overwrite: true);
+        }
+        catch (Exception e) when (e is IOException or UnauthorizedAccessException)
+        {
+            // The settings are not the kind of thing worth bringing the application down for
+            return;
+        }
+
+        SettingsSignal.NotifyAgent();
+    }
+
+    public void Dispose()
+    {
+        _saveTimer.Tick -= OnSaveTimerTick;
+        _saveTimer.Dispose();
+
+        if (_settings is not null && _isTrackingChanges)
+        {
+            _settings.PropertyChanged -= OnSettingsChanged;
+            _isTrackingChanges = false;
+            Save();
+        }
+    }
+
+    /// <summary>
+    /// The folder the application writes its settings to.
+    /// </summary>
+    private static string GetSettingsFolder()
+    {
+        if (!PackageIdentityNative.IsPackaged)
+        {
+            return GetSeparateInstallFolder();
+        }
+
+        // A package has a data folder of its own, which Windows creates and removes
+        // itself. The application name is not appended to it: the folder belongs to it alone anyway
+        return ApplicationData.Current.LocalFolder.Path;
+    }
+
+    /// <summary>
+    /// The settings folder of a separately installed application — the same source
+    /// the package inherits the settings from on the first launch.
+    /// </summary>
+    private static string GetSeparateInstallFolder() => Path.Combine(
+        Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+        "CursorLang");
+
+    private static AppSettings? ReadFile(string path)
+    {
+        try
+        {
+            if (!File.Exists(path))
+            {
+                return null;
+            }
+
+            return JsonSerializer.Deserialize<AppSettings>(File.ReadAllText(path), SerializerOptions);
+        }
+        catch (Exception e) when (e is IOException or UnauthorizedAccessException or JsonException)
+        {
+            return null;
+        }
+    }
+
+    private static AppSettings CreateDefault()
+    {
+        string uiLanguage = System.Globalization.CultureInfo.CurrentUICulture.TwoLetterISOLanguageName;
+        return new AppSettings { Language = uiLanguage == "ru" ? "ru" : "en" };
+    }
+
+    private void OnSettingsChanged(object? sender, PropertyChangedEventArgs e)
+    {
+        _saveTimer.Stop();
+        _saveTimer.Start();
+    }
+
+    private void OnSaveTimerTick(object? sender, EventArgs e) => Save();
+}

CursorLang.Core/Services/SettingsSignal.cs

@@ -0,0 +1,52 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Tells the agent that settings.json has changed.
+/// </summary>
+/// <remarks>
+/// The message carries nothing but the fact. The temptation to send the changed values
+/// along has to be resisted: the file would stop being the only source of truth, and
+/// the two would part company the first time somebody edits it by hand. Nothing else
+/// tells the agent — it does not watch the file — so a message that goes missing means
+/// settings it does not pick up until it is restarted.
+///
+/// Order is what makes it safe. The settings window writes the file whole, moves it
+/// into place in one step and only then signals, so by the time the agent reads there
+/// is nothing half-written to read.
+///
+/// A registered message rather than <c>WM_APP + n</c>: the identifier is unique across
+/// the system, so it cannot be confused with anything else that finds its way to that
+/// window.
+/// </remarks>
+internal static class SettingsSignal
+{
+    /// <summary>The window class the agent registers for its hidden window.</summary>
+    internal const string AgentWindowClass = "CursorLang.Agent.Window";
+
+    /// <summary>The message both sides agree on.</summary>
+    internal static uint Message { get; } = RegisterWindowMessage("CursorLang.SettingsChanged");
+
+    /// <summary>
+    /// Wakes the agent, if one is running in this session. Silence is a normal
+    /// answer: the settings window is perfectly usable with no agent behind it.
+    /// </summary>
+    internal static void NotifyAgent()
+    {
+        IntPtr agent = FindWindow(AgentWindowClass, null);
+        if (agent != IntPtr.Zero)
+        {
+            PostMessage(agent, Message, IntPtr.Zero, IntPtr.Zero);
+        }
+    }
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "RegisterWindowMessageW")]
+    private static extern uint RegisterWindowMessage(string lpString);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "FindWindowW")]
+    private static extern IntPtr FindWindow(string lpClassName, string? lpWindowName);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "PostMessageW")]
+    private static extern bool PostMessage(IntPtr hWnd, uint message, IntPtr wParam, IntPtr lParam);
+}

CursorLang.Core/Services/SingleInstanceGate.cs

@@ -0,0 +1,139 @@
+using CursorLang.Core.Interop;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Lets only one instance of the application run: a second launch does not bring up
+/// a second window but shows the window of the one already running.
+/// </summary>
+/// <remarks>
+/// The kernel object names are left without the Global prefix, that is, they live in
+/// the session namespace. A single instance for the whole machine would make for an
+/// odd picture with fast user switching: the second user would be left without the
+/// application, and showing them the window of the first one is impossible anyway —
+/// windows belong to a session.
+///
+/// Two processes use this now, and each guards its own slot: the agent so that one
+/// background process runs, the settings window so that a second "Settings" from the
+/// tray raises the window already open instead of a second one. Hence the name part.
+/// </remarks>
+public sealed class SingleInstanceGate : IDisposable
+{
+    /// <summary>The agent's slot — one background process per session.</summary>
+    public const string AgentName = ".Agent";
+
+    /// <summary>The settings window's slot — one window per session.</summary>
+    public const string SettingsName = ".Settings";
+
+    private const string MutexName = "CursorLang.SingleInstance";
+    private const string ActivationEventName = "CursorLang.ActivationRequest";
+
+    private readonly string _mutexName;
+    private readonly string _activationEventName;
+
+    private Mutex? _mutex;
+    private EventWaitHandle? _activationRequest;
+    private RegisteredWaitHandle? _activationWait;
+    private bool _isOwner;
+
+    /// <summary>
+    /// Takes a named slot. The name tells the agent's slot from the settings window's,
+    /// and the tests use one of their own: otherwise they would share a slot with the
+    /// running application and get in its way.
+    /// </summary>
+    public SingleInstanceGate(string nameSuffix)
+    {
+        _mutexName = MutexName + nameSuffix;
+        _activationEventName = ActivationEventName + nameSuffix;
+    }
+
+    /// <summary>
+    /// Another launch asks for the window to be shown.
+    /// </summary>
+    /// <remarks>
+    /// Raised on a thread pool thread, wherever the wait happened to be answered. The
+    /// two hosts get back to their own thread differently — one through the dispatcher,
+    /// one by posting to its window — so neither is assumed here.
+    /// </remarks>
+    public event EventHandler? ActivationRequested;
+
+    /// <summary>
+    /// Takes the single-instance slot. When the application is already running, asks
+    /// it to show itself and returns <c>false</c> — the caller is left to exit.
+    /// </summary>
+    public bool TryAcquire() => TryAcquire(showRunningInstance: true);
+
+    /// <summary>
+    /// The same, with a say in what is to happen to the application already running.
+    /// </summary>
+    /// <param name="showRunningInstance">
+    /// Whether the running application is to be brought up. A launch by Windows
+    /// itself passes <c>false</c>: it was not asked for a window, and the
+    /// application already in the tray is answer enough.
+    /// </param>
+    public bool TryAcquire(bool showRunningInstance)
+    {
+        _mutex = new Mutex(initiallyOwned: false, _mutexName);
+
+        try
+        {
+            _isOwner = _mutex.WaitOne(TimeSpan.Zero, exitContext: false);
+        }
+        catch (AbandonedMutexException)
+        {
+            // The previous instance crashed and did not release the mutex.
+            // It has no owner now, which means the slot is free
+            _isOwner = true;
+        }
+
+        // The event is opened by both instances: the first one to wait for a request,
+        // the second one to make it. Which of them creates the object depends on who
+        // came first and does not affect the work
+        _activationRequest = new EventWaitHandle(false, EventResetMode.AutoReset, _activationEventName);
+
+        if (!_isOwner)
+        {
+            if (showRunningInstance)
+            {
+                ForegroundPermissionNative.GrantToAnyProcess();
+                _activationRequest.Set();
+            }
+
+            return false;
+        }
+
+        // The wait is handed over to the thread pool: there is no reason to hold a
+        // thread of our own for it, and the request may never come
+        _activationWait = ThreadPool.RegisterWaitForSingleObject(
+            _activationRequest,
+            OnActivationSignalled,
+            state: null,
+            Timeout.Infinite,
+            executeOnlyOnce: false);
+
+        return true;
+    }
+
+    public void Dispose()
+    {
+        _activationWait?.Unregister(null);
+        _activationWait = null;
+
+        _activationRequest?.Dispose();
+        _activationRequest = null;
+
+        // The mutex is released by the same thread that took it: both happen
+        // on the user interface thread
+        if (_isOwner)
+        {
+            _mutex?.ReleaseMutex();
+            _isOwner = false;
+        }
+
+        _mutex?.Dispose();
+        _mutex = null;
+    }
+
+    private void OnActivationSignalled(object? state, bool timedOut) =>
+        ActivationRequested?.Invoke(this, EventArgs.Empty);
+}

CursorLang.Core/Services/StartupLaunch.cs

@@ -0,0 +1,54 @@
+using System.Runtime.InteropServices;
+using CursorLang.Core.Interop;
+using Windows.ApplicationModel;
+using Windows.ApplicationModel.Activation;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Whether Windows started the application by itself.
+/// </summary>
+/// <remarks>
+/// A launch of its own accord ends up in the tray without a window: the user asked
+/// for the application to be there when they sign in, not for a window to greet them
+/// every morning. A launch by the user is another matter — the window is what they
+/// clicked for.
+///
+/// The two builds tell the launches apart differently. A build in a folder is
+/// started from the registry, and the command written there carries an argument of
+/// its own — see <see cref="RegistryStartup"/>. A package has no say in its command
+/// line, and Windows is asked about the activation instead.
+/// </remarks>
+internal static class StartupLaunch
+{
+    /// <summary>What the registry entry adds to the path of the application.</summary>
+    internal const string Argument = "--startup";
+
+    /// <summary>Whether this launch is the doing of Windows rather than of the user.</summary>
+    internal static bool IsAutomatic(IReadOnlyList<string> arguments) =>
+        HasArgument(arguments) || IsStartupActivation();
+
+    /// <summary>The command line says the launch comes from the startup entry.</summary>
+    internal static bool HasArgument(IReadOnlyList<string> arguments) =>
+        arguments.Any(argument => string.Equals(argument, Argument, StringComparison.OrdinalIgnoreCase));
+
+    private static bool IsStartupActivation()
+    {
+        if (!PackageIdentityNative.IsPackaged)
+        {
+            return false;
+        }
+
+        try
+        {
+            return AppInstance.GetActivatedEventArgs() is { Kind: ActivationKind.StartupTask };
+        }
+        catch (Exception e) when (e is COMException or InvalidOperationException or NotSupportedException)
+        {
+            // Windows has nothing to say about the activation. A window shown when it
+            // was not asked for is a smaller mishap than an application that hides
+            // when the user has just started it
+            return false;
+        }
+    }
+}

