fixed tests

LFS turn on

.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 CursorLang.Tests/coverage.runsettings

.gitea/workflows/release.yml

@@ -0,0 +1,166 @@
+# 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 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
+
+      - 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 CursorLang.Tests/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
+
+      - name: Keep the packages
+        uses: actions/upload-artifact@v4
+        with:
+          name: msix-${{ steps.version.outputs.version }}
+          path: artifacts/packages/
+          if-no-files-found: error
+
+      # Gitea creates a release of its own for a pushed tag, so the release is
+      # looked up first and only made when it is not there
+      - 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" }
+
+          $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/packages -File) {
+              # 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
+          }

CursorLang.Tests/AppTests.cs

@@ -0,0 +1,119 @@
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.ViewModels;
+using CursorLang.Views;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace CursorLang.Tests;
+
+/// <summary>
+/// What the container is made of. The tests cannot build the application whole
+/// — it would raise windows 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(IUpdateService))]
+    [InlineData(typeof(UpdateOptions))]
+    [InlineData(typeof(IKeyboardLayoutService))]
+    [InlineData(typeof(ILayoutPopupService))]
+    [InlineData(typeof(ICapsLockHotkeyService))]
+    [InlineData(typeof(ILayoutPopupWindow))]
+    [InlineData(typeof(LayoutNotificationCoordinator))]
+    [InlineData(typeof(CapsLockSwitchCoordinator))]
+    [InlineData(typeof(LayoutPopupViewModel))]
+    [InlineData(typeof(SettingsViewModel))]
+    [InlineData(typeof(UpdateViewModel))]
+    [InlineData(typeof(LayoutPopupWindow))]
+    [InlineData(typeof(MainWindow))]
+    [InlineData(typeof(KeyboardLayoutOptions))]
+    [InlineData(typeof(AppSettings))]
+    public void Everything_the_app_needs_is_declared_in_the_container(Type service)
+    {
+        Assert.Contains(Describe(), descriptor => descriptor.ServiceType == service);
+    }
+
+    // The settings, the tooltip and the layout watch have to be shared by the
+    // whole application: a second copy of them would mean a second
+    // tooltip or lost settings
+    [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_tooltip_window_and_its_interface_are_one_window()
+    {
+        ServiceDescriptor window = Describe()
+            .Single(descriptor => descriptor.ServiceType == typeof(ILayoutPopupWindow));
+
+        Assert.NotNull(window.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.Tests/CursorLang.Tests.csproj

@@ -0,0 +1,61 @@
+<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>$(NoWarn);CS1591</NoWarn>
+        <IsPackable>false</IsPackable>
+        <IsTestProject>true</IsTestProject>
+        <NoWarn>$(NoWarn);IDE0130</NoWarn>
+    </PropertyGroup>
+
+    <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\CursorLang.csproj" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <AssemblyAttribute Include="System.Reflection.AssemblyMetadataAttribute">
+            <_Parameter1>CursorLangExecutable</_Parameter1>
+            <_Parameter2>$(MSBuildThisFileDirectory)..\CursorLang\bin\$(Configuration)\$(TargetFramework)\CursorLang.exe</_Parameter2>
+        </AssemblyAttribute>
+    </ItemGroup>
+
+    <ItemGroup>
+        <Content Include="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
+    </ItemGroup>
+
+    <PropertyGroup>
+        <MainWindowMarkup>$(MSBuildThisFileDirectory)..\CursorLang\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.Tests/EndToEndTests.cs

@@ -0,0 +1,192 @@
+using System.Diagnostics;
+using System.IO;
+using System.Reflection;
+using System.Runtime.InteropServices;
+using System.Text;
+
+namespace CursorLang.Tests;
+
+/// <summary>
+/// The application as a whole: the start, the single instance and the exit.
+/// </summary>
+/// <remarks>
+/// The application cannot be built inside the tests — it raises windows 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 class EndToEndTests
+{
+    private static readonly TimeSpan StartTimeout = TimeSpan.FromSeconds(30);
+    private static readonly TimeSpan ExitTimeout = TimeSpan.FromSeconds(15);
+
+    [Fact]
+    public void The_application_starts_and_shows_the_settings_window()
+    {
+        using Launch launch = Launch.Start();
+
+        Assert.NotEqual(IntPtr.Zero, launch.WaitForWindow());
+        Assert.False(launch.Process.HasExited);
+    }
+
+    // A second run raises no second window but shows the window of the running one
+    [Fact]
+    public void The_second_run_ends_by_itself()
+    {
+        using Launch launch = Launch.Start();
+        launch.WaitForWindow();
+
+        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);
+    }
+
+    [Fact]
+    public void Closing_the_window_ends_the_application()
+    {
+        using Launch launch = Launch.Start();
+        launch.WaitForWindow();
+
+        Assert.True(launch.Process.CloseMainWindow(), "the window did not accept the request to close");
+        Assert.True(launch.Process.WaitForExit(ExitTimeout), "the application did not end after the window closed");
+        Assert.Equal(0, launch.Process.ExitCode);
+    }
+
+    /// <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 application first — making sure the place is free.</summary>
+        internal static Launch Start()
+        {
+            if (!HasInteractiveDesktop())
+            {
+                Assert.Skip("There is no interactive desktop here — the application has nowhere to show its window");
+            }
+
+            if (Process.GetProcessesByName("CursorLang").Length > 0)
+            {
+                Assert.Skip("The application is already running — this check keeps out of someone else's run");
+            }
+
+            return new Launch(StartProcess());
+        }
+
+        /// <summary>Starts the application the way the user does.</summary>
+        internal static Process StartProcess()
+        {
+            string path = ExecutablePath();
+
+            if (!File.Exists(path))
+            {
+                Assert.Skip($"The application is not built: {path}");
+            }
+
+            return Process.Start(new ProcessStartInfo(path) { UseShellExecute = true })!;
+        }
+
+        /// <summary>
+        /// Waits for the settings window: by the time it appears the application
+        /// has raised its whole cast.
+        /// </summary>
+        internal IntPtr WaitForWindow()
+        {
+            DateTime deadline = DateTime.UtcNow + StartTimeout;
+
+            while (DateTime.UtcNow < deadline)
+            {
+                Process.Refresh();
+
+                if (Process.HasExited)
+                {
+                    Assert.Fail($"The application exited while starting with code {Process.ExitCode}");
+                }
+
+                if (Process.MainWindowHandle != IntPtr.Zero)
+                {
+                    return Process.MainWindowHandle;
+                }
+
+                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()
+        {
+            try
+            {
+                if (!Process.HasExited)
+                {
+                    // The polite way first — that way the app gets to save its settings
+                    if (!Process.CloseMainWindow() || !Process.WaitForExit(ExitTimeout))
+                    {
+                        Process.Kill(entireProcessTree: true);
+                        Process.WaitForExit(ExitTimeout);
+                    }
+                }
+            }
+            catch (InvalidOperationException)
+            {
+                // The process has already ended on its own
+            }
+            finally
+            {
+                Process.Dispose();
+            }
+        }
+    }
+}

CursorLang.Tests/GlobalUsings.cs

@@ -0,0 +1 @@
+global using Xunit;

CursorLang.Tests/Infrastructure/FakeHttp.cs

@@ -0,0 +1,41 @@
+using System.Net;
+using System.Net.Http;
+using System.Text;
+
+namespace CursorLang.Tests.Infrastructure;
+
+/// <summary>
+/// The network as the test writes it: the answer is decided here rather than
+/// by a repository somewhere.
+/// </summary>
+internal sealed class FakeHttpHandler : HttpMessageHandler
+{
+    private readonly Func<HttpRequestMessage, HttpResponseMessage> _reply;
+
+    internal FakeHttpHandler(Func<HttpRequestMessage, HttpResponseMessage> reply) => _reply = reply;
+
+    internal List<HttpRequestMessage> Requests { get; } = [];
+
+    internal static FakeHttpHandler Json(string json) => new(_ => new HttpResponseMessage(HttpStatusCode.OK)
+    {
+        Content = new StringContent(json, Encoding.UTF8, "application/json"),
+    });
+
+    internal static FakeHttpHandler Status(HttpStatusCode status) =>
+        new(_ => new HttpResponseMessage(status));
+
+    internal static FakeHttpHandler Bytes(byte[] content) => new(_ => new HttpResponseMessage(HttpStatusCode.OK)
+    {
+        Content = new ByteArrayContent(content),
+    });
+
+    internal HttpClient CreateClient() => new(this);
+
+    protected override Task<HttpResponseMessage> SendAsync(
+        HttpRequestMessage request,
+        CancellationToken cancellationToken)
+    {
+        Requests.Add(request);
+        return Task.FromResult(_reply(request));
+    }
+}