CursorLang.Core/Services/StartupService.cs

@@ -0,0 +1,85 @@
+using System.Runtime.InteropServices;
+using CursorLang.Core.Interop;
+using CursorLang.Core.Models;
+using Windows.ApplicationModel;
+
+namespace CursorLang.Core.Services;
+
+/// <summary>
+/// Startup, arranged by whatever means the current build has.
+/// </summary>
+/// <remarks>
+/// Startup used to be a scheduled task with the highest rights — otherwise an app
+/// that wanted administrator rights would not start from the startup folder. The
+/// app needs no such rights any more, and the two ways left are simpler.
+///
+/// A package declares the task in its manifest, and Windows lists it for the user
+/// next to the rest under Settings — Apps — Startup. Turned off there, it can no
+/// longer be turned back on by the app. Outside a package the same setting is kept
+/// in the registry: see <see cref="RegistryStartup"/>.
+/// </remarks>
+public sealed class StartupService : IStartupService
+{
+    /// <summary>Matches TaskId in the package manifest.</summary>
+    private const string TaskId = "CursorLangStartup";
+
+    private readonly RegistryStartup _registry = new();
+
+    public async Task<StartupState> GetStateAsync()
+    {
+        if (!PackageIdentityNative.IsPackaged)
+        {
+            return _registry.GetState();
+        }
+
+        try
+        {
+            StartupTask task = await StartupTask.GetAsync(TaskId);
+            return Translate(task.State);
+        }
+        catch (Exception e) when (e is COMException or ArgumentException or InvalidOperationException)
+        {
+            // No task by that name in the manifest: that happens to a package put
+            // together by hand. The setting simply will not show
+            return StartupState.Unavailable;
+        }
+    }
+
+    public async Task<StartupState> SetEnabledAsync(bool enabled)
+    {
+        if (!PackageIdentityNative.IsPackaged)
+        {
+            return _registry.SetEnabled(enabled);
+        }
+
+        try
+        {
+            StartupTask task = await StartupTask.GetAsync(TaskId);
+
+            if (!enabled)
+            {
+                task.Disable();
+                return Translate(task.State);
+            }
+
+            // Windows answers with a state rather than with success: once the user
+            // has forbidden startup, the ban stays
+            return Translate(await task.RequestEnableAsync());
+        }
+        catch (Exception e) when (e is COMException or ArgumentException or InvalidOperationException)
+        {
+            return StartupState.Unavailable;
+        }
+    }
+
+    /// <summary>The state of a Windows task in the app's own terms.</summary>
+    internal static StartupState Translate(StartupTaskState state) => state switch
+    {
+        StartupTaskState.Enabled => StartupState.Enabled,
+        StartupTaskState.EnabledByPolicy => StartupState.EnabledByPolicy,
+        StartupTaskState.Disabled => StartupState.Disabled,
+        StartupTaskState.DisabledByUser => StartupState.DisabledByUser,
+        StartupTaskState.DisabledByPolicy => StartupState.DisabledByPolicy,
+        _ => StartupState.Unavailable,
+    };
+}

CursorLang.Core/Threading/MessageLoop.cs

@@ -0,0 +1,66 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Threading;
+
+/// <summary>
+/// The Win32 message loop — what the application has instead of a dispatcher.
+/// </summary>
+/// <remarks>
+/// It lives in Core rather than in the agent because the things that need a pumping
+/// thread do: <see cref="MessageTimer"/> is used by the layout polling and by saving
+/// the settings, and both are Core's. The settings window has a loop of its own, run
+/// by WPF, and everything here works inside it just the same.
+/// </remarks>
+public static class MessageLoop
+{
+    /// <summary>
+    /// Pumps messages until <c>WM_QUIT</c> and returns its exit code.
+    /// </summary>
+    /// <remarks>
+    /// A -1 from GetMessage means the window handle has already gone; going round
+    /// again would spin forever, so the loop gives up instead.
+    /// </remarks>
+    public static int Run()
+    {
+        while (true)
+        {
+            int result = GetMessage(out Message message, IntPtr.Zero, 0, 0);
+            if (result is 0 or -1)
+            {
+                return result == 0 ? (int)message.wParam : 1;
+            }
+
+            TranslateMessage(ref message);
+            DispatchMessage(ref message);
+        }
+    }
+
+    /// <summary>Asks the loop on this thread to finish.</summary>
+    public static void Quit(int exitCode = 0) => PostQuitMessage(exitCode);
+
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "GetMessageW")]
+    private static extern int GetMessage(out Message lpMsg, IntPtr hWnd, uint filterMin, uint filterMax);
+
+    [DllImport("user32.dll")]
+    private static extern bool TranslateMessage(ref Message lpMsg);
+
+    [DllImport("user32.dll", CharSet = CharSet.Unicode, EntryPoint = "DispatchMessageW")]
+    private static extern IntPtr DispatchMessage(ref Message lpMsg);
+
+    [DllImport("user32.dll")]
+    private static extern void PostQuitMessage(int exitCode);
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct Message
+    {
+        public IntPtr hwnd;
+        public uint message;
+        public IntPtr wParam;
+        public IntPtr lParam;
+        public uint time;
+        public int x;
+        public int y;
+        public uint lPrivate;
+    }
+}

CursorLang.Core/Threading/MessageTimer.cs

@@ -0,0 +1,70 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Core.Threading;
+
+/// <summary>
+/// A timer that ticks on the message loop — the agent's stand-in for
+/// <c>DispatcherTimer</c>.
+/// </summary>
+/// <remarks>
+/// <c>SetTimer</c> with a null window binds the timer to the thread rather than to a
+/// window, and <c>DispatchMessage</c> calls the callback straight from the loop. The
+/// upshot is the same as with a dispatcher timer: the tick arrives on the thread that
+/// owns the hook and the popup, so nothing needs marshalling and nothing races.
+///
+/// The callback lives in a field for the reason a hook procedure does: the only
+/// reference to it is held by Win32, and a collected delegate takes the process down
+/// with it at the first tick.
+/// </remarks>
+internal sealed class MessageTimer : IDisposable
+{
+    /// <summary>Windows will not go below this, and pretending otherwise misleads.</summary>
+    private const uint MinimumIntervalMilliseconds = 10;
+
+    private readonly TimerProc _callback;
+
+    private nuint _id;
+
+    internal MessageTimer() => _callback = OnTimer;
+
+    internal event EventHandler? Tick;
+
+    internal TimeSpan Interval { get; set; } = TimeSpan.FromMilliseconds(100);
+
+    internal bool IsRunning => _id != 0;
+
+    /// <summary>Starts the timer, or restarts it from zero when it is already running.</summary>
+    internal void Start()
+    {
+        Stop();
+
+        var milliseconds = (uint)Math.Clamp(
+            Math.Round(Interval.TotalMilliseconds), MinimumIntervalMilliseconds, int.MaxValue);
+
+        _id = SetTimer(IntPtr.Zero, 0, milliseconds, _callback);
+    }
+
+    internal void Stop()
+    {
+        if (_id == 0)
+        {
+            return;
+        }
+
+        KillTimer(IntPtr.Zero, _id);
+        _id = 0;
+    }
+
+    public void Dispose() => Stop();
+
+    private void OnTimer(IntPtr window, uint message, nuint id, uint time) =>
+        Tick?.Invoke(this, EventArgs.Empty);
+
+    private delegate void TimerProc(IntPtr hWnd, uint message, nuint idEvent, uint time);
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern nuint SetTimer(IntPtr hWnd, nuint nIDEvent, uint uElapse, TimerProc lpTimerFunc);
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern bool KillTimer(IntPtr hWnd, nuint uIDEvent);
+}

CursorLang.Settings.Tests/AppTests.cs

@@ -0,0 +1,120 @@
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+using CursorLang.Settings.Services;
+using CursorLang.Settings.ViewModels;
+using CursorLang.Settings.Views;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace CursorLang.Settings.Tests;
+
+/// <summary>
+/// What the container of the settings process is made of. The tests cannot build the
+/// application whole — it would raise a window and take the place of the single
+/// instance — but checking that everything needed is declared and resolvable works
+/// without that.
+/// </summary>
+public sealed class AppTests
+{
+    [Theory]
+    [InlineData(typeof(SettingsService))]
+    [InlineData(typeof(ThemeService))]
+    [InlineData(typeof(IThemeService))]
+    [InlineData(typeof(MainWindowPlacement))]
+    [InlineData(typeof(ILocalizationService))]
+    [InlineData(typeof(IStartupService))]
+    [InlineData(typeof(Version))]
+    [InlineData(typeof(SettingsViewModel))]
+    [InlineData(typeof(MainWindow))]
+    [InlineData(typeof(AppSettings))]
+    public void Everything_the_window_needs_is_declared_in_the_container(Type service)
+    {
+        Assert.Contains(Describe(), descriptor => descriptor.ServiceType == service);
+    }
+
+    /// <summary>
+    /// The background half is not in here, and must not be.
+    /// </summary>
+    /// <remarks>
+    /// The hook, the popup and the layout polling belong to the agent process now. A
+    /// registration of any of them here would mean two applications watching the
+    /// keyboard at once — and the second of them holding WPF while it did so.
+    /// </remarks>
+    [Theory]
+    [InlineData("KeyboardLayoutService")]
+    [InlineData("LayoutPopupService")]
+    [InlineData("CapsLockHotkeyService")]
+    [InlineData("LayoutNotificationCoordinator")]
+    [InlineData("CapsLockSwitchCoordinator")]
+    [InlineData("TrayIcon")]
+    public void The_background_half_is_not_in_the_settings_container(string name)
+    {
+        Assert.DoesNotContain(Describe(), descriptor => descriptor.ServiceType.Name.Contains(name));
+    }
+
+    // The settings and the theme have to be shared by the whole window: a second copy
+    // of them would mean lost edits or half the controls in the wrong colours
+    [Fact]
+    public void Everything_in_the_container_is_declared_as_a_single_copy()
+    {
+        Assert.All(Describe(), descriptor =>
+            Assert.Equal(ServiceLifetime.Singleton, descriptor.Lifetime));
+    }
+
+    [Fact]
+    public void Every_service_is_declared_once()
+    {
+        var types = Describe().Select(descriptor => descriptor.ServiceType).ToList();
+
+        Assert.Equal(types.Count, types.Distinct().Count());
+    }
+
+    /// <summary>
+    /// The dependencies of every service have to be resolvable. The check runs
+    /// while the container is built and creates no services itself.
+    /// </summary>
+    [Fact]
+    public void The_service_dependencies_come_together_with_nothing_missing()
+    {
+        var services = new ServiceCollection();
+        App.ConfigureServices(services);
+
+        ServiceProvider provider = services.BuildServiceProvider(new ServiceProviderOptions
+        {
+            ValidateOnBuild = true,
+            ValidateScopes = true,
+        });
+
+        // Ensure the provider was built successfully — this serves as an assertion
+        // so the test framework recognizes this as a meaningful test.
+        Assert.NotNull(provider);
+
+        provider.Dispose();
+    }
+
+    [Fact]
+    public void The_settings_come_from_the_settings_service()
+    {
+        ServiceDescriptor settings = Describe()
+            .Single(descriptor => descriptor.ServiceType == typeof(AppSettings));
+
+        // The settings are not created anew but read from disk by the service
+        Assert.NotNull(settings.ImplementationFactory);
+    }
+
+    [Fact]
+    public void The_theme_and_its_interface_are_one_service()
+    {
+        ServiceDescriptor theme = Describe()
+            .Single(descriptor => descriptor.ServiceType == typeof(IThemeService));
+
+        Assert.NotNull(theme.ImplementationFactory);
+    }
+
+    private static ServiceCollection Describe()
+    {
+        var services = new ServiceCollection();
+        App.ConfigureServices(services);
+
+        return services;
+    }
+}