CursorLang.Tests/Infrastructure/Fakes.cs

@@ -0,0 +1,289 @@
+using System.ComponentModel;
+using System.Net.Http;
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.ViewModels;
+
+namespace CursorLang.Tests.Infrastructure;
+
+/// <summary>
+/// Parts every test needs but few tests care about.
+/// </summary>
+internal static class Fake
+{
+    internal static UpdateViewModel Updates() =>
+        new(new FakeUpdateService(), new FakeLocalizationService(), new AppSettings(), new UpdateOptions());
+}
+
+/// <summary>
+/// The keyboard layout, with the test in charge of it.
+/// </summary>
+internal sealed class FakeKeyboardLayoutService : IKeyboardLayoutService
+{
+    internal int StartCalls { get; private set; }
+
+    internal int StopCalls { get; private set; }
+
+    internal int SwitchCalls { get; private set; }
+
+    internal KeyboardLayout CurrentLayout { get; set; } = KeyboardLayout.FromLocaleId(0x0409);
+
+    public KeyboardLayout Current => CurrentLayout;
+
+    public event EventHandler<LayoutChangedEventArgs>? LayoutChanged;
+
+    public void Start() => StartCalls++;
+
+    public void Stop() => StopCalls++;
+
+    public void SwitchToNext() => SwitchCalls++;
+
+    internal void RaiseLayoutChanged(KeyboardLayout layout, LayoutChangeReason reason) =>
+        LayoutChanged?.Invoke(this, new LayoutChangedEventArgs(layout, reason));
+
+    internal bool HasSubscribers => LayoutChanged is not null;
+}
+
+/// <summary>
+/// A tooltip that pops up nowhere and merely remembers what it was asked for.
+/// </summary>
+internal sealed class FakeLayoutPopupService : ILayoutPopupService
+{
+    internal List<KeyboardLayout> Shown { get; } = [];
+
+    internal List<KeyboardLayout> ShownUntilHidden { get; } = [];
+
+    internal int HideCalls { get; private set; }
+
+    public void Show(KeyboardLayout layout) => Shown.Add(layout);
+
+    public void ShowUntilHidden(KeyboardLayout layout) => ShownUntilHidden.Add(layout);
+
+    public void Hide() => HideCalls++;
+}
+
+/// <summary>
+/// The Caps Lock interception without intercepting anything: the test supplies the presses.
+/// </summary>
+internal sealed class FakeCapsLockHotkeyService : ICapsLockHotkeyService
+{
+    public event EventHandler? Tapped;
+
+    public event EventHandler? HoldStarted;
+
+    public event EventHandler? HoldEnded;
+
+    public bool IsRunning { get; private set; }
+
+    internal int StartCalls { get; private set; }
+
+    internal int StopCalls { get; private set; }
+
+    public void Start()
+    {
+        StartCalls++;
+        IsRunning = true;
+    }
+
+    public void Stop()
+    {
+        StopCalls++;
+        IsRunning = false;
+    }
+
+    internal void RaiseTapped() => Tapped?.Invoke(this, EventArgs.Empty);
+
+    internal void RaiseHoldStarted() => HoldStarted?.Invoke(this, EventArgs.Empty);
+
+    internal void RaiseHoldEnded() => HoldEnded?.Invoke(this, EventArgs.Empty);
+
+    internal bool HasSubscribers => Tapped is not null || HoldStarted is not null || HoldEnded is not null;
+}
+
+/// <summary>
+/// Startup whose state the test assigns.
+/// </summary>
+internal sealed class FakeStartupService : IStartupService
+{
+    internal StartupState State { get; set; } = StartupState.Disabled;
+
+    internal StartupState? AnswerOnEnable { get; set; }
+
+    internal List<bool> Requests { get; } = [];
+
+    internal int GetStateCalls { get; private set; }
+
+    public Task<StartupState> GetStateAsync()
+    {
+        GetStateCalls++;
+        return Task.FromResult(State);
+    }
+
+    public Task<StartupState> SetEnabledAsync(bool enabled)
+    {
+        Requests.Add(enabled);
+
+        State = enabled
+            ? AnswerOnEnable ?? StartupState.Enabled
+            : StartupState.Disabled;
+
+        return Task.FromResult(State);
+    }
+}
+
+/// <summary>
+/// Releases the test writes itself, with no repository behind them.
+/// </summary>
+internal sealed class FakeUpdateService : IUpdateService
+{
+    internal ReleaseInfo? Release { get; set; }
+
+    internal Exception? Failure { get; set; }
+
+    internal TaskCompletionSource? DownloadGate { get; set; }
+
+    internal string PackagePath { get; set; } = string.Empty;
+
+    internal int CheckCalls { get; private set; }
+
+    internal List<string> Installed { get; } = [];
+
+    public bool IsSupported { get; set; } = true;
+
+    public Version CurrentVersion { get; set; } = new(1, 0, 0, 0);
+
+    public Task<ReleaseInfo?> CheckAsync(CancellationToken cancellationToken)
+    {
+        CheckCalls++;
+
+        return Failure is null
+            ? Task.FromResult(Release)
+            : Task.FromException<ReleaseInfo?>(Failure);
+    }
+
+    public async Task<string> DownloadAsync(
+        ReleaseInfo release,
+        IProgress<double>? progress,
+        CancellationToken cancellationToken)
+    {
+        if (DownloadGate is not null)
+        {
+            await DownloadGate.Task.WaitAsync(cancellationToken);
+        }
+
+        if (Failure is not null)
+        {
+            throw Failure;
+        }
+
+        progress?.Report(0.5);
+        return PackagePath;
+    }
+
+    public void Install(string packagePath) => Installed.Add(packagePath);
+}
+
+/// <summary>
+/// A release list the test fills in, with no repository behind it.
+/// </summary>
+internal sealed class FakeReleaseFeed : IReleaseFeed
+{
+    internal ReleaseInfo? Release { get; set; }
+
+    internal Exception? Failure { get; set; }
+
+    internal List<HttpRequestMessage> Authorized { get; } = [];
+
+    public Task<ReleaseInfo?> GetLatestAsync(CancellationToken cancellationToken) =>
+        Failure is null ? Task.FromResult(Release) : Task.FromException<ReleaseInfo?>(Failure);
+
+    public void Authorize(HttpRequestMessage request) => Authorized.Add(request);
+}
+
+/// <summary>
+/// Interface strings without resources: the key comes back as is, tagged with the language.
+/// </summary>
+internal sealed class FakeLocalizationService : ILocalizationService
+{
+    private string _currentLanguage = "en";
+
+    public event PropertyChangedEventHandler? PropertyChanged;
+
+    internal List<string> RequestedKeys { get; } = [];
+
+    public string this[string key]
+    {
+        get
+        {
+            RequestedKeys.Add(key);
+            return $"{_currentLanguage}:{key}";
+        }
+    }
+
+    public IReadOnlyList<LanguageOption> AvailableLanguages { get; } =
+    [
+        new("en", "English"),
+        new("ru", "Русский"),
+    ];
+
+    public string CurrentLanguage
+    {
+        get => _currentLanguage;
+        set
+        {
+            if (string.IsNullOrWhiteSpace(value) || value == _currentLanguage)
+            {
+                return;
+            }
+
+            _currentLanguage = value;
+
+            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(CurrentLanguage)));
+            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(System.Windows.Data.Binding.IndexerName));
+        }
+    }
+}
+
+/// <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);
+}
+
+/// <summary>
+/// A tooltip window that shows nothing.
+/// </summary>
+internal sealed class FakeLayoutPopupWindow : ILayoutPopupWindow
+{
+    internal int ShowCalls { get; private set; }
+
+    internal int HideCalls { get; private set; }
+
+    internal int CloseCalls { get; private set; }
+
+    internal List<string> Calls { get; } = [];
+
+    public void ShowPopup()
+    {
+        ShowCalls++;
+        Calls.Add("show");
+    }
+
+    public void Hide()
+    {
+        HideCalls++;
+        Calls.Add("hide");
+    }
+
+    public void Close()
+    {
+        CloseCalls++;
+        Calls.Add("close");
+    }
+}

CursorLang.Tests/Infrastructure/Sta.cs

@@ -0,0 +1,179 @@
+using System.Windows;
+using System.Windows.Threading;
+
+namespace CursorLang.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;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.Tests/Infrastructure/TempFolder.cs

@@ -0,0 +1,39 @@
+using System.IO;
+
+namespace CursorLang.Tests.Infrastructure;
+
+/// <summary>
+/// An empty folder for the lifetime of one test. Settings live in a file, and
+/// working with that file has to be verified where nothing is worth losing.
+/// </summary>
+internal sealed class TempFolder : IDisposable
+{
+    internal TempFolder()
+    {
+        Path = System.IO.Path.Combine(
+            System.IO.Path.GetTempPath(),
+            "CursorLang.Tests",
+            Guid.NewGuid().ToString("N"));
+
+        Directory.CreateDirectory(Path);
+    }
+
+    internal string Path { get; }
+
+    internal string File(string name) => System.IO.Path.Combine(Path, name);
+
+    public void Dispose()
+    {
+        try
+        {
+            if (Directory.Exists(Path))
+            {
+                Directory.Delete(Path, recursive: true);
+            }
+        }
+        catch (Exception e) when (e is IOException or UnauthorizedAccessException)
+        {
+            // Litter in the temp folder is no reason to fail a test that passed
+        }
+    }
+}

CursorLang.Tests/Infrastructure/TempRegistryKey.cs

@@ -0,0 +1,36 @@
+using Microsoft.Win32;
+
+namespace CursorLang.Tests.Infrastructure;
+
+/// <summary>
+/// A registry key of one test's own. Startup outside a package lives in the
+/// registry, and the real startup list of the user is no place to experiment.
+/// </summary>
+internal sealed class TempRegistryKey : IDisposable
+{
+    private const string Parent = @"Software\CursorLang.Tests";
+
+    private readonly string _path;
+
+    internal TempRegistryKey()
+    {
+        _path = $@"{Parent}\{Guid.NewGuid():N}";
+        Key = Registry.CurrentUser.CreateSubKey(_path);
+    }
+
+    internal RegistryKey Key { get; }
+
+    public void Dispose()
+    {
+        Key.Dispose();
+
+        try
+        {
+            Registry.CurrentUser.DeleteSubKeyTree(_path, throwOnMissingSubKey: false);
+        }
+        catch (Exception e) when (e is System.Security.SecurityException or UnauthorizedAccessException)
+        {
+            // A leftover key is no reason to fail a test that passed
+        }
+    }
+}

CursorLang.Tests/Interop/CaretNativeTests.cs

@@ -0,0 +1,106 @@
+using CursorLang.Interop;
+using CursorLang.Tests.Infrastructure;
+
+namespace CursorLang.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 = Sta.Run(CaretNative.TryGetCaretRect);
+
+        if (caret is not null)
+        {
+            Assert.True(caret.Value.Bottom > caret.Value.Top);
+        }
+    }
+}

CursorLang.Tests/Interop/ForegroundWindowTests.cs

@@ -0,0 +1,181 @@
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Interop;
+using CursorLang.Interop;
+using CursorLang.Models;
+using CursorLang.Tests.Infrastructure;
+using CursorLang.ViewModels;
+using CursorLang.Views;
+
+namespace CursorLang.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");
+            }
+
+            var settings = new AppSettings
+            {
+                PlacementMode = PopupPlacementMode.AtCaret,
+                CaretSide = AnchorSide.BottomRight,
+                CaretOffset = 8,
+            };
+
+            var viewModel = new LayoutPopupViewModel(settings) { ShortName = "RU" };
+            var popup = new LayoutPopupWindow(viewModel, settings);
+
+            try
+            {
+                popup.ShowPopup();
+
+                PopupWindowNative.Rect bounds =
+                    WindowPlacementNative.TryGetBounds(new WindowInteropHelper(popup).Handle)!.Value;
+
+                // The tooltip landed to the right of and below the caret — as asked
+                Assert.True(bounds.Left >= caret.Value.Right);
+                Assert.True(bounds.Top >= caret.Value.Bottom);
+            }
+            finally
+            {
+                popup.Close();
+            }
+        });
+    }
+
+    // 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.Tests/Interop/LowLevelKeyboardHookTests.cs

@@ -0,0 +1,209 @@
+using System.Reflection;
+using System.Runtime.InteropServices;
+using CursorLang.Interop;
+using CursorLang.Tests.Infrastructure;
+
+namespace CursorLang.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()
+    {
+        Sta.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()
+    {
+        Sta.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()
+    {
+        Sta.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.Tests/Interop/NativeWrappersTests.cs

@@ -0,0 +1,243 @@
+using System.Windows;
+using System.Windows.Interop;
+using CursorLang.Interop;
+using CursorLang.Tests.Infrastructure;
+
+namespace CursorLang.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_window_bounds_are_set_as_a_whole()
+    {
+        Sta.Run(() =>
+        {
+            using var window = new HandleWindow();
+
+            PopupWindowNative.SetBounds(window.Handle, 60, 70, 320, 240);
+
+            PopupWindowNative.Rect bounds = WindowPlacementNative.TryGetBounds(window.Handle)!.Value;
+            Assert.Equal(60, bounds.Left);
+            Assert.Equal(70, bounds.Top);
+            Assert.Equal(380, bounds.Right);
+            Assert.Equal(310, bounds.Bottom);
+        });
+    }
+
+    [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 A_window_becomes_invisible_to_the_focus_and_the_switcher()
+    {
+        Sta.Run(() =>
+        {
+            using var window = new HandleWindow();
+
+            PopupWindowNative.MakePassive(window.Handle);
+
+            // Checked through the same wrapper: the style has to stick and not
+            // be reset by a repeated call
+            PopupWindowNative.MakePassive(window.Handle);
+        });
+    }
+
+    [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.Tests/Models/AppSettingsTests.cs

@@ -0,0 +1,165 @@
+using System.ComponentModel;
+using System.Reflection;
+using System.Text.Json;
+using System.Windows.Media;
+using CursorLang.Models;
+
+namespace CursorLang.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(AnchorSide.BottomRight, settings.CursorSide);
+        Assert.Equal(16, settings.CursorOffset);
+        Assert.Equal(AnchorSide.BottomRight, settings.CaretSide);
+        Assert.Equal(16, settings.CaretOffset);
+        Assert.Equal(ScreenPosition.BottomRight, settings.ScreenPosition);
+        Assert.Equal(24, settings.ScreenMargin);
+        Assert.Equal(20, settings.FontSize);
+        Assert.Equal(0.9, settings.Opacity);
+        Assert.Equal(500, settings.DurationMilliseconds);
+        Assert.Equal(300, settings.CapsLockHoldMilliseconds);
+        Assert.Equal(Color.FromRgb(0x20, 0x20, 0x20), settings.BackgroundColor);
+        Assert.Equal(Color.FromRgb(0xFF, 0xFF, 0xFF), settings.ForegroundColor);
+    }
+
+    // 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);
+    }
+
+    // Duration and CapsLockHoldDelay are derived from other settings and have
+    // no business being in the file
+    [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);
+
+        // 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);
+    }
+
+    [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>
+    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.FromRgb((byte)(color.R + 1), color.G, color.B),
+            Enum value => NextEnumValue(value),
+            DateTimeOffset moment => moment.AddDays(1),
+
+            // A setting never set yet: the app has not checked for updates once
+            null when property.PropertyType == typeof(DateTimeOffset?) => DateTimeOffset.UnixEpoch,
+
+            _ => 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.Tests/Models/KeyboardLayoutTests.cs

@@ -0,0 +1,76 @@
+using System.Globalization;
+using CursorLang.Models;
+
+namespace CursorLang.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.Tests/Models/LayoutChangedEventArgsTests.cs

@@ -0,0 +1,27 @@
+using CursorLang.Models;
+
+namespace CursorLang.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.Tests/Resources/StringsTests.cs

@@ -0,0 +1,178 @@
+using System.Collections;
+using System.Globalization;
+using System.IO;
+using System.Reflection;
+using System.Resources;
+using System.Text.RegularExpressions;
+using CursorLang.Models;
+
+namespace CursorLang.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 partial class StringsTests
+{
+    private static readonly ResourceManager Resources =
+        new("CursorLang.Resources.Strings", typeof(App).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>
+    /// Every key the settings window markup asks for has to exist in the
+    /// resources: otherwise the user sees the key itself in its place.
+    /// </summary>
+    [Fact]
+    public void Every_key_from_the_settings_window_markup_exists_in_the_resources()
+    {
+        HashSet<string> known = [.. NeutralKeys()];
+        List<string> missing = [];
+
+        foreach (Match match in LocalizationBinding().Matches(ReadSettingsWindowMarkup()))
+        {
+            string key = match.Groups["key"].Value;
+            if (!known.Contains(key))
+            {
+                missing.Add(key);
+            }
+        }
+
+        Assert.Empty(missing);
+    }
+
+    /// <summary>
+    /// The version of an update is put into the string by the app, so the place
+    /// for it has to be there in both languages.
+    /// </summary>
+    [Fact]
+    public void The_string_about_an_available_update_has_room_for_the_version()
+    {
+        Assert.Contains("{0}", Resources.GetString("UpdateAvailable", English), StringComparison.Ordinal);
+        Assert.Contains("{0}", Resources.GetString("UpdateAvailable", Russian), StringComparison.Ordinal);
+    }
+
+    /// <summary>The markup does ask for strings — otherwise the check above means nothing.</summary>
+    [Fact]
+    public void The_settings_window_markup_asks_for_resource_strings()
+    {
+        Assert.NotEmpty(LocalizationBinding().Matches(ReadSettingsWindowMarkup()));
+    }
+
+    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<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)!;
+
+    private static string ReadSettingsWindowMarkup()
+    {
+        using Stream stream = Assembly.GetExecutingAssembly().GetManifestResourceStream("MainWindow.xaml")
+            ?? throw new InvalidOperationException("The settings window markup is not embedded in the test assembly");
+
+        using var reader = new StreamReader(stream);
+        return reader.ReadToEnd();
+    }
+
+    [GeneratedRegex(@"Localization\[(?<key>\w+)\]")]
+    private static partial Regex LocalizationBinding();
+}