CursorLang.Settings.Tests/CursorLang.Settings.Tests.csproj

@@ -0,0 +1,58 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net10.0-windows10.0.19041.0</TargetFramework>
+        <SupportedOSPlatformVersion>10.0.17763.0</SupportedOSPlatformVersion>
+        <Nullable>enable</Nullable>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <UseWPF>true</UseWPF>
+        <OutputType>Exe</OutputType>
+        <PlatformTarget>AnyCPU</PlatformTarget>
+        <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
+        <GenerateDocumentationFile>true</GenerateDocumentationFile>
+        <NoWarn>$(NoWarn);CS1591</NoWarn>
+        <IsPackable>false</IsPackable>
+        <IsTestProject>true</IsTestProject>
+        <RootNamespace>CursorLang.Settings.Tests</RootNamespace>
+        <AssemblyName>CursorLang.Settings.Tests</AssemblyName>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <Using Include="Xunit" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <PackageReference Include="xunit.v3" Version="3.2.2" />
+        <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.8.1" />
+        <PackageReference Include="coverlet.collector" Version="10.0.1" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\CursorLang.Core\CursorLang.Core.csproj" />
+        <ProjectReference Include="..\CursorLang.Tests.Shared\CursorLang.Tests.Shared.csproj" />
+        <ProjectReference Include="..\CursorLang.Settings\CursorLang.Settings.csproj" />
+    </ItemGroup>
+
+    <!-- The markup is embedded as plain text so that the check for missing resource
+         keys reads what the window really says, without walking the folder tree -->
+    <PropertyGroup>
+        <MainWindowMarkup>$(MSBuildThisFileDirectory)..\CursorLang.Settings\Views\MainWindow.xaml</MainWindowMarkup>
+    </PropertyGroup>
+
+    <Target Name="EmbedMainWindowMarkup" BeforeTargets="PrepareForBuild">
+        <PropertyGroup>
+            <MainWindowMarkupCopy>$(IntermediateOutputPath)MainWindow.xaml.txt</MainWindowMarkupCopy>
+        </PropertyGroup>
+
+        <Copy SourceFiles="$(MainWindowMarkup)"
+              DestinationFiles="$(MainWindowMarkupCopy)"
+              SkipUnchangedFiles="true" />
+
+        <ItemGroup>
+            <EmbeddedResource Include="$(MainWindowMarkupCopy)" LogicalName="MainWindow.xaml" />
+            <FileWrites Include="$(MainWindowMarkupCopy)" />
+        </ItemGroup>
+    </Target>
+
+</Project>

CursorLang.Settings.Tests/Infrastructure/SettingsFakes.cs

@@ -0,0 +1,16 @@
+using CursorLang.Core.Models;
+using CursorLang.Settings.Services;
+
+namespace CursorLang.Settings.Tests.Infrastructure;
+
+/// <summary>
+/// A theme that paints nothing and only remembers the windows attached to it.
+/// </summary>
+internal sealed class FakeThemeService : IThemeService
+{
+    public AppTheme CurrentTheme { get; set; } = AppTheme.Light;
+
+    internal List<System.Windows.Window> Registered { get; } = [];
+
+    public void Register(System.Windows.Window window) => Registered.Add(window);
+}

CursorLang.Settings.Tests/Infrastructure/Sta.cs

@@ -0,0 +1,179 @@
+using System.Windows;
+using System.Windows.Threading;
+
+namespace CursorLang.Settings.Tests.Infrastructure;
+
+/// <summary>
+/// The user interface thread for the tests.
+/// </summary>
+/// <remarks>
+/// Half of the application — windows, the dispatcher and the timers on it —
+/// only works on an STA thread with a message queue, while tests run on a pool
+/// thread. The thread is therefore started once for the whole run: a process
+/// may hold only one <see cref="Application"/>, and recreating it between tests
+/// is not possible.
+///
+/// The queue on that thread is pumped for real, so timers fire on their own:
+/// the test only has to await the consequences through <see cref="WaitFor"/>.
+/// </remarks>
+internal static class Sta
+{
+    private static readonly Lock Gate = new();
+
+    private static readonly TimeSpan DefaultTimeout = TimeSpan.FromSeconds(5);
+
+    internal static Dispatcher Dispatcher
+    {
+        get
+        {
+            lock (Gate)
+            {
+                return field ??= Start();
+            }
+        }
+    }
+
+    /// <summary>Runs an action on the interface thread and waits for it to finish.</summary>
+    internal static void Run(Action action) => Dispatcher.Invoke(action);
+
+    /// <summary>The same for an action that returns a result.</summary>
+    internal static TResult Run<TResult>(Func<TResult> action) => Dispatcher.Invoke(action);
+
+    /// <summary>
+    /// Runs an action on a separate STA thread and waits for it to finish.
+    /// </summary>
+    /// <remarks>
+    /// Needed where the foreignness of the thread is the point: kernel objects
+    /// such as a mutex let their own owner in again, so a "second instance" of
+    /// the application on the same thread does not count as second.
+    /// </remarks>
+    internal static void RunApart(Action action)
+    {
+        Exception? failure = null;
+
+        var thread = new Thread(() =>
+        {
+            try
+            {
+                action();
+            }
+            catch (Exception e)
+            {
+                failure = e;
+            }
+        })
+        {
+            IsBackground = true,
+            Name = "CursorLang.Tests apart",
+        };
+
+        thread.SetApartmentState(ApartmentState.STA);
+        thread.Start();
+        thread.Join();
+
+        if (failure is not null)
+        {
+            throw new InvalidOperationException("The action on the separate thread failed", failure);
+        }
+    }
+
+    /// <summary>
+    /// Waits until the dispatcher queue drains: calls deferred through
+    /// <c>BeginInvoke</c> have run by that time.
+    /// </summary>
+    internal static void Drain() =>
+        Dispatcher.Invoke(static () => { }, DispatcherPriority.ApplicationIdle);
+
+    /// <summary>
+    /// Waits for a condition without getting in the way of the timers.
+    /// </summary>
+    /// <remarks>
+    /// Waiting is allowed from any thread, including the interface thread
+    /// itself: there a plain wait would stop the message queue, and with it
+    /// everything being waited for. On the interface thread the queue therefore
+    /// keeps being pumped by a nested loop.
+    /// </remarks>
+    internal static void WaitFor(Func<bool> condition, string because, TimeSpan? timeout = null)
+    {
+        DateTime deadline = DateTime.UtcNow + (timeout ?? DefaultTimeout);
+
+        while (!condition())
+        {
+            Assert.True(DateTime.UtcNow < deadline, $"Waited in vain: {because}");
+            Idle(TimeSpan.FromMilliseconds(5));
+        }
+    }
+
+    /// <summary>
+    /// Waits for the given time while still pumping the queue: that is how
+    /// "nothing happened during this time" is verified.
+    /// </summary>
+    internal static void Pause(TimeSpan duration)
+    {
+        Idle(duration);
+        Drain();
+    }
+
+    // A wait during which the dispatcher queue gets its chance to run
+    private static void Idle(TimeSpan duration)
+    {
+        if (Dispatcher.CheckAccess())
+        {
+            var frame = new DispatcherFrame();
+            var timer = new DispatcherTimer(
+                duration,
+                DispatcherPriority.Background,
+                (_, _) => frame.Continue = false,
+                Dispatcher);
+
+            try
+            {
+                Dispatcher.PushFrame(frame);
+            }
+            finally
+            {
+                timer.Stop();
+            }
+
+            return;
+        }
+
+        Thread.Sleep(duration);
+    }
+
+    private static Dispatcher Start()
+    {
+        var ready = new TaskCompletionSource<Dispatcher>();
+
+        var thread = new Thread(() =>
+        {
+            Dispatcher dispatcher = Dispatcher.CurrentDispatcher;
+
+            // The application has no reason to shut down after its windows:
+            // tests open and close them by the dozen
+            var application = new Application { ShutdownMode = ShutdownMode.OnExplicitShutdown };
+
+            // The shared style dictionary is merged by App.xaml, which the tests
+            // do not have. Without it the settings window still builds, but not
+            // the way the user will see it
+            application.Resources.MergedDictionaries.Add(new ResourceDictionary
+            {
+                Source = new Uri(
+                    "pack://application:,,,/CursorLang.Settings;component/Themes/Controls.xaml",
+                    UriKind.Absolute),
+            });
+
+            ready.SetResult(dispatcher);
+            Dispatcher.Run();
+        })
+        {
+            IsBackground = true,
+            Name = "CursorLang.Tests UI",
+        };
+
+        thread.SetApartmentState(ApartmentState.STA);
+        thread.Start();
+
+        return ready.Task.GetAwaiter().GetResult();
+    }
+}