CursorLang.Tests/Services/CapsLockHotkeyServiceTests.cs

@@ -0,0 +1,274 @@
+using System.Collections.Concurrent;
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+
+namespace CursorLang.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(Sta.Run(() => harness.Service.HandleKeyEvent(CapsLock, isKeyDown: true)));
+        Assert.True(Sta.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(Sta.Run(() => harness.Service.HandleKeyEvent(LetterA, isKeyDown: true)));
+        Assert.False(Sta.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();
+
+        Sta.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();
+
+        Sta.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();
+        Sta.WaitFor(() => harness.Events.Contains("hold-start"), "the hold threshold was passed");
+
+        harness.Release();
+
+        Sta.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++)
+        {
+            Sta.Pause(TimeSpan.FromMilliseconds(10));
+            harness.Press();
+        }
+
+        Sta.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();
+
+        Sta.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();
+        Sta.WaitFor(() => harness.Events.Contains("hold-start"), "the hold threshold was passed");
+
+        Sta.Run(harness.Service.Stop);
+
+        Sta.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();
+        Sta.Run(harness.Service.Stop);
+        Sta.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();
+        Sta.Run(harness.Service.Stop);
+
+        Sta.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();
+        Sta.Run(harness.Service.Stop);
+
+        harness.Press();
+        harness.Release();
+
+        Sta.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();
+        Sta.WaitFor(() => harness.Events.Contains("hold-start"), "the hold threshold was passed");
+
+        Sta.Run(harness.Service.Dispose);
+        Sta.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();
+        Sta.WaitFor(() => harness.Events.Count == 1, "the first press counted as short");
+
+        harness.Settings.CapsLockHoldMilliseconds = 20;
+        harness.Press();
+
+        Sta.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);
+        Sta.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);
+
+        Sta.Run(() =>
+        {
+            harness.Service.Start();
+            harness.Service.Dispose();
+
+            Assert.False(harness.Service.IsRunning);
+        });
+    }
+
+    /// <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 };
+
+            // The service remembers the dispatcher of the thread it was created on
+            CapsLockHotkeyService service = Sta.Run(() => new CapsLockHotkeyService(settings));
+            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() => Sta.Run(() => Service.HandleKeyEvent(CapsLock, isKeyDown: true));
+
+        internal void Release() => Sta.Run(() => Service.HandleKeyEvent(CapsLock, isKeyDown: false));
+
+        public void Dispose() => Sta.Run(Service.Dispose);
+    }
+}

CursorLang.Tests/Services/CapsLockSwitchCoordinatorTests.cs

@@ -0,0 +1,192 @@
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+
+namespace CursorLang.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.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.Tests/Services/GiteaReleaseFeedTests.cs

@@ -0,0 +1,305 @@
+using System.Net;
+using System.Net.Http;
+using System.Runtime.InteropServices;
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+
+namespace CursorLang.Tests.Services;
+
+/// <summary>
+/// Reading the release list of Gitea. The answer of the server is not ours to
+/// shape, so what matters is what the app makes of it.
+/// </summary>
+public sealed class GiteaReleaseFeedTests
+{
+    private const string Releases = """
+        [
+          {
+            "tag_name": "v1.2.0",
+            "draft": false,
+            "prerelease": false,
+            "html_url": "https://git.alrakis.kz/alrakis/cursor-lang/releases/tag/v1.2.0",
+            "assets": [
+              {
+                "name": "CursorLang-1.2.0.0.msixbundle",
+                "browser_download_url": "https://git.alrakis.kz/attachments/CursorLang-1.2.0.0.msixbundle",
+                "size": 4096
+              }
+            ]
+          }
+        ]
+        """;
+
+    [Fact]
+    public async Task A_release_is_read_whole()
+    {
+        ReleaseInfo? release = await Read(Releases);
+
+        Assert.NotNull(release);
+        Assert.Equal(new Version(1, 2, 0, 0), release.Version);
+        Assert.Equal("v1.2.0", release.Tag);
+        Assert.Equal("https://git.alrakis.kz/alrakis/cursor-lang/releases/tag/v1.2.0", release.PageUrl?.ToString());
+        Assert.Equal("CursorLang-1.2.0.0.msixbundle", release.Package.FileName);
+        Assert.Equal(
+            "https://git.alrakis.kz/attachments/CursorLang-1.2.0.0.msixbundle",
+            release.Package.Url.ToString());
+        Assert.Equal(4096, release.Package.Size);
+    }
+
+    // The API of Gitea lives on the server itself, next to the pages of the
+    // repository
+    [Fact]
+    public async Task The_request_goes_to_the_releases_of_the_project()
+    {
+        var handler = FakeHttpHandler.Json(Releases);
+        var feed = new GiteaReleaseFeed(handler.CreateClient(), Options());
+
+        await feed.GetLatestAsync(TestContext.Current.CancellationToken);
+
+        Uri asked = Assert.Single(handler.Requests).RequestUri!;
+        Assert.Equal("git.alrakis.kz", asked.Host);
+        Assert.StartsWith(
+            "/api/v1/repos/alrakis/cursor-lang/releases", asked.AbsolutePath, StringComparison.Ordinal);
+    }
+
+    // A server sitting under a path of its own keeps that path: dropping it
+    // would send the request to a place that answers nothing
+    [Fact]
+    public async Task A_server_behind_a_path_keeps_it()
+    {
+        var handler = FakeHttpHandler.Json(Releases);
+        var feed = new GiteaReleaseFeed(
+            handler.CreateClient(),
+            new UpdateOptions { ServiceUri = new Uri("https://host.example.com/gitea"), Project = "team/app" });
+
+        await feed.GetLatestAsync(TestContext.Current.CancellationToken);
+
+        Uri asked = Assert.Single(handler.Requests).RequestUri!;
+        Assert.StartsWith("/gitea/api/v1/repos/team/app/releases", asked.AbsolutePath, StringComparison.Ordinal);
+    }
+
+    // «token» is the scheme of Gitea for keys of access
+    [Fact]
+    public void A_closed_repository_gets_the_token_it_asks_for()
+    {
+        var feed = new GiteaReleaseFeed(new HttpClient(), Options("secret"));
+        using var request = new HttpRequestMessage();
+
+        feed.Authorize(request);
+
+        Assert.Equal("token", request.Headers.Authorization?.Scheme);
+        Assert.Equal("secret", request.Headers.Authorization?.Parameter);
+    }
+
+    [Fact]
+    public void An_open_repository_is_asked_without_a_token()
+    {
+        var feed = new GiteaReleaseFeed(new HttpClient(), Options());
+        using var request = new HttpRequestMessage();
+
+        feed.Authorize(request);
+
+        Assert.Null(request.Headers.Authorization);
+    }
+
+    [Theory]
+    [InlineData("1.2.3", "1.2.3.0")]
+    [InlineData("v1.2.3", "1.2.3.0")]
+    [InlineData("V1.2", "1.2.0.0")]
+    [InlineData("1.2.3.4", "1.2.3.4")]
+    public async Task A_version_is_read_out_of_the_tag(string tag, string expected)
+    {
+        ReleaseInfo? release = await Read(WithTag(tag));
+
+        Assert.NotNull(release);
+        Assert.Equal(Version.Parse(expected), release.Version);
+    }
+
+    // A pre-release version is not something the app offers by itself:
+    // such a version is asked for on purpose
+    [Theory]
+    [InlineData("v1.2.3-beta")]
+    [InlineData("nightly")]
+    [InlineData("release-1")]
+    public async Task A_tag_that_is_not_a_version_is_passed_over(string tag)
+    {
+        Assert.Null(await Read(WithTag(tag)));
+    }
+
+    [Fact]
+    public async Task A_draft_and_a_pre_release_are_passed_over()
+    {
+        const string releases = """
+            [
+              { "tag_name": "v3.0.0", "draft": true, "assets": [
+                { "name": "a.msixbundle", "browser_download_url": "https://host/a.msixbundle" } ] },
+              { "tag_name": "v2.0.0", "prerelease": true, "assets": [
+                { "name": "b.msixbundle", "browser_download_url": "https://host/b.msixbundle" } ] },
+              { "tag_name": "v1.0.0", "assets": [
+                { "name": "c.msixbundle", "browser_download_url": "https://host/c.msixbundle" } ] }
+            ]
+            """;
+
+        ReleaseInfo? release = await Read(releases);
+
+        Assert.NotNull(release);
+        Assert.Equal(new Version(1, 0, 0, 0), release.Version);
+    }
+
+    // The order of the releases belongs to the server, the highest number to
+    // the app: a fix to an older branch can be the freshest release
+    [Fact]
+    public async Task The_highest_version_wins_over_the_order_of_the_answer()
+    {
+        const string releases = """
+            [
+              { "tag_name": "v1.0.5", "assets": [
+                { "name": "a.msixbundle", "browser_download_url": "https://host/a.msixbundle" } ] },
+              { "tag_name": "v2.0.0", "assets": [
+                { "name": "b.msixbundle", "browser_download_url": "https://host/b.msixbundle" } ] }
+            ]
+            """;
+
+        ReleaseInfo? release = await Read(releases);
+
+        Assert.NotNull(release);
+        Assert.Equal(new Version(2, 0, 0, 0), release.Version);
+    }
+
+    [Fact]
+    public async Task A_release_without_a_package_is_passed_over()
+    {
+        const string releases = """
+            [
+              { "tag_name": "v2.0.0", "assets": [
+                { "name": "notes.txt", "browser_download_url": "https://host/notes.txt" } ] },
+              { "tag_name": "v1.0.0", "assets": [
+                { "name": "a.msixbundle", "browser_download_url": "https://host/a.msixbundle" } ] }
+            ]
+            """;
+
+        ReleaseInfo? release = await Read(releases);
+
+        Assert.NotNull(release);
+        Assert.Equal(new Version(1, 0, 0, 0), release.Version);
+    }
+
+    // The signature is what Windows checks, but a package offered over an open
+    // connection is not worth downloading in the first place
+    [Fact]
+    public async Task A_package_offered_over_an_open_connection_is_passed_over()
+    {
+        const string releases = """
+            [
+              { "tag_name": "v2.0.0", "assets": [
+                { "name": "a.msixbundle", "browser_download_url": "http://host/a.msixbundle" } ] }
+            ]
+            """;
+
+        Assert.Null(await Read(releases));
+    }
+
+    [Fact]
+    public async Task A_bundle_wins_over_the_packages_of_single_architectures()
+    {
+        const string releases = """
+            [
+              { "tag_name": "v2.0.0", "assets": [
+                { "name": "CursorLang-2.0.0.0-x64.msix", "browser_download_url": "https://host/x64.msix" },
+                { "name": "CursorLang-2.0.0.0.msixbundle", "browser_download_url": "https://host/all.msixbundle" } ] }
+            ]
+            """;
+
+        ReleaseInfo? release = await Read(releases);
+
+        Assert.NotNull(release);
+        Assert.Equal("https://host/all.msixbundle", release.Package.Url.ToString());
+    }
+
+    [Fact]
+    public async Task Out_of_several_packages_the_one_for_this_machine_is_taken()
+    {
+        const string releases = """
+            [
+              { "tag_name": "v2.0.0", "assets": [
+                { "name": "CursorLang-2.0.0.0-arm64.msix", "browser_download_url": "https://host/arm64.msix" },
+                { "name": "CursorLang-2.0.0.0-x64.msix", "browser_download_url": "https://host/x64.msix" } ] }
+            ]
+            """;
+
+        string expected = RuntimeInformation.ProcessArchitecture == Architecture.Arm64
+            ? "https://host/arm64.msix"
+            : "https://host/x64.msix";
+
+        ReleaseInfo? release = await Read(releases);
+
+        Assert.NotNull(release);
+        Assert.Equal(expected, release.Package.Url.ToString());
+    }
+
+    // Without the architecture in the name there is no telling which package is
+    // for this machine — unless it is the only one there
+    [Fact]
+    public async Task A_package_without_an_architecture_is_taken_only_when_alone()
+    {
+        const string alone = """
+            [
+              { "tag_name": "v2.0.0", "assets": [
+                { "name": "CursorLang.msix", "browser_download_url": "https://host/one.msix" } ] }
+            ]
+            """;
+
+        const string ambiguous = """
+            [
+              { "tag_name": "v2.0.0", "assets": [
+                { "name": "CursorLang.msix", "browser_download_url": "https://host/one.msix" },
+                { "name": "CursorLang-other.msix", "browser_download_url": "https://host/other.msix" } ] }
+            ]
+            """;
+
+        Assert.NotNull(await Read(alone));
+        Assert.Null(await Read(ambiguous));
+    }
+
+    [Fact]
+    public async Task An_empty_list_of_releases_means_nothing_to_offer()
+    {
+        Assert.Null(await Read("[]"));
+    }
+
+    // A server answering with something else is no reason to fail
+    [Fact]
+    public async Task An_answer_that_is_not_a_list_leaves_the_app_with_nothing()
+    {
+        Assert.Null(await Read("""{ "message": "Not Found" }"""));
+    }
+
+    [Fact]
+    public async Task A_refusal_of_the_server_is_raised()
+    {
+        var handler = FakeHttpHandler.Status(HttpStatusCode.Unauthorized);
+        var feed = new GiteaReleaseFeed(handler.CreateClient(), Options());
+
+        await Assert.ThrowsAsync<HttpRequestException>(
+            () => feed.GetLatestAsync(TestContext.Current.CancellationToken));
+    }
+
+    private static string WithTag(string tag) => $$"""
+        [
+          { "tag_name": "{{tag}}", "assets": [
+            { "name": "CursorLang.msixbundle", "browser_download_url": "https://host/a.msixbundle" } ] }
+        ]
+        """;
+
+    private static UpdateOptions Options(string? token = null) => new()
+    {
+        ServiceUri = new Uri("https://git.alrakis.kz/"),
+        Project = "alrakis/cursor-lang",
+        AccessToken = token,
+    };
+
+    private static Task<ReleaseInfo?> Read(string json) =>
+        new GiteaReleaseFeed(FakeHttpHandler.Json(json).CreateClient(), Options())
+            .GetLatestAsync(TestContext.Current.CancellationToken);
+}