CursorLang.Settings.Tests/Interop/ForegroundWindowTests.cs

@@ -0,0 +1,159 @@
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Interop;
+using CursorLang.Core.Interop;
+using CursorLang.Settings.Interop;
+using CursorLang.Settings.Tests.Infrastructure;
+
+namespace CursorLang.Settings.Tests.Interop;
+
+/// <summary>
+/// Checks that need a real foreground window with an input field: the caret
+/// and the layout switch live exactly there.
+/// </summary>
+/// <remarks>
+/// Windows does not always allow a window to come forward — when the screen is
+/// locked, say, or when the run happens in a session without a desktop. In
+/// those cases the check reports itself as skipped rather than failed: there
+/// would be nothing to verify.
+/// </remarks>
+public sealed class ForegroundWindowTests
+{
+    [Fact]
+    public void The_caret_in_an_input_field_is_found()
+    {
+        Sta.Run(() =>
+        {
+            using var input = new InputWindow();
+            input.RequireForeground();
+
+            PopupWindowNative.Rect? caret = CaretNative.TryGetCaretRect();
+
+            if (caret is null)
+            {
+                Assert.Skip("The input field did not report the caret position");
+            }
+
+            // The caret has to sit inside the input window and to have a height
+            PopupWindowNative.Rect bounds = WindowPlacementNative.TryGetBounds(input.Handle)!.Value;
+
+            Assert.True(caret.Value.Bottom > caret.Value.Top);
+            Assert.InRange(caret.Value.Left, bounds.Left, bounds.Right);
+            Assert.InRange(caret.Value.Top, bounds.Top, bounds.Bottom);
+        });
+    }
+
+    [Fact]
+    public void The_tooltip_at_the_caret_lands_next_to_it()
+    {
+        Sta.Run(() =>
+        {
+            using var input = new InputWindow();
+            input.RequireForeground();
+
+            PopupWindowNative.Rect? caret = CaretNative.TryGetCaretRect();
+            if (caret is null)
+            {
+                Assert.Skip("The input field did not report the caret position");
+            }
+
+            PopupWindowNative.Rect field =
+                WindowPlacementNative.TryGetBounds(input.Handle)!.Value;
+
+            Assert.True(CaretNative.IsInside(caret.Value, field),
+                "the caret reported by the input field is outside that field");
+        });
+    }
+
+    // The request to switch the layout goes to the window holding the input
+    // focus, so it only concerns the test window itself
+    [Fact]
+    public void The_request_to_change_the_layout_reaches_its_own_window()
+    {
+        Sta.Run(() =>
+        {
+            using var input = new InputWindow();
+            input.RequireForeground();
+
+            int before = KeyboardLayoutNative.GetActiveLocaleId();
+
+            KeyboardLayoutNative.RequestNextLayout();
+            Sta.Pause(TimeSpan.FromMilliseconds(150));
+
+            int after = KeyboardLayoutNative.GetActiveLocaleId();
+
+            Assert.InRange(after, 1, 0xFFFF);
+
+            if (after == before)
+            {
+                // The system may hold a single layout — there is nothing to switch to
+                return;
+            }
+
+            // Bring the layout back around the circle to where it was
+            for (int i = 0; i < 8 && KeyboardLayoutNative.GetActiveLocaleId() != before; i++)
+            {
+                KeyboardLayoutNative.RequestNextLayout();
+                Sta.Pause(TimeSpan.FromMilliseconds(150));
+            }
+        });
+    }
+
+    /// <summary>A window with an input field brought to the foreground.</summary>
+    private sealed class InputWindow : IDisposable
+    {
+        private readonly Window _window;
+
+        internal InputWindow()
+        {
+            var box = new TextBox { Text = "check", FontSize = 20 };
+
+            _window = new Window
+            {
+                Width = 400,
+                Height = 200,
+                ShowInTaskbar = false,
+                WindowStartupLocation = WindowStartupLocation.Manual,
+                Left = 100,
+                Top = 100,
+                Topmost = true,
+                Content = box,
+            };
+
+            _window.Show();
+            Handle = new WindowInteropHelper(_window).Handle;
+
+            // Windows grants the right to bring a window forward neither to
+            // everyone nor at once, so it takes a few attempts
+            for (int attempt = 0; attempt < 10; attempt++)
+            {
+                _window.Activate();
+                box.Focus();
+                box.CaretIndex = box.Text.Length;
+
+                Sta.Pause(TimeSpan.FromMilliseconds(50));
+
+                if (KeyboardLayoutNative.GetForegroundWindow() == Handle)
+                {
+                    break;
+                }
+            }
+
+            // The caret does not appear at the same instant as the focus
+            Sta.Pause(TimeSpan.FromMilliseconds(100));
+        }
+
+        internal IntPtr Handle { get; }
+
+        /// <summary>Skips the check if the window never became the foreground one.</summary>
+        internal void RequireForeground()
+        {
+            if (KeyboardLayoutNative.GetForegroundWindow() != Handle)
+            {
+                Assert.Skip("The window could not be brought to the foreground");
+            }
+        }
+
+        public void Dispose() => _window.Close();
+    }
+}

CursorLang.Settings.Tests/Interop/NativeWrappersTests.cs

@@ -0,0 +1,212 @@
+using System.Windows;
+using System.Windows.Interop;
+using CursorLang.Core.Interop;
+using CursorLang.Settings.Interop;
+using CursorLang.Settings.Tests.Infrastructure;
+
+namespace CursorLang.Settings.Tests.Interop;
+
+/// <summary>
+/// The Win32 wrappers: what is checked is that the calls are put together
+/// right — structures of the expected size, flags in place, and the answers
+/// of the system read correctly.
+/// </summary>
+public sealed class NativeWrappersTests
+{
+    [Fact]
+    public void The_cursor_position_is_read()
+    {
+        PopupWindowNative.Point cursor = PopupWindowNative.GetCursorPosition();
+
+        // The virtual screen may run into negative coordinates, but not beyond
+        // reason: a misread structure would give garbage
+        Assert.InRange(cursor.X, -32_000, 32_000);
+        Assert.InRange(cursor.Y, -32_000, 32_000);
+    }
+
+    [Fact]
+    public void The_scale_of_the_monitor_under_the_cursor_is_positive()
+    {
+        double scale = PopupWindowNative.GetScaleAt(PopupWindowNative.GetCursorPosition());
+
+        Assert.InRange(scale, 0.5, 8.0);
+    }
+
+    [Fact]
+    public void The_work_area_of_the_active_monitor_is_not_empty()
+    {
+        (PopupWindowNative.Rect work, double scale) = PopupWindowNative.GetActiveMonitorWorkArea();
+
+        Assert.True(work.Right > work.Left);
+        Assert.True(work.Bottom > work.Top);
+        Assert.InRange(scale, 0.5, 8.0);
+    }
+
+    [Fact]
+    public void The_window_bounds_are_read_from_the_system()
+    {
+        Sta.Run(() =>
+        {
+            using var window = new HandleWindow();
+
+            PopupWindowNative.Rect? bounds = WindowPlacementNative.TryGetBounds(window.Handle);
+
+            Assert.NotNull(bounds);
+            Assert.True(bounds.Value.Right > bounds.Value.Left);
+            Assert.True(bounds.Value.Bottom > bounds.Value.Top);
+        });
+    }
+
+    [Fact]
+    public void A_window_that_does_not_exist_has_no_bounds()
+    {
+        Assert.Null(WindowPlacementNative.TryGetBounds(IntPtr.Zero));
+    }
+
+    [Fact]
+    public void A_window_is_moved_to_the_given_point()
+    {
+        Sta.Run(() =>
+        {
+            using var window = new HandleWindow();
+
+            PopupWindowNative.MoveTo(window.Handle, 120, 90);
+
+            PopupWindowNative.Rect bounds = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+            Assert.Equal(120, bounds.Left);
+            Assert.Equal(90, bounds.Top);
+        });
+    }
+
+    [Fact]
+    public void Moving_does_not_change_the_window_size()
+    {
+        Sta.Run(() =>
+        {
+            using var window = new HandleWindow();
+
+            PopupWindowNative.Rect before = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+            PopupWindowNative.MoveTo(window.Handle, 200, 150);
+            PopupWindowNative.Rect after = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+
+            Assert.Equal(before.Right - before.Left, after.Right - after.Left);
+            Assert.Equal(before.Bottom - before.Top, after.Bottom - after.Top);
+        });
+    }
+
+    [Fact]
+    public void The_work_area_is_found_by_the_rectangle_of_the_window()
+    {
+        Sta.Run(() =>
+        {
+            using var window = new HandleWindow();
+            PopupWindowNative.Rect bounds = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+
+            PopupWindowNative.Rect? work = WindowPlacementNative.TryGetWorkAreaNear(bounds);
+
+            Assert.NotNull(work);
+            Assert.True(work.Value.Right > work.Value.Left);
+            Assert.True(work.Value.Bottom > work.Value.Top);
+        });
+    }
+
+    // The nearest monitor is picked, so an area is found even for a point far off screen
+    [Fact]
+    public void For_a_rectangle_off_every_screen_the_nearest_monitor_is_taken()
+    {
+        var far = new PopupWindowNative.Rect { Left = 30_000, Top = 30_000, Right = 30_100, Bottom = 30_100 };
+
+        PopupWindowNative.Rect? work = WindowPlacementNative.TryGetWorkAreaNear(far);
+
+        Assert.NotNull(work);
+        Assert.True(work.Value.Right > work.Value.Left);
+    }
+
+    [Fact]
+    public void The_layout_of_the_foreground_window_is_read()
+    {
+        int localeId = KeyboardLayoutNative.GetActiveLocaleId();
+
+        // The low word of the HKL is the locale identifier, and it is never zero
+        Assert.NotEqual(0, localeId);
+        Assert.InRange(localeId, 1, 0xFFFF);
+    }
+
+    [Fact]
+    public void The_layout_is_read_for_any_window()
+    {
+        int localeId = KeyboardLayoutNative.GetLocaleIdOf(KeyboardLayoutNative.GetForegroundWindow());
+
+        Assert.InRange(localeId, 0, 0xFFFF);
+    }
+
+    [Fact]
+    public void The_input_state_of_the_foreground_is_read()
+    {
+        bool received = ForegroundInputNative.TryGetInfo(out ForegroundInputNative.GuiThreadInfo info);
+
+        if (received)
+        {
+            // The structure size is filled in by the wrapper itself, and it has
+            // to match what Windows expects
+            Assert.Equal(System.Runtime.InteropServices.Marshal.SizeOf<ForegroundInputNative.GuiThreadInfo>(),
+                info.cbSize);
+        }
+    }
+
+    [Fact]
+    public void The_right_to_show_a_window_is_given_away_without_errors()
+    {
+        ForegroundPermissionNative.GrantToAnyProcess();
+    }
+
+    [Fact]
+    public void The_window_title_bar_is_repainted()
+    {
+        Sta.Run(() =>
+        {
+            using var window = new HandleWindow();
+
+            WindowThemeNative.SetDarkTitleBar(window.Handle, isDark: true);
+            WindowThemeNative.SetDarkTitleBar(window.Handle, isDark: false);
+        });
+    }
+
+    // The window may be gone by the time of the repaint
+    [Fact]
+    public void Repainting_a_window_that_does_not_exist_passes_silently()
+    {
+        WindowThemeNative.SetDarkTitleBar(IntPtr.Zero, isDark: true);
+    }
+
+    [Fact]
+    public void The_package_flag_is_computed_once_and_does_not_change()
+    {
+        bool first = PackageIdentityNative.IsPackaged;
+
+        Assert.Equal(first, PackageIdentityNative.IsPackaged);
+    }
+
+    /// <summary>A window with a created handle that never appears on screen.</summary>
+    private sealed class HandleWindow : IDisposable
+    {
+        private readonly Window _window;
+
+        internal HandleWindow()
+        {
+            _window = new Window
+            {
+                Width = 300,
+                Height = 200,
+                ShowInTaskbar = false,
+                WindowStartupLocation = WindowStartupLocation.Manual,
+            };
+
+            Handle = new WindowInteropHelper(_window).EnsureHandle();
+        }
+
+        internal IntPtr Handle { get; }
+
+        public void Dispose() => _window.Close();
+    }
+}

CursorLang.Settings.Tests/Services/MainWindowPlacementTests.cs

@@ -0,0 +1,288 @@
+using System.Windows;
+using System.Windows.Interop;
+using CursorLang.Core.Interop;
+using CursorLang.Settings.Interop;
+using CursorLang.Settings.Services;
+using CursorLang.Settings.Tests.Infrastructure;
+
+namespace CursorLang.Settings.Tests.Services;
+
+/// <summary>
+/// Placing the settings window: the centre maths and bringing the window back
+/// into the work area.
+/// </summary>
+public sealed class MainWindowPlacementTests
+{
+    private static readonly PopupWindowNative.Rect Work = new()
+    {
+        Left = 0,
+        Top = 0,
+        Right = 1000,
+        Bottom = 800,
+    };
+
+    private static readonly PopupWindowNative.Rect Bounds = new()
+    {
+        Left = 0,
+        Top = 0,
+        Right = 400,
+        Bottom = 300,
+    };
+
+    [Fact]
+    public void The_centre_follows_the_window_size_and_the_work_area()
+    {
+        PopupWindowNative.Point point = MainWindowPlacement.Center(Bounds, Work);
+
+        Assert.Equal((1000 - 400) / 2, point.X);
+        Assert.Equal((800 - 300) / 2, point.Y);
+    }
+
+    // The work area of a second monitor does not start at zero
+    [Fact]
+    public void The_centre_of_a_neighbouring_monitor_is_measured_from_its_left_edge()
+    {
+        var work = new PopupWindowNative.Rect { Left = 1920, Top = 100, Right = 3520, Bottom = 1000 };
+
+        PopupWindowNative.Point point = MainWindowPlacement.Center(Bounds, work);
+
+        Assert.Equal(1920 + ((1600 - 400) / 2), point.X);
+        Assert.Equal(100 + ((900 - 300) / 2), point.Y);
+    }
+
+    [Fact]
+    public void A_window_inside_the_work_area_stays_where_it_is()
+    {
+        var position = new PopupWindowNative.Point { X = 120, Y = 90 };
+
+        PopupWindowNative.Point clamped = MainWindowPlacement.Clamp(position, Bounds, Work);
+
+        Assert.Equal(120, clamped.X);
+        Assert.Equal(90, clamped.Y);
+    }
+
+    [Fact]
+    public void A_window_past_the_right_edge_is_pulled_back_in()
+    {
+        var position = new PopupWindowNative.Point { X = 900, Y = 700 };
+
+        PopupWindowNative.Point clamped = MainWindowPlacement.Clamp(position, Bounds, Work);
+
+        Assert.Equal(1000 - 400, clamped.X);
+        Assert.Equal(800 - 300, clamped.Y);
+    }
+
+    [Fact]
+    public void A_window_past_the_left_and_top_edges_is_pulled_back_in()
+    {
+        var position = new PopupWindowNative.Point { X = -500, Y = -400 };
+
+        PopupWindowNative.Point clamped = MainWindowPlacement.Clamp(position, Bounds, Work);
+
+        Assert.Equal(Work.Left, clamped.X);
+        Assert.Equal(Work.Top, clamped.Y);
+    }
+
+    // The window height matches its content and on a short monitor exceeds the
+    // work area. The title bar matters more than the bottom of the window
+    [Fact]
+    public void A_window_taller_than_the_work_area_is_pinned_to_its_top()
+    {
+        var tall = new PopupWindowNative.Rect { Left = 0, Top = 0, Right = 400, Bottom = 900 };
+        var position = new PopupWindowNative.Point { X = 0, Y = 300 };
+
+        PopupWindowNative.Point clamped = MainWindowPlacement.Clamp(position, tall, Work);
+
+        Assert.Equal(Work.Top, clamped.Y);
+    }
+
+    [Fact]
+    public void A_window_wider_than_the_work_area_is_pinned_to_its_left_edge()
+    {
+        var wide = new PopupWindowNative.Rect { Left = 0, Top = 0, Right = 1200, Bottom = 300 };
+        var position = new PopupWindowNative.Point { X = 400, Y = 0 };
+
+        PopupWindowNative.Point clamped = MainWindowPlacement.Clamp(position, wide, Work);
+
+        Assert.Equal(Work.Left, clamped.X);
+    }
+
+    [Theory]
+    [InlineData(0, 0, 0, 0, true)]
+    [InlineData(0, 0, 100, 0, true)]
+    [InlineData(0, 0, 0, 100, true)]
+    [InlineData(100, 100, 100, 200, true)]
+    [InlineData(0, 0, 1, 1, false)]
+    [InlineData(-100, -100, 100, 100, false)]
+    public void An_area_without_width_or_height_counts_as_empty(
+        int left, int top, int right, int bottom, bool expected)
+    {
+        var rect = new PopupWindowNative.Rect { Left = left, Top = top, Right = right, Bottom = bottom };
+
+        Assert.Equal(expected, MainWindowPlacement.IsEmpty(rect));
+    }
+
+    [Fact]
+    public void The_first_time_in_a_session_the_window_lands_centred_on_the_active_monitor()
+    {
+        Sta.Run(() =>
+        {
+            var placement = new MainWindowPlacement();
+            using var window = new TestWindow();
+
+            (PopupWindowNative.Rect before, _) = PopupWindowNative.GetActiveMonitorWorkArea();
+            placement.Apply(window);
+            (PopupWindowNative.Rect work, _) = PopupWindowNative.GetActiveMonitorWorkArea();
+
+            if (!before.Equals(work))
+            {
+                // The user moved to another monitor right during the check
+                Assert.Skip("The active monitor changed while the check was running");
+            }
+
+            PopupWindowNative.Rect bounds = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+
+            // Pixel precision: the window goes exactly where it was computed to go
+            PopupWindowNative.Point expected = MainWindowPlacement.Clamp(
+                MainWindowPlacement.Center(bounds, work), bounds, work);
+
+            Assert.Equal(expected.X, bounds.Left);
+            Assert.Equal(expected.Y, bounds.Top);
+        });
+    }
+
+    [Fact]
+    public void The_window_returns_where_the_user_moved_it()
+    {
+        Sta.Run(() =>
+        {
+            var placement = new MainWindowPlacement();
+            using var window = new TestWindow();
+
+            placement.Attach(window);
+            placement.Apply(window);
+
+            // Move the window the way the user does it with the mouse
+            PopupWindowNative.Rect centered = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+            PopupWindowNative.MoveTo(window.Handle, centered.Left + 40, centered.Top + 30);
+            window.RaiseLocationChanged();
+
+            // Showing the window again — it has to stay where it was left
+            placement.Apply(window);
+
+            PopupWindowNative.Rect after = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+            Assert.Equal(centered.Left + 40, after.Left);
+            Assert.Equal(centered.Top + 30, after.Top);
+        });
+    }
+
+    // While we move the window ourselves its position must not drift from repeats
+    [Fact]
+    public void Placing_again_does_not_move_the_window()
+    {
+        Sta.Run(() =>
+        {
+            var placement = new MainWindowPlacement();
+            using var window = new TestWindow();
+
+            placement.Attach(window);
+            placement.Apply(window);
+
+            PopupWindowNative.Rect first = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+
+            placement.Apply(window);
+            placement.Apply(window);
+
+            PopupWindowNative.Rect third = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+            Assert.Equal(first.Left, third.Left);
+            Assert.Equal(first.Top, third.Top);
+        });
+    }
+
+    [Fact]
+    public void A_minimised_window_is_not_placed()
+    {
+        Sta.Run(() =>
+        {
+            var placement = new MainWindowPlacement();
+            using var window = new TestWindow();
+
+            PopupWindowNative.MoveTo(window.Handle, 7, 9);
+            window.WindowState = WindowState.Minimized;
+
+            placement.Apply(window);
+
+            PopupWindowNative.Rect bounds = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+            Assert.Equal(7, bounds.Left);
+            Assert.Equal(9, bounds.Top);
+        });
+    }
+
+    [Fact]
+    public void A_window_without_a_handle_is_not_placed()
+    {
+        Sta.Run(() =>
+        {
+            var placement = new MainWindowPlacement();
+            var window = new Window { Width = 200, Height = 150 };
+
+            // There must be no exception: the window does not exist yet,
+            // so there is nothing to place
+            placement.Apply(window);
+
+            Assert.Equal(IntPtr.Zero, new WindowInteropHelper(window).Handle);
+        });
+    }
+
+    // The place of the window lives in memory only: the set of monitors may be
+    // different by the next run
+    [Fact]
+    public void Every_placement_starts_its_session_afresh()
+    {
+        Sta.Run(() =>
+        {
+            using var window = new TestWindow();
+
+            var first = new MainWindowPlacement();
+            first.Attach(window);
+            first.Apply(window);
+
+            PopupWindowNative.Rect centered = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+
+            PopupWindowNative.MoveTo(window.Handle, centered.Left + 60, centered.Top + 60);
+            window.RaiseLocationChanged();
+
+            // A new placement knows nothing of the earlier move and centres the window again
+            var second = new MainWindowPlacement();
+            second.Apply(window);
+
+            PopupWindowNative.Rect bounds = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+            Assert.Equal(centered.Left, bounds.Left);
+            Assert.Equal(centered.Top, bounds.Top);
+        });
+    }
+
+    /// <summary>
+    /// A window with a ready handle that never appears on screen: placement
+    /// works with the system bounds, and a created window is enough for those.
+    /// </summary>
+    private sealed class TestWindow : Window, IDisposable
+    {
+        internal TestWindow()
+        {
+            Width = 400;
+            Height = 300;
+            ShowInTaskbar = false;
+            WindowStartupLocation = WindowStartupLocation.Manual;
+
+            Handle = new WindowInteropHelper(this).EnsureHandle();
+        }
+
+        internal IntPtr Handle { get; }
+
+        /// <summary>Reports a move the way WPF does after the user acts.</summary>
+        internal void RaiseLocationChanged() => OnLocationChanged(EventArgs.Empty);
+
+        public void Dispose() => Close();
+    }
+}