CursorLang.Tests/Services/KeyboardLayoutServiceTests.cs

@@ -0,0 +1,288 @@
+using System.Collections.Concurrent;
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+
+namespace CursorLang.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();
+
+        Sta.Run(service.Start);
+        world.LocaleId = Russian;
+        Sta.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();
+
+        Sta.Run(service.Start);
+
+        world.ForegroundWindow = SecondWindow;
+        world.LocaleId = Russian;
+        Sta.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();
+
+        Sta.Run(service.Start);
+
+        world.ForegroundWindow = SecondWindow;
+        Sta.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();
+
+        Sta.Run(service.Start);
+
+        for (int i = 0; i < 5; i++)
+        {
+            Sta.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();
+
+        Sta.Run(service.Start);
+
+        world.ForegroundWindow = IntPtr.Zero;
+        world.LocaleId = Russian;
+        Sta.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;
+        Sta.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();
+
+        Sta.Run(service.Start);
+
+        world.LocaleId = Russian;
+        Sta.Run(service.Poll);
+        Sta.Run(service.Poll);
+        Sta.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;
+        Sta.Run(service.Start);
+        Sta.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));
+
+        Sta.Run(service.Start);
+        world.LocaleId = Russian;
+
+        Sta.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));
+
+        Sta.Run(service.Start);
+        Sta.Run(service.Stop);
+
+        world.LocaleId = Russian;
+        Sta.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));
+
+        Sta.Run(service.Start);
+        Sta.Run(service.Dispose);
+
+        world.LocaleId = Russian;
+        Sta.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));
+
+        Sta.Run(service.Start);
+        Sta.Run(service.Stop);
+        Sta.Run(service.Start);
+
+        world.LocaleId = Russian;
+
+        Sta.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 = Sta.Run(() =>
+            new KeyboardLayoutService(new KeyboardLayoutOptions { PollInterval = TimeSpan.FromHours(1) }));
+
+        try
+        {
+            Sta.Run(service.Start);
+            Sta.Run(service.Poll);
+
+            Assert.InRange(service.Current.LocaleId, 1, 0xFFFF);
+
+            Sta.Run(service.Stop);
+        }
+        finally
+        {
+            Sta.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)
+        {
+            // An hour between ticks means the poll only runs when the test asks for it
+            var options = new KeyboardLayoutOptions
+            {
+                PollInterval = pollInterval ?? TimeSpan.FromHours(1),
+            };
+
+            _service = Sta.Run(() => new KeyboardLayoutService(
+                options,
+                () => ForegroundWindow,
+                () => LocaleId,
+                () => SwitchRequests++));
+
+            _service.LayoutChanged += (_, e) => Changes.Enqueue(e);
+
+            return _service;
+        }
+
+        public void Dispose()
+        {
+            if (_service is not null)
+            {
+                Sta.Run(_service.Dispose);
+            }
+        }
+    }
+}

CursorLang.Tests/Services/LayoutNotificationCoordinatorTests.cs

@@ -0,0 +1,116 @@
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+
+namespace CursorLang.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.Tests/Services/LayoutPopupServiceTests.cs

@@ -0,0 +1,212 @@
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+using CursorLang.ViewModels;
+
+namespace CursorLang.Tests.Services;
+
+/// <summary>
+/// The lifetime of the tooltip. Its timer lives on the interface thread,
+/// so everything happens there 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 };
+        var viewModel = new LayoutPopupViewModel(settings);
+
+        Sta.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, viewModel, settings);
+            service.Show(Russian);
+        });
+
+        Assert.Equal("RU", viewModel.ShortName);
+        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 };
+        var viewModel = new LayoutPopupViewModel(settings);
+
+        Sta.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, viewModel, settings);
+            service.Show(Russian);
+
+            Assert.Equal(0, window.HideCalls);
+            Sta.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 };
+        var viewModel = new LayoutPopupViewModel(settings);
+
+        Sta.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, viewModel, settings);
+            service.Show(Russian);
+
+            settings.DurationMilliseconds = 30;
+            service.Show(English);
+
+            Sta.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 };
+        var viewModel = new LayoutPopupViewModel(settings);
+
+        Sta.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, viewModel, settings);
+
+            for (int i = 0; i < 5; i++)
+            {
+                service.Show(i % 2 == 0 ? Russian : English);
+                Sta.Pause(TimeSpan.FromMilliseconds(20));
+                Assert.Equal(0, window.HideCalls);
+            }
+
+            Sta.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 };
+        var viewModel = new LayoutPopupViewModel(settings);
+
+        Sta.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, viewModel, settings);
+            service.ShowUntilHidden(Russian);
+
+            Sta.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 };
+        var viewModel = new LayoutPopupViewModel(settings);
+
+        Sta.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, viewModel, settings);
+            service.Show(Russian);
+            service.ShowUntilHidden(English);
+
+            Sta.Pause(TimeSpan.FromMilliseconds(100));
+
+            Assert.Equal(0, window.HideCalls);
+            Assert.Equal("EN", viewModel.ShortName);
+        });
+    }
+
+    [Fact]
+    public void Hiding_cancels_a_running_countdown()
+    {
+        var window = new FakeLayoutPopupWindow();
+        var settings = new AppSettings { DurationMilliseconds = 30 };
+        var viewModel = new LayoutPopupViewModel(settings);
+
+        Sta.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, viewModel, settings);
+            service.Show(Russian);
+            service.Hide();
+
+            Sta.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();
+        var viewModel = new LayoutPopupViewModel(settings);
+
+        Sta.Run(() =>
+        {
+            var service = new LayoutPopupService(window, viewModel, settings);
+            service.Show(Russian);
+            service.Dispose();
+
+            Sta.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 };
+        var viewModel = new LayoutPopupViewModel(settings);
+
+        Sta.Run(() =>
+        {
+            var service = new LayoutPopupService(window, viewModel, settings);
+            service.Show(Russian);
+            service.Dispose();
+
+            Sta.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 };
+        var viewModel = new LayoutPopupViewModel(settings);
+
+        Sta.Run(() =>
+        {
+            using var service = new LayoutPopupService(window, viewModel, settings);
+            service.Show(Russian);
+
+            Sta.WaitFor(() => window.HideCalls == 1, "the tooltip hid itself");
+
+            Assert.Equal(["show", "hide"], window.Calls);
+        });
+    }
+}