CursorLang.Settings.Tests/Services/ThemeServiceTests.cs

@@ -0,0 +1,458 @@
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Interop;
+using System.Windows.Media;
+using System.Windows.Shapes;
+using CursorLang.Core.Models;
+using CursorLang.Settings.Services;
+using CursorLang.Settings.Tests.Infrastructure;
+using Microsoft.Win32;
+
+namespace CursorLang.Settings.Tests.Services;
+
+/// <summary>
+/// The look of the windows. The palette lives in the application resources,
+/// so everything happens on the interface thread.
+/// </summary>
+public sealed class ThemeServiceTests
+{
+    [Theory]
+    [InlineData(AppTheme.Light)]
+    [InlineData(AppTheme.Dark)]
+    public void A_chosen_theme_is_applied_as_is(AppTheme theme)
+    {
+        var settings = new AppSettings { Theme = theme };
+
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(settings, static () => AppTheme.Dark);
+
+            Assert.Equal(theme, service.CurrentTheme);
+        });
+    }
+
+    [Theory]
+    [InlineData(AppTheme.Light)]
+    [InlineData(AppTheme.Dark)]
+    public void The_system_theme_is_taken_from_Windows(AppTheme system)
+    {
+        var settings = new AppSettings { Theme = AppTheme.System };
+
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(settings, () => system);
+
+            Assert.Equal(system, service.CurrentTheme);
+        });
+    }
+
+    [Fact]
+    public void Changing_the_theme_in_the_settings_repaints_the_windows()
+    {
+        var settings = new AppSettings { Theme = AppTheme.Light };
+
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(settings, static () => AppTheme.Light);
+
+            Color light = WindowBackground();
+
+            settings.Theme = AppTheme.Dark;
+
+            Assert.Equal(AppTheme.Dark, service.CurrentTheme);
+            Assert.NotEqual(light, WindowBackground());
+        });
+    }
+
+    // The palette is replaced rather than piled up: otherwise the light one
+    // would still sit under the dark one
+    [Fact]
+    public void The_palette_does_not_pile_up_in_the_resources()
+    {
+        var settings = new AppSettings { Theme = AppTheme.Light };
+
+        Sta.Run(() =>
+        {
+            int before = Application.Current.Resources.MergedDictionaries.Count;
+
+            using var service = new ThemeService(settings, static () => AppTheme.Light);
+
+            settings.Theme = AppTheme.Dark;
+            settings.Theme = AppTheme.Light;
+            settings.Theme = AppTheme.Dark;
+
+            Assert.Equal(before + 1, Application.Current.Resources.MergedDictionaries.Count);
+        });
+    }
+
+    [Fact]
+    public void Choosing_the_same_theme_again_leaves_the_resources_alone()
+    {
+        var settings = new AppSettings { Theme = AppTheme.Dark };
+
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(settings, static () => AppTheme.Light);
+
+            int count = Application.Current.Resources.MergedDictionaries.Count;
+            ResourceDictionary palette = Application.Current.Resources.MergedDictionaries[^1];
+
+            settings.Theme = AppTheme.Dark;
+
+            Assert.Equal(count, Application.Current.Resources.MergedDictionaries.Count);
+            Assert.Same(palette, Application.Current.Resources.MergedDictionaries[^1]);
+        });
+    }
+
+    // The other settings have nothing to do with the look
+    [Fact]
+    public void Other_settings_do_not_change_the_theme()
+    {
+        var settings = new AppSettings { Theme = AppTheme.Light };
+
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(settings, static () => AppTheme.Dark);
+
+            settings.Current.FontSize = 40;
+            settings.Language = "ru";
+
+            Assert.Equal(AppTheme.Light, service.CurrentTheme);
+        });
+    }
+
+    [Fact]
+    public void A_window_that_already_exists_is_attached_at_once()
+    {
+        var settings = new AppSettings { Theme = AppTheme.Dark };
+
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(settings, static () => AppTheme.Light);
+            var window = new Window();
+
+            try
+            {
+                _ = new WindowInteropHelper(window).EnsureHandle();
+
+                // The title bar is painted by Windows, and the only way to check
+                // this is that the call goes through without an error
+                service.Register(window);
+            }
+            finally
+            {
+                window.Close();
+            }
+        });
+    }
+
+    [Fact]
+    public void A_window_without_a_handle_is_attached_once_it_appears()
+    {
+        var settings = new AppSettings { Theme = AppTheme.Dark };
+
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(settings, static () => AppTheme.Light);
+            var window = new Window { Opacity = 0, ShowInTaskbar = false, ShowActivated = false };
+
+            try
+            {
+                service.Register(window);
+
+                // The window is created on show — and the look comes with it
+                window.Show();
+            }
+            finally
+            {
+                window.Close();
+            }
+        });
+    }
+
+    // The ordinary service takes the theme from the Windows settings
+    [Fact]
+    public void The_service_can_work_with_the_real_Windows_theme()
+    {
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(new AppSettings { Theme = AppTheme.System });
+
+            Assert.True(service.CurrentTheme is AppTheme.Light or AppTheme.Dark);
+            Assert.Equal(ThemeService.DetectSystemTheme(), service.CurrentTheme);
+        });
+    }
+
+    // Windows reports a look change from a thread other than the interface one
+    [Fact]
+    public void A_look_change_in_Windows_repaints_the_windows()
+    {
+        var settings = new AppSettings { Theme = AppTheme.System };
+        AppTheme system = AppTheme.Light;
+
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(settings, () => system);
+            Assert.Equal(AppTheme.Light, service.CurrentTheme);
+
+            system = AppTheme.Dark;
+            RaiseUserPreferenceChanged(service);
+
+            Sta.WaitFor(() => service.CurrentTheme == AppTheme.Dark, "the theme was recomputed at the request of Windows");
+        });
+    }
+
+    // Windows reports a look change even when nothing changed for the app:
+    // in that case there is nothing to repaint
+    [Fact]
+    public void The_same_theme_does_not_replace_the_resources()
+    {
+        var settings = new AppSettings { Theme = AppTheme.System };
+
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(settings, static () => AppTheme.Dark);
+
+            ResourceDictionary palette = Application.Current.Resources.MergedDictionaries[^1];
+            int count = Application.Current.Resources.MergedDictionaries.Count;
+
+            RaiseUserPreferenceChanged(service);
+            Sta.Pause(TimeSpan.FromMilliseconds(30));
+
+            Assert.Same(palette, Application.Current.Resources.MergedDictionaries[^1]);
+            Assert.Equal(count, Application.Current.Resources.MergedDictionaries.Count);
+        });
+    }
+
+    [Fact]
+    public void A_closed_window_is_forgotten()
+    {
+        var settings = new AppSettings { Theme = AppTheme.Light };
+
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(settings, static () => AppTheme.Light);
+            var window = new Window();
+            _ = new WindowInteropHelper(window).EnsureHandle();
+
+            service.Register(window);
+            window.Close();
+
+            // A theme change must no longer concern the closed window
+            settings.Theme = AppTheme.Dark;
+
+            Assert.Equal(AppTheme.Dark, service.CurrentTheme);
+        });
+    }
+
+    [Fact]
+    public void Closing_the_service_lets_go_of_the_attached_windows()
+    {
+        var settings = new AppSettings { Theme = AppTheme.Light };
+
+        Sta.Run(() =>
+        {
+            var service = new ThemeService(settings, static () => AppTheme.Light);
+            var window = new Window { Opacity = 0, ShowInTaskbar = false, ShowActivated = false };
+
+            try
+            {
+                window.Show();
+                service.Register(window);
+
+                service.Dispose();
+
+                // The window now closes on its own, with no regard for the theme
+                window.Close();
+            }
+            finally
+            {
+                window.Close();
+            }
+        });
+    }
+
+    [Fact]
+    public void After_the_service_is_closed_the_setting_no_longer_changes_the_theme()
+    {
+        var settings = new AppSettings { Theme = AppTheme.Light };
+
+        Sta.Run(() =>
+        {
+            var service = new ThemeService(settings, static () => AppTheme.Light);
+            service.Dispose();
+
+            settings.Theme = AppTheme.Dark;
+
+            Assert.Equal(AppTheme.Light, service.CurrentTheme);
+        });
+    }
+
+    [Fact]
+    public void After_the_service_is_closed_requests_from_Windows_go_unanswered()
+    {
+        var settings = new AppSettings { Theme = AppTheme.System };
+        AppTheme system = AppTheme.Light;
+
+        Sta.Run(() =>
+        {
+            var service = new ThemeService(settings, () => system);
+            service.Dispose();
+
+            system = AppTheme.Dark;
+            Sta.Pause(TimeSpan.FromMilliseconds(50));
+
+            Assert.Equal(AppTheme.Light, service.CurrentTheme);
+        });
+    }
+
+    [Theory]
+    [InlineData(AppTheme.Light)]
+    [InlineData(AppTheme.Dark)]
+    public void The_palette_of_each_theme_lives_in_the_application_assembly(AppTheme theme)
+    {
+        Sta.Run(() =>
+        {
+            var palette = new ResourceDictionary { Source = ThemeService.PaletteUri(theme) };
+
+            Assert.NotEmpty(palette.Keys);
+            Assert.True(palette.Contains("Theme.WindowBackground"));
+        });
+    }
+
+    // The palette address names the application assembly rather than the one
+    // the process started from
+    [Fact]
+    public void The_palette_address_names_the_application_assembly()
+    {
+        // The palettes live with the settings window, not with the agent next to it
+        Assert.Contains(
+            "CursorLang.Settings;component",
+            ThemeService.PaletteUri(AppTheme.Dark).ToString(),
+            StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void The_light_and_dark_palettes_share_one_set_of_keys()
+    {
+        Sta.Run(() =>
+        {
+            var light = new ResourceDictionary { Source = ThemeService.PaletteUri(AppTheme.Light) };
+            var dark = new ResourceDictionary { Source = ThemeService.PaletteUri(AppTheme.Dark) };
+
+            Assert.Equal(light.Keys.Cast<object>().OrderBy(key => key.ToString()),
+                dark.Keys.Cast<object>().OrderBy(key => key.ToString()));
+        });
+    }
+
+    [Fact]
+    public void The_Windows_theme_is_read_without_errors()
+    {
+        AppTheme theme = ThemeService.DetectSystemTheme();
+
+        // The "follow the system" setting has to yield something definite
+        Assert.True(theme is AppTheme.Light or AppTheme.Dark);
+    }
+
+    // A control the shared dictionary says nothing about keeps the look Windows
+    // gives it and stays light in the dark theme
+    [Theory]
+    [InlineData(typeof(Button))]
+    [InlineData(typeof(ComboBox))]
+    [InlineData(typeof(CheckBox))]
+    [InlineData(typeof(GroupBox))]
+    [InlineData(typeof(ProgressBar))]
+    [InlineData(typeof(Slider))]
+    public void A_control_of_the_window_is_repainted_together_with_the_theme(Type control)
+    {
+        var settings = new AppSettings { Theme = AppTheme.Light };
+
+        Sta.Run(() =>
+        {
+            using var service = new ThemeService(settings, static () => AppTheme.Light);
+
+            var window = new Window
+            {
+                Opacity = 0,
+                ShowInTaskbar = false,
+                ShowActivated = false,
+                Width = 200,
+                Height = 100,
+                Content = (Control)Activator.CreateInstance(control)!,
+            };
+
+            try
+            {
+                window.Show();
+                window.UpdateLayout();
+
+                IReadOnlyList<Color> light = PaintOf(window);
+                Assert.NotEmpty(light);
+
+                settings.Theme = AppTheme.Dark;
+                window.UpdateLayout();
+
+                Assert.NotEqual(light, PaintOf(window));
+            }
+            finally
+            {
+                window.Close();
+            }
+        });
+    }
+
+    // Windows reports a look change through an event that cannot be synthesised:
+    // the test goes straight to the handler that event arrives at
+    private static void RaiseUserPreferenceChanged(ThemeService service)
+    {
+        System.Reflection.MethodInfo handler = typeof(ThemeService)
+            .GetMethod("OnUserPreferenceChanged", System.Reflection.BindingFlags.Instance
+                | System.Reflection.BindingFlags.NonPublic)!;
+
+        handler.Invoke(service, [null, new UserPreferenceChangedEventArgs(UserPreferenceCategory.General)]);
+    }
+
+    private static Color WindowBackground() =>
+        ((SolidColorBrush)Application.Current.Resources["Theme.WindowBackground"]).Color;
+
+    /// <summary>
+    /// Every colour the element tree is painted with. What is compared is the
+    /// whole set: which part of a control the palette reaches is its own business.
+    /// </summary>
+    private static IReadOnlyList<Color> PaintOf(DependencyObject root)
+    {
+        var colours = new List<Color>();
+        Collect(root, colours);
+
+        return colours;
+    }
+
+    private static void Collect(DependencyObject node, List<Color> colours)
+    {
+        switch (node)
+        {
+            case Control control:
+                Add(colours, control.Background, control.BorderBrush, control.Foreground);
+                break;
+            case Border border:
+                Add(colours, border.Background, border.BorderBrush);
+                break;
+            case Shape shape:
+                Add(colours, shape.Fill, shape.Stroke);
+                break;
+            case TextBlock text:
+                Add(colours, text.Background, text.Foreground);
+                break;
+        }
+
+        int count = VisualTreeHelper.GetChildrenCount(node);
+
+        for (int i = 0; i < count; i++)
+        {
+            Collect(VisualTreeHelper.GetChild(node, i), colours);
+        }
+    }
+
+    private static void Add(List<Color> colours, params Brush?[] brushes) =>
+        colours.AddRange(brushes.OfType<SolidColorBrush>().Select(brush => brush.Color));
+}