CursorLang.Tests/Services/LocalizationServiceTests.cs

@@ -0,0 +1,149 @@
+using System.ComponentModel;
+using System.Globalization;
+using System.Windows.Data;
+using CursorLang.Services;
+
+namespace CursorLang.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.Tests/Services/MainWindowPlacementTests.cs

@@ -0,0 +1,287 @@
+using System.Windows;
+using System.Windows.Interop;
+using CursorLang.Interop;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+
+namespace CursorLang.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.Tests/Services/PopupLayoutTests.cs

@@ -0,0 +1,229 @@
+using CursorLang.Interop;
+using CursorLang.Models;
+using CursorLang.Services;
+
+namespace CursorLang.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.Tests/Services/RegistryStartupTests.cs

@@ -0,0 +1,156 @@
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+using Microsoft.Win32;
+
+namespace CursorLang.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.Tests/Services/SettingsServiceTests.cs

@@ -0,0 +1,454 @@
+using System.Globalization;
+using System.IO;
+using System.Text.Json;
+using System.Windows.Media;
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+using CursorLang.Tests.Models;
+
+namespace CursorLang.Tests.Services;
+
+/// <summary>
+/// Keeping the settings in a file. 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 = Sta.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(AppTheme.System, settings.Theme);
+        Assert.Equal(20, settings.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 = Sta.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();
+
+        Sta.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+
+            settings.FontSize = 42;
+            settings.Theme = AppTheme.Dark;
+            settings.PlacementMode = PopupPlacementMode.AtCaret;
+            settings.BackgroundColor = Color.FromRgb(0x11, 0x22, 0x33);
+            settings.UseCapsLockHotkey = true;
+
+            service.Save();
+        });
+
+        AppSettings restored = Sta.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(42, restored.FontSize);
+        Assert.Equal(AppTheme.Dark, restored.Theme);
+        Assert.Equal(PopupPlacementMode.AtCaret, restored.PlacementMode);
+        Assert.Equal(Color.FromRgb(0x11, 0x22, 0x33), restored.BackgroundColor);
+        Assert.True(restored.UseCapsLockHotkey);
+    }
+
+    [Fact]
+    public void The_settings_land_in_the_file_in_a_readable_form()
+    {
+        using var folder = new TempFolder();
+
+        Sta.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+            settings.Theme = AppTheme.Dark;
+            settings.BackgroundColor = Color.FromRgb(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");
+
+        Sta.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+
+            settings.FontSize = 33;
+
+            // Right after the edit there is nothing on disk yet: the write is deferred
+            Assert.False(File.Exists(path));
+
+            Sta.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");
+
+        Sta.Run(() =>
+        {
+            using SettingsService service = new(path, folder.File("inherited.json"), TimeSpan.FromMilliseconds(150));
+            AppSettings settings = service.Load();
+
+            for (int i = 0; i < 10; i++)
+            {
+                settings.Opacity = 0.5 + (i * 0.01);
+                Assert.False(File.Exists(path));
+                Sta.Pause(TimeSpan.FromMilliseconds(10));
+            }
+
+            Sta.WaitFor(() => File.Exists(path), "the write happened after the pause in edits");
+        });
+    }
+
+    [Fact]
+    public void Closing_the_service_saves_the_latest_edits()
+    {
+        using var folder = new TempFolder();
+
+        Sta.Run(() =>
+        {
+            SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+            settings.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");
+
+        Sta.Run(() =>
+        {
+            SettingsService service = Create(folder);
+            AppSettings settings = service.Load();
+            service.Dispose();
+
+            string afterDispose = File.ReadAllText(path);
+
+            settings.FontSize = 99;
+            Sta.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, """{"FontSize": 31, "Language": "ru"}""");
+
+        AppSettings settings = Sta.Run(() =>
+        {
+            using SettingsService service = new(own, inherited, SaveDelay);
+            return service.Load();
+        });
+
+        Assert.Equal(31, settings.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 = """{"FontSize": 31}""";
+
+        File.WriteAllText(inherited, original);
+
+        Sta.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, """{"FontSize": 12}""");
+        File.WriteAllText(inherited, """{"FontSize": 31}""");
+
+        AppSettings settings = Sta.Run(() =>
+        {
+            using SettingsService service = new(own, inherited, SaveDelay);
+            return service.Load();
+        });
+
+        Assert.Equal(12, settings.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");
+
+        Sta.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 = Sta.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(20, settings.FontSize);
+    }
+
+    [Fact]
+    public void Settings_with_unknown_fields_are_still_read()
+    {
+        using var folder = new TempFolder();
+        File.WriteAllText(folder.File("settings.json"), """{"FontSize": 15, "SomethingNew": true}""");
+
+        AppSettings settings = Sta.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(15, settings.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"), $$"""{"BackgroundColor": {{stored}}}""");
+
+        AppSettings settings = Sta.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(Color.FromRgb(r, g, b), settings.BackgroundColor);
+    }
+
+    [Theory]
+    [InlineData("\"\"")]
+    [InlineData("\"   \"")]
+    [InlineData("\"not a colour\"")]
+    public void An_unintelligible_colour_becomes_black(string stored)
+    {
+        using var folder = new TempFolder();
+        File.WriteAllText(folder.File("settings.json"), $$"""{"BackgroundColor": {{stored}}}""");
+
+        AppSettings settings = Sta.Run(() =>
+        {
+            using SettingsService service = Create(folder);
+            return service.Load();
+        });
+
+        Assert.Equal(Colors.Black, settings.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");
+
+        Sta.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);
+
+        Sta.Run(() =>
+        {
+            using SettingsService service = new(path, folder.File("inherited.json"), SaveDelay);
+            AppSettings settings = service.Load();
+
+            settings.FontSize = 18;
+            service.Save();
+        });
+
+        Assert.True(Directory.Exists(path));
+    }
+
+    [Fact]
+    public void Saving_without_loading_writes_nothing()
+    {
+        using var folder = new TempFolder();
+
+        Sta.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();
+
+        Sta.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();
+
+        Sta.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);
+        }
+    }
+
+    // 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()
+    {
+        Sta.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.Tests/Services/SingleInstanceGateTests.cs