CursorLang.Settings.Tests/ViewModels/EnumOptionTests.cs

@@ -0,0 +1,56 @@
+using CursorLang.Core.Models;
+using CursorLang.Settings.ViewModels;
+
+namespace CursorLang.Settings.Tests.ViewModels;
+
+public sealed class EnumOptionTests
+{
+    [Fact]
+    public void An_option_remembers_its_value_and_caption()
+    {
+        var option = new EnumOption<AppTheme>(AppTheme.Dark, "Dark theme");
+
+        Assert.Equal(AppTheme.Dark, option.Value);
+        Assert.Equal("Dark theme", option.Display);
+    }
+
+    [Fact]
+    public void A_changed_caption_is_announced_to_subscribers()
+    {
+        var option = new EnumOption<AppTheme>(AppTheme.Dark, "Dark");
+        List<string?> changed = [];
+        option.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        option.Display = "Dark theme";
+
+        Assert.Equal("Dark theme", option.Display);
+        Assert.Equal([nameof(EnumOption<>.Display)], changed);
+    }
+
+    [Fact]
+    public void The_same_caption_is_not_announced_again()
+    {
+        var option = new EnumOption<AppTheme>(AppTheme.Dark, "Dark theme");
+        List<string?> changed = [];
+        option.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        option.Display = "Dark theme";
+
+        Assert.Empty(changed);
+    }
+
+    // Accessibility tools take the name of a list item from here
+    [Fact]
+    public void An_option_presents_itself_by_its_caption()
+    {
+        Assert.Equal("Dark theme", new EnumOption<AppTheme>(AppTheme.Dark, "Dark theme").ToString());
+    }
+
+    [Fact]
+    public void A_changed_caption_changes_the_presentation_too()
+    {
+        var option = new EnumOption<ScreenPosition>(ScreenPosition.Center, "Center") { Display = "In the centre" };
+
+        Assert.Equal("In the centre", option.ToString());
+    }
+}

CursorLang.Settings.Tests/ViewModels/SettingsViewModelTests.cs

@@ -0,0 +1,304 @@
+using CursorLang.Core.Models;
+using CursorLang.Core.Services;
+using CursorLang.Settings.ViewModels;
+using CursorLang.Tests.Shared;
+
+namespace CursorLang.Settings.Tests.ViewModels;
+
+/// <summary>
+/// The settings window in terms of what it shows and what it is in charge of.
+/// </summary>
+public sealed class SettingsViewModelTests
+{
+    [Fact]
+    public void The_interface_language_comes_from_the_settings()
+    {
+        var settings = new AppSettings { Language = "ru" };
+        var localization = new FakeLocalizationService();
+
+        using var viewModel = Create(settings, localization);
+
+        Assert.Equal("ru", localization.CurrentLanguage);
+        Assert.Same(settings, viewModel.Settings);
+        Assert.Same(localization, viewModel.Localization);
+    }
+
+    [Fact]
+    public void Changing_the_language_setting_switches_the_interface()
+    {
+        var settings = new AppSettings { Language = "en" };
+        var localization = new FakeLocalizationService();
+        using var viewModel = Create(settings, localization);
+
+        settings.Language = "ru";
+
+        Assert.Equal("ru", localization.CurrentLanguage);
+    }
+
+    [Fact]
+    public void Other_settings_leave_the_language_alone()
+    {
+        var settings = new AppSettings { Language = "en" };
+        var localization = new FakeLocalizationService();
+        using var viewModel = Create(settings, localization);
+
+        settings.Current.FontSize = 30;
+
+        Assert.Equal("en", localization.CurrentLanguage);
+    }
+
+    [Fact]
+    public void The_lists_are_built_from_every_value_of_the_enums()
+    {
+        using SettingsViewModel viewModel = Create();
+
+        Assert.Equal(Enum.GetValues<AppTheme>(), viewModel.Themes.Select(option => option.Value));
+        Assert.Equal(Enum.GetValues<PopupPlacementMode>(), viewModel.PlacementModes.Select(option => option.Value));
+        Assert.Equal(Enum.GetValues<AnchorSide>(), viewModel.AnchorSides.Select(option => option.Value));
+        Assert.Equal(Enum.GetValues<ScreenPosition>(), viewModel.ScreenPositions.Select(option => option.Value));
+    }
+
+    // Next to the caret the popup only goes beside it: above or below is where the next
+    // line of the text is
+    [Fact]
+    public void The_caret_is_offered_two_sides_and_no_more()
+    {
+        using SettingsViewModel viewModel = Create();
+
+        Assert.Equal(
+            [CaretSide.Left, CaretSide.Right],
+            viewModel.CaretSides.Select(option => option.Value));
+    }
+
+    // A caption key is built from the type name and the value
+    [Fact]
+    public void The_option_captions_come_from_the_resources()
+    {
+        var localization = new FakeLocalizationService();
+        using var viewModel = Create(localization: localization);
+
+        Assert.Equal("en:AppTheme_System", viewModel.Themes[0].Display);
+        Assert.Contains("PopupPlacementMode_AtCursor", localization.RequestedKeys);
+    }
+
+    // If the list items were recreated, the ComboBox would drop the selected value
+    [Fact]
+    public void Changing_the_language_changes_the_captions_not_the_options()
+    {
+        var settings = new AppSettings { Language = "en" };
+        var localization = new FakeLocalizationService();
+        using var viewModel = Create(settings, localization);
+
+        EnumOption<AppTheme> first = viewModel.Themes[0];
+
+        settings.Language = "ru";
+
+        Assert.Same(first, viewModel.Themes[0]);
+        Assert.Equal("ru:AppTheme_System", first.Display);
+        Assert.Equal("ru:AnchorSide_TopLeft", viewModel.AnchorSides[0].Display);
+        Assert.Equal("ru:CaretSide_Left", viewModel.CaretSides[0].Display);
+        Assert.Equal("ru:ScreenPosition_TopLeft", viewModel.ScreenPositions[0].Display);
+        Assert.Equal("ru:PopupPlacementMode_AtCursor", viewModel.PlacementModes[0].Display);
+    }
+
+    [Fact]
+    public void The_background_and_text_palettes_are_non_empty_and_different()
+    {
+        using SettingsViewModel viewModel = Create();
+
+        Assert.NotEmpty(viewModel.BackgroundPalette);
+        Assert.NotEmpty(viewModel.TextPalette);
+        Assert.NotEqual(viewModel.BackgroundPalette, viewModel.TextPalette);
+    }
+
+    [Fact]
+    public void The_palettes_hold_no_duplicates()
+    {
+        using SettingsViewModel viewModel = Create();
+
+        Assert.Equal(viewModel.BackgroundPalette.Count, viewModel.BackgroundPalette.Distinct().Count());
+        Assert.Equal(viewModel.TextPalette.Count, viewModel.TextPalette.Distinct().Count());
+    }
+
+    [Fact]
+    public void The_default_colours_are_present_in_the_palettes()
+    {
+        var settings = new AppSettings();
+        using var viewModel = Create(settings);
+
+        Assert.Contains(settings.Current.BackgroundColor, viewModel.BackgroundPalette);
+        Assert.Contains(settings.Current.ForegroundColor, viewModel.TextPalette);
+    }
+
+    [Fact]
+    public void The_palettes_are_the_same_for_every_window()
+    {
+        using SettingsViewModel first = Create();
+        using SettingsViewModel second = Create();
+
+        Assert.Same(first.BackgroundPalette, second.BackgroundPalette);
+        Assert.Same(first.TextPalette, second.TextPalette);
+    }
+
+    [Fact]
+    public void The_background_palette_consists_of_colours()
+    {
+        using SettingsViewModel viewModel = Create();
+
+        Assert.All(viewModel.BackgroundPalette, color => Assert.IsType<System.Drawing.Color>(color));
+    }
+
+    [Fact]
+    public void Until_Windows_answers_the_startup_setting_stays_hidden()
+    {
+        using SettingsViewModel viewModel = Create();
+
+        Assert.False(viewModel.IsStartupAvailable);
+        Assert.False(viewModel.CanChangeStartup);
+        Assert.False(viewModel.IsStartupLocked);
+        Assert.False(viewModel.RunAtStartup);
+    }
+
+    [Theory]
+    [InlineData(StartupState.Enabled, true, true, false, true)]
+    [InlineData(StartupState.Disabled, true, true, false, false)]
+    [InlineData(StartupState.DisabledByUser, true, false, true, false)]
+    [InlineData(StartupState.DisabledByPolicy, true, false, true, false)]
+    [InlineData(StartupState.EnabledByPolicy, true, false, true, true)]
+    [InlineData(StartupState.Unavailable, false, false, false, false)]
+    public async Task The_startup_state_decides_how_the_setting_looks(
+        StartupState state, bool available, bool canChange, bool locked, bool enabled)
+    {
+        var startup = new FakeStartupService { State = state };
+        using var viewModel = Create(startup: startup);
+
+        await viewModel.InitializeAsync();
+
+        Assert.Equal(available, viewModel.IsStartupAvailable);
+        Assert.Equal(canChange, viewModel.CanChangeStartup);
+        Assert.Equal(locked, viewModel.IsStartupLocked);
+        Assert.Equal(enabled, viewModel.RunAtStartup);
+    }
+
+    [Fact]
+    public async Task The_startup_setting_is_announced_after_Windows_answers()
+    {
+        var startup = new FakeStartupService { State = StartupState.Enabled };
+        using var viewModel = Create(startup: startup);
+
+        List<string?> changed = [];
+        viewModel.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        await viewModel.InitializeAsync();
+
+        Assert.Contains(nameof(SettingsViewModel.RunAtStartup), changed);
+        Assert.Contains(nameof(SettingsViewModel.IsStartupAvailable), changed);
+        Assert.Contains(nameof(SettingsViewModel.CanChangeStartup), changed);
+        Assert.Contains(nameof(SettingsViewModel.IsStartupLocked), changed);
+    }
+
+    [Fact]
+    public async Task Enabling_startup_reaches_Windows()
+    {
+        var startup = new FakeStartupService { State = StartupState.Disabled };
+        using var viewModel = Create(startup: startup);
+        await viewModel.InitializeAsync();
+
+        viewModel.RunAtStartup = true;
+
+        await WaitForStartupRequests(startup, 1);
+        Assert.Equal([true], startup.Requests);
+        Assert.True(viewModel.RunAtStartup);
+    }
+
+    [Fact]
+    public async Task Disabling_startup_reaches_Windows()
+    {
+        var startup = new FakeStartupService { State = StartupState.Enabled };
+        using var viewModel = Create(startup: startup);
+        await viewModel.InitializeAsync();
+
+        viewModel.RunAtStartup = false;
+
+        await WaitForStartupRequests(startup, 1);
+        Assert.Equal([false], startup.Requests);
+        Assert.False(viewModel.RunAtStartup);
+    }
+
+    [Fact]
+    public async Task Setting_the_same_value_again_leaves_Windows_alone()
+    {
+        var startup = new FakeStartupService { State = StartupState.Enabled };
+        using var viewModel = Create(startup: startup);
+        await viewModel.InitializeAsync();
+
+        viewModel.RunAtStartup = true;
+
+        Assert.Empty(startup.Requests);
+    }
+
+    // A ban by the user is not for the app to argue with: the tick has to come back
+    [Fact]
+    public async Task A_refused_request_puts_the_tick_back()
+    {
+        var startup = new FakeStartupService
+        {
+            State = StartupState.Disabled,
+            AnswerOnEnable = StartupState.DisabledByUser,
+        };
+
+        using var viewModel = Create(startup: startup);
+        await viewModel.InitializeAsync();
+
+        List<string?> changed = [];
+        viewModel.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        viewModel.RunAtStartup = true;
+
+        await WaitForStartupRequests(startup, 1);
+
+        Assert.False(viewModel.RunAtStartup);
+        Assert.True(viewModel.IsStartupLocked);
+        Assert.Contains(nameof(SettingsViewModel.RunAtStartup), changed);
+    }
+
+    [Fact]
+    public void Closing_unsubscribes_from_the_settings_and_the_language()
+    {
+        var settings = new AppSettings { Language = "en" };
+        var localization = new FakeLocalizationService();
+        SettingsViewModel viewModel = Create(settings, localization);
+
+        EnumOption<AppTheme> option = viewModel.Themes[0];
+        string display = option.Display;
+
+        viewModel.Dispose();
+
+        settings.Language = "ru";
+
+        // Neither the interface language nor the option captions change any more
+        Assert.Equal("en", localization.CurrentLanguage);
+        Assert.Equal(display, option.Display);
+    }
+
+    private static SettingsViewModel Create(
+        AppSettings? settings = null,
+        ILocalizationService? localization = null,
+        IStartupService? startup = null,
+        Version? version = null) =>
+        new(settings ?? new AppSettings(),
+            localization ?? new FakeLocalizationService(),
+            startup ?? new FakeStartupService(),
+            version ?? new Version(1, 0, 0, 0));
+
+    // The setting travels to Windows without being awaited: the window must not freeze
+    private static async Task WaitForStartupRequests(FakeStartupService startup, int count)
+    {
+        for (int i = 0; i < 100 && startup.Requests.Count < count; i++)
+        {
+            await Task.Delay(5);
+        }
+
+        Assert.Equal(count, startup.Requests.Count);
+    }
+}

CursorLang.Settings.Tests/Views/ConvertersTests.cs

@@ -0,0 +1,171 @@
+using System.Globalization;
+using System.Windows;
+using System.Windows.Data;
+using System.Windows.Media;
+using CursorLang.Core.Models;
+using CursorLang.Settings.Views;
+using DrawingColor = System.Drawing.Color;
+
+namespace CursorLang.Settings.Tests.Views;
+
+/// <summary>
+/// The binding converters: they decide what the settings window shows and what
+/// it keeps out of sight — and they are the border between the colours the
+/// settings hold, which are GDI ones, and the brushes WPF paints with.
+/// </summary>
+public sealed class ConvertersTests
+{
+    private static readonly CultureInfo Culture = CultureInfo.InvariantCulture;
+
+    [Fact]
+    public void A_match_with_the_single_listed_value_shows_the_element()
+    {
+        var converter = new EnumToVisibilityConverter();
+
+        Assert.Equal(
+            Visibility.Visible,
+            converter.Convert(PopupPlacementMode.AtCursor, typeof(Visibility), "AtCursor", Culture));
+    }
+
+    // The anchor settings suit two placement modes at once
+    [Fact]
+    public void A_match_with_one_of_the_listed_values_shows_the_element()
+    {
+        var converter = new EnumToVisibilityConverter();
+
+        Assert.Equal(
+            Visibility.Visible,
+            converter.Convert(PopupPlacementMode.AtCaret, typeof(Visibility), "AtCursor, AtCaret", Culture));
+    }
+
+    [Fact]
+    public void No_match_hides_the_element()
+    {
+        var converter = new EnumToVisibilityConverter();
+
+        Assert.Equal(
+            Visibility.Collapsed,
+            converter.Convert(PopupPlacementMode.FixedPoint, typeof(Visibility), "AtCursor, AtCaret", Culture));
+    }
+
+    [Theory]
+    [InlineData(null, "AtCursor")]
+    [InlineData(PopupPlacementMode.AtCursor, null)]
+    [InlineData(PopupPlacementMode.AtCursor, "")]
+    [InlineData(PopupPlacementMode.AtCursor, ",,")]
+    [InlineData(null, null)]
+    public void Without_a_value_or_without_a_list_the_element_is_hidden(object? value, string? parameter)
+    {
+        var converter = new EnumToVisibilityConverter();
+
+        Assert.Equal(Visibility.Collapsed, converter.Convert(value, typeof(Visibility), parameter, Culture));
+    }
+
+    [Fact]
+    public void Extra_spaces_in_the_list_do_not_get_in_the_way()
+    {
+        var converter = new EnumToVisibilityConverter();
+
+        Assert.Equal(
+            Visibility.Visible,
+            converter.Convert(ScreenPosition.Center, typeof(Visibility), "  Top ,  Center  ", Culture));
+    }
+
+    [Fact]
+    public void Visibility_does_not_convert_back()
+    {
+        var converter = new EnumToVisibilityConverter();
+
+        Assert.Equal(
+            Binding.DoNothing,
+            converter.ConvertBack(Visibility.Visible, typeof(PopupPlacementMode), "AtCursor", Culture));
+    }
+
+    // The alpha is not shown: transparency is a setting of its own
+    [Fact]
+    public void A_colour_is_shown_with_six_digits()
+    {
+        var converter = new ColorToHexConverter();
+
+        Assert.Equal("#0A1B2C", converter.Convert(DrawingColor.FromArgb(0x0A, 0x1B, 0x2C), typeof(string), null, Culture));
+    }
+
+    [Fact]
+    public void A_semi_transparent_colour_is_shown_without_its_alpha()
+    {
+        var converter = new ColorToHexConverter();
+
+        Assert.Equal(
+            "#102030",
+            converter.Convert(DrawingColor.FromArgb(0x80, 0x10, 0x20, 0x30), typeof(string), null, Culture));
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("not a colour")]
+    [InlineData(42)]
+    public void Anything_that_is_not_a_colour_shows_as_an_empty_string(object? value)
+    {
+        var converter = new ColorToHexConverter();
+
+        Assert.Equal(string.Empty, converter.Convert(value, typeof(string), null, Culture));
+    }
+
+    [Fact]
+    public void The_colour_notation_does_not_convert_back()
+    {
+        var converter = new ColorToHexConverter();
+
+        Assert.Equal(Binding.DoNothing, converter.ConvertBack("#102030", typeof(DrawingColor), null, Culture));
+    }
+
+    [Fact]
+    public void A_colour_turns_into_a_brush()
+    {
+        var converter = new ColorToBrushConverter();
+        DrawingColor color = DrawingColor.FromArgb(0x10, 0x20, 0x30);
+
+        var brush = Assert.IsType<SolidColorBrush>(converter.Convert(color, typeof(Brush), null, Culture));
+
+        Assert.Equal(Color.FromArgb(color.A, color.R, color.G, color.B), brush.Color);
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("not a colour")]
+    public void Anything_that_is_not_a_colour_turns_into_a_transparent_brush(object? value)
+    {
+        var converter = new ColorToBrushConverter();
+
+        Assert.Same(Brushes.Transparent, converter.Convert(value, typeof(Brush), null, Culture));
+    }
+
+    // Picking a swatch in the list sends the colour back into the settings
+    [Fact]
+    public void A_brush_converts_back_into_a_colour()
+    {
+        var converter = new ColorToBrushConverter();
+        DrawingColor color = DrawingColor.FromArgb(0x10, 0x20, 0x30);
+        var brush = new SolidColorBrush(Color.FromArgb(color.A, color.R, color.G, color.B));
+
+        Assert.Equal(color, converter.ConvertBack(brush, typeof(DrawingColor), null, Culture));
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("not a brush")]
+    public void Anything_that_is_not_a_brush_does_not_convert_back(object? value)
+    {
+        var converter = new ColorToBrushConverter();
+
+        Assert.Equal(Binding.DoNothing, converter.ConvertBack(value, typeof(DrawingColor), null, Culture));
+    }
+
+    [Fact]
+    public void The_converters_are_fit_for_bindings()
+    {
+        Assert.IsAssignableFrom<IValueConverter>(new EnumToVisibilityConverter());
+        Assert.IsAssignableFrom<IValueConverter>(new ColorToHexConverter());
+        Assert.IsAssignableFrom<IValueConverter>(new ColorToBrushConverter());
+    }
+}