@@ -0,0 +1,204 @@
+using System.Collections.Concurrent;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+
+namespace CursorLang.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 = Sta.Run(() => new SingleInstanceGate(suffix));
+
+        try
+        {
+            Assert.True(Sta.Run(gate.TryAcquire));
+        }
+        finally
+        {
+            Sta.Run(gate.Dispose);
+        }
+    }
+
+    [Fact]
+    public void The_second_run_does_not_get_the_place()
+    {
+        string suffix = UniqueSuffix();
+        SingleInstanceGate first = Sta.Run(() => new SingleInstanceGate(suffix));
+
+        try
+        {
+            Assert.True(Sta.Run(first.TryAcquire));
+            Assert.False(TryAcquireApart(suffix));
+        }
+        finally
+        {
+            Sta.Run(first.Dispose);
+        }
+    }
+
+    [Fact]
+    public void The_second_run_asks_the_running_one_to_show_its_window()
+    {
+        string suffix = UniqueSuffix();
+        SingleInstanceGate first = Sta.Run(() => new SingleInstanceGate(suffix));
+
+        ConcurrentQueue<EventArgs> requests = new();
+        first.ActivationRequested += (_, e) => requests.Enqueue(e);
+
+        try
+        {
+            Sta.Run(first.TryAcquire);
+            Assert.False(TryAcquireApart(suffix));
+
+            Sta.WaitFor(() => !requests.IsEmpty, "the running instance got the request to show itself");
+        }
+        finally
+        {
+            Sta.Run(first.Dispose);
+        }
+    }
+
+    [Fact]
+    public void Without_a_second_run_no_request_arrives()
+    {
+        string suffix = UniqueSuffix();
+        SingleInstanceGate gate = Sta.Run(() => new SingleInstanceGate(suffix));
+
+        ConcurrentQueue<EventArgs> requests = new();
+        gate.ActivationRequested += (_, e) => requests.Enqueue(e);
+
+        try
+        {
+            Sta.Run(gate.TryAcquire);
+            Sta.Pause(TimeSpan.FromMilliseconds(80));
+
+            Assert.Empty(requests);
+        }
+        finally
+        {
+            Sta.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 = Sta.Run(() => new SingleInstanceGate(suffix));
+        Assert.True(Sta.Run(first.TryAcquire));
+        Sta.Run(first.Dispose);
+
+        Assert.True(TryAcquireApart(suffix));
+    }
+
+    [Fact]
+    public void No_requests_arrive_after_the_exit()
+    {
+        string suffix = UniqueSuffix();
+        SingleInstanceGate first = Sta.Run(() => new SingleInstanceGate(suffix));
+
+        ConcurrentQueue<EventArgs> requests = new();
+        first.ActivationRequested += (_, e) => requests.Enqueue(e);
+
+        Sta.Run(first.TryAcquire);
+        Sta.Run(first.Dispose);
+
+        // The place is free, so the new run simply takes it for itself
+        Assert.True(TryAcquireApart(suffix));
+        Sta.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
+        Sta.RunApart(() =>
+        {
+            var abandoned = new Mutex(initiallyOwned: false, "CursorLang.SingleInstance" + suffix);
+            abandoned.WaitOne(TimeSpan.Zero, exitContext: false);
+        });
+
+        SingleInstanceGate gate = Sta.Run(() => new SingleInstanceGate(suffix));
+
+        try
+        {
+            Assert.True(Sta.Run(gate.TryAcquire));
+        }
+        finally
+        {
+            Sta.Run(gate.Dispose);
+        }
+    }
+
+    [Fact]
+    public void Closing_without_taking_the_place_passes_without_consequence()
+    {
+        SingleInstanceGate gate = Sta.Run(() => new SingleInstanceGate(UniqueSuffix()));
+
+        Sta.Run(gate.Dispose);
+    }
+
+    // The application takes the place under its ordinary name
+    [Fact]
+    public void The_ordinary_application_takes_the_place_under_its_own_name()
+    {
+        SingleInstanceGate gate = Sta.Run(() => new SingleInstanceGate());
+
+        // The place may be held by a running application — then it is simply not taken
+        Sta.Run(gate.Dispose);
+    }
+
+    [Fact]
+    public void Closing_twice_passes_without_consequence()
+    {
+        SingleInstanceGate gate = Sta.Run(() => new SingleInstanceGate(UniqueSuffix()));
+
+        Sta.Run(gate.TryAcquire);
+        Sta.Run(gate.Dispose);
+        Sta.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;
+
+        Sta.RunApart(() =>
+        {
+            var gate = new SingleInstanceGate(suffix);
+
+            try
+            {
+                acquired = gate.TryAcquire();
+            }
+            finally
+            {
+                gate.Dispose();
+            }
+        });
+
+        return acquired;
+    }
+}

CursorLang.Tests/Services/StartupServiceTests.cs

@@ -0,0 +1,55 @@
+using Windows.ApplicationModel;
+using CursorLang.Interop;
+using CursorLang.Models;
+using CursorLang.Services;
+
+namespace CursorLang.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;
+        }
+
+        // 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.Tests/Services/ThemeServiceTests.cs

@@ -0,0 +1,454 @@
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Interop;
+using System.Windows.Media;
+using System.Windows.Shapes;
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+using Microsoft.Win32;
+
+namespace CursorLang.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.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()
+    {
+        Assert.Contains("CursorLang;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.Tests/Services/UpdateOptionsTests.cs

@@ -0,0 +1,38 @@
+using CursorLang.Services;
+
+namespace CursorLang.Tests.Services;
+
+/// <summary>
+/// Where the app looks for its releases. The values belong to the build, and a
+/// wrong one shows only as an update that never arrives.
+/// </summary>
+public sealed class UpdateOptionsTests
+{
+    [Fact]
+    public void Out_of_the_box_the_releases_are_looked_for_in_the_repository_of_the_app()
+    {
+        var options = new UpdateOptions();
+
+        Assert.Equal("git.alrakis.kz", options.ServiceUri.Host);
+        Assert.Equal("alrakis/cursor-lang", options.Project);
+    }
+
+    [Fact]
+    public void Another_server_is_taken_as_it_is_given()
+    {
+        var options = new UpdateOptions
+        {
+            ServiceUri = new Uri("https://git.example.com/"),
+            Project = "team/app",
+        };
+
+        Assert.Equal("git.example.com", options.ServiceUri.Host);
+        Assert.Equal("team/app", options.Project);
+    }
+
+    [Fact]
+    public void The_app_asks_about_releases_no_more_than_once_a_day()
+    {
+        Assert.Equal(TimeSpan.FromDays(1), new UpdateOptions().CheckInterval);
+    }
+}

CursorLang.Tests/Services/UpdateServiceTests.cs

@@ -0,0 +1,158 @@
+using System.IO;
+using System.Net;
+using System.Net.Http;
+using System.Text;
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+
+namespace CursorLang.Tests.Services;
+
+/// <summary>
+/// What the app does with a release once it has found one: whether it is newer
+/// at all, and what ends up on disk.
+/// </summary>
+public sealed class UpdateServiceTests
+{
+    [Theory]
+    [InlineData("1.0.0.0", "1.0.1.0", true)]
+    [InlineData("1.0.0.0", "2.0.0.0", true)]
+    [InlineData("1.0.0.0", "1.0.0.0", false)]
+    [InlineData("1.0.1.0", "1.0.0.0", false)]
+    public async Task Only_a_higher_version_counts_as_an_update(string current, string found, bool offered)
+    {
+        var feed = new FakeReleaseFeed { Release = Release(found) };
+        using TempFolder folder = new();
+        using UpdateService service = Create(feed, folder, current);
+
+        ReleaseInfo? update = await service.CheckAsync(TestContext.Current.CancellationToken);
+
+        Assert.Equal(offered, update is not null);
+    }
+
+    [Fact]
+    public async Task An_empty_repository_leaves_the_app_with_nothing()
+    {
+        var feed = new FakeReleaseFeed { Release = null };
+        using TempFolder folder = new();
+        using UpdateService service = Create(feed, folder);
+
+        Assert.Null(await service.CheckAsync(TestContext.Current.CancellationToken));
+    }
+
+    [Fact]
+    public async Task The_package_ends_up_on_disk_whole()
+    {
+        byte[] content = Encoding.UTF8.GetBytes(new string('p', 300_000));
+        using TempFolder folder = new();
+        using UpdateService service = Create(new FakeReleaseFeed(), folder, client: FakeHttpHandler.Bytes(content));
+
+        string path = await service.DownloadAsync(Release("2.0.0.0"), null, TestContext.Current.CancellationToken);
+
+        Assert.Equal(content, await File.ReadAllBytesAsync(path, TestContext.Current.CancellationToken));
+    }
+
+    // The name comes from the version, not from the answer: the app creates a
+    // file with it, and the answer comes from the other side
+    [Fact]
+    public async Task The_name_of_the_file_is_built_by_the_app_itself()
+    {
+        using TempFolder folder = new();
+        using UpdateService service = Create(new FakeReleaseFeed(), folder, client: FakeHttpHandler.Bytes([1, 2, 3]));
+
+        var release = new ReleaseInfo(
+            new Version(2, 0, 0, 0),
+            "v2.0.0",
+            null,
+            new ReleaseAsset(@"..\..\evil.msixbundle", new Uri("https://host/a"), 3));
+
+        string path = await service.DownloadAsync(release, null, TestContext.Current.CancellationToken);
+
+        Assert.Equal("CursorLang-2.0.0.0.msixbundle", Path.GetFileName(path));
+        Assert.Equal(folder.Path, Path.GetDirectoryName(path));
+    }
+
+    [Fact]
+    public async Task The_download_reports_how_far_it_has_come()
+    {
+        byte[] content = new byte[500_000];
+        using TempFolder folder = new();
+        using UpdateService service = Create(new FakeReleaseFeed(), folder, client: FakeHttpHandler.Bytes(content));
+
+        var reported = new CollectingProgress();
+        await service.DownloadAsync(Release("2.0.0.0"), reported, TestContext.Current.CancellationToken);
+
+        Assert.NotEmpty(reported.Values);
+        Assert.All(reported.Values, value => Assert.InRange(value, 0, 1));
+        Assert.Equal(reported.Values, [.. reported.Values.Order()]);
+        Assert.Equal(1, reported.Values[^1]);
+    }
+
+    [Fact]
+    public async Task A_closed_repository_gets_the_token_with_the_download_too()
+    {
+        var feed = new FakeReleaseFeed();
+        using TempFolder folder = new();
+        using UpdateService service = Create(feed, folder, client: FakeHttpHandler.Bytes([1]));
+
+        await service.DownloadAsync(Release("2.0.0.0"), null, TestContext.Current.CancellationToken);
+
+        Assert.Single(feed.Authorized);
+    }
+
+    [Fact]
+    public async Task A_refusal_of_the_hosting_service_leaves_no_package_behind()
+    {
+        using TempFolder folder = new();
+        using UpdateService service = Create(
+            new FakeReleaseFeed(), folder, client: FakeHttpHandler.Status(HttpStatusCode.NotFound));
+
+        await Assert.ThrowsAsync<HttpRequestException>(
+            () => service.DownloadAsync(Release("2.0.0.0"), null, TestContext.Current.CancellationToken));
+
+        Assert.Empty(Directory.GetFiles(folder.Path));
+    }
+
+    // A package left from an earlier download takes up room and is of no use
+    // once it has been installed
+    [Fact]
+    public async Task An_older_download_is_cleared_away()
+    {
+        using TempFolder folder = new();
+        string leftover = folder.File("CursorLang-1.5.0.0.msixbundle");
+        await File.WriteAllTextAsync(leftover, "old", TestContext.Current.CancellationToken);
+
+        using UpdateService service = Create(new FakeReleaseFeed(), folder, client: FakeHttpHandler.Bytes([1]));
+        string path = await service.DownloadAsync(Release("2.0.0.0"), null, TestContext.Current.CancellationToken);
+
+        Assert.False(File.Exists(leftover));
+        Assert.True(File.Exists(path));
+    }
+
+    /// <summary>
+    /// The reports as the download makes them. <c>Progress&lt;T&gt;</c> would
+    /// hand them over to another thread, and a test has nowhere to wait for that.
+    /// </summary>
+    private sealed class CollectingProgress : IProgress<double>
+    {
+        internal List<double> Values { get; } = [];
+
+        public void Report(double value) => Values.Add(value);
+    }
+
+    private static ReleaseInfo Release(string version) => new(
+        Version.Parse(version),
+        $"v{version}",
+        new Uri("https://host/releases/tag"),
+        new ReleaseAsset($"CursorLang-{version}.msixbundle", new Uri("https://host/package"), 0));
+
+    private static UpdateService Create(
+        IReleaseFeed feed,
+        TempFolder folder,
+        string current = "1.0.0.0",
+        FakeHttpHandler? client = null) =>
+        new(feed,
+            (client ?? FakeHttpHandler.Bytes([])).CreateClient(),
+            Version.Parse(current),
+            folder.Path);
+}

CursorLang.Tests/ViewModels/EnumOptionTests.cs

@@ -0,0 +1,56 @@
+using CursorLang.Models;
+using CursorLang.ViewModels;
+
+namespace CursorLang.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.Tests/ViewModels/LayoutPopupViewModelTests.cs

@@ -0,0 +1,48 @@
+using CursorLang.Models;
+using CursorLang.ViewModels;
+
+namespace CursorLang.Tests.ViewModels;
+
+public sealed class LayoutPopupViewModelTests
+{
+    // Before the first layout change there is nothing to show, yet the window
+    // is already being built
+    [Fact]
+    public void Before_the_first_layout_a_dash_is_shown()
+    {
+        Assert.Equal("—", new LayoutPopupViewModel(new AppSettings()).ShortName);
+    }
+
+    [Fact]
+    public void The_look_comes_straight_from_the_settings()
+    {
+        var settings = new AppSettings();
+
+        Assert.Same(settings, new LayoutPopupViewModel(settings).Settings);
+    }
+
+    [Fact]
+    public void A_changed_layout_name_is_announced_to_subscribers()
+    {
+        var viewModel = new LayoutPopupViewModel(new AppSettings());
+        List<string?> changed = [];
+        viewModel.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        viewModel.ShortName = "RU";
+
+        Assert.Equal("RU", viewModel.ShortName);
+        Assert.Equal([nameof(LayoutPopupViewModel.ShortName)], changed);
+    }
+
+    [Fact]
+    public void The_same_layout_name_is_not_announced_again()
+    {
+        var viewModel = new LayoutPopupViewModel(new AppSettings()) { ShortName = "RU" };
+        List<string?> changed = [];
+        viewModel.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        viewModel.ShortName = "RU";
+
+        Assert.Empty(changed);
+    }
+}

CursorLang.Tests/ViewModels/SettingsViewModelTests.cs

@@ -0,0 +1,291 @@
+using System.Windows.Media;
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+using CursorLang.ViewModels;
+
+namespace CursorLang.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.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));
+    }
+
+    // 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: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.BackgroundColor, viewModel.BackgroundPalette);
+        Assert.Contains(settings.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<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) =>
+        new(settings ?? new AppSettings(),
+            localization ?? new FakeLocalizationService(),
+            startup ?? new FakeStartupService(),
+            Fake.Updates());
+
+    // 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.Tests/ViewModels/UpdateViewModelTests.cs

@@ -0,0 +1,292 @@
+using System.IO;
+using System.Net.Http;
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+using CursorLang.ViewModels;
+
+namespace CursorLang.Tests.ViewModels;
+
+/// <summary>
+/// The updates section of the settings window: what it shows at every step and
+/// what it asks of the service behind it.
+/// </summary>
+public sealed class UpdateViewModelTests
+{
+    [Fact]
+    public void Before_the_first_check_the_section_says_nothing()
+    {
+        using UpdateViewModel viewModel = Create(new FakeUpdateService());
+
+        Assert.Equal(UpdateStatus.Idle, viewModel.Status);
+        Assert.False(viewModel.HasStatus);
+        Assert.False(viewModel.IsDownloadOffered);
+        Assert.False(viewModel.IsInstallOffered);
+        Assert.True(viewModel.CanCheck);
+    }
+
+    [Fact]
+    public async Task An_update_found_is_offered_for_download()
+    {
+        var updates = new FakeUpdateService { Release = Release("2.0.0.0") };
+        using UpdateViewModel viewModel = Create(updates);
+
+        await viewModel.CheckCommand.ExecuteAsync(null);
+
+        Assert.Equal(UpdateStatus.Available, viewModel.Status);
+        Assert.True(viewModel.IsDownloadOffered);
+        Assert.False(viewModel.IsInstallOffered);
+        Assert.Equal("https://host/releases/tag", viewModel.ReleaseUrl?.ToString());
+        Assert.True(viewModel.IsReleaseLinkShown);
+        Assert.Equal("en:UpdateAvailable", viewModel.StatusText);
+    }
+
+    [Fact]
+    public async Task With_the_latest_version_installed_there_is_nothing_to_offer()
+    {
+        using UpdateViewModel viewModel = Create(new FakeUpdateService());
+
+        await viewModel.CheckCommand.ExecuteAsync(null);
+
+        Assert.Equal(UpdateStatus.UpToDate, viewModel.Status);
+        Assert.False(viewModel.IsDownloadOffered);
+        Assert.False(viewModel.IsReleaseLinkShown);
+    }
+
+    [Fact]
+    public async Task A_check_by_the_button_says_when_it_did_not_work_out()
+    {
+        var updates = new FakeUpdateService { Failure = new HttpRequestException("no network") };
+        using UpdateViewModel viewModel = Create(updates);
+
+        await viewModel.CheckCommand.ExecuteAsync(null);
+
+        Assert.Equal(UpdateStatus.Failed, viewModel.Status);
+        Assert.True(viewModel.HasStatus);
+    }
+
+    // The app does not always start with a live network, and the user who never
+    // asked about updates has no use for the complaint
+    [Fact]
+    public async Task A_check_at_startup_keeps_a_failure_to_itself()
+    {
+        var updates = new FakeUpdateService { Failure = new HttpRequestException("no network") };
+        using UpdateViewModel viewModel = Create(updates);
+
+        await viewModel.StartAsync();
+
+        Assert.Equal(UpdateStatus.Idle, viewModel.Status);
+        Assert.False(viewModel.HasStatus);
+    }
+
+    [Fact]
+    public async Task A_successful_check_is_remembered_in_the_settings()
+    {
+        var settings = new AppSettings();
+        var updates = new FakeUpdateService { Release = Release("2.0.0.0") };
+        using UpdateViewModel viewModel = Create(updates, settings);
+
+        await viewModel.CheckCommand.ExecuteAsync(null);
+
+        Assert.NotNull(settings.LastUpdateCheck);
+    }
+
+    [Fact]
+    public async Task A_check_that_did_not_work_out_is_not_remembered()
+    {
+        var settings = new AppSettings();
+        var updates = new FakeUpdateService { Failure = new HttpRequestException("no network") };
+        using UpdateViewModel viewModel = Create(updates, settings);
+
+        await viewModel.CheckCommand.ExecuteAsync(null);
+
+        Assert.Null(settings.LastUpdateCheck);
+    }
+
+    [Fact]
+    public async Task A_recent_check_is_not_repeated_at_startup()
+    {
+        var settings = new AppSettings { LastUpdateCheck = DateTimeOffset.UtcNow };
+        var updates = new FakeUpdateService();
+        using UpdateViewModel viewModel = Create(updates, settings);
+
+        await viewModel.StartAsync();
+
+        Assert.Equal(0, updates.CheckCalls);
+    }
+
+    [Fact]
+    public async Task A_check_of_yesterday_is_repeated_at_startup()
+    {
+        var settings = new AppSettings { LastUpdateCheck = DateTimeOffset.UtcNow - TimeSpan.FromDays(2) };
+        var updates = new FakeUpdateService();
+        using UpdateViewModel viewModel = Create(updates, settings);
+
+        await viewModel.StartAsync();
+
+        Assert.Equal(1, updates.CheckCalls);
+    }
+
+    [Fact]
+    public async Task A_ban_on_checking_by_itself_is_obeyed()
+    {
+        var settings = new AppSettings { CheckForUpdates = false };
+        var updates = new FakeUpdateService();
+        using UpdateViewModel viewModel = Create(updates, settings);
+
+        await viewModel.StartAsync();
+
+        Assert.Equal(0, updates.CheckCalls);
+
+        // The button still works: the setting is about the app doing it on its own
+        await viewModel.CheckCommand.ExecuteAsync(null);
+        Assert.Equal(1, updates.CheckCalls);
+    }
+
+    [Fact]
+    public void The_ban_on_checking_travels_to_the_settings()
+    {
+        var settings = new AppSettings { CheckForUpdates = true };
+        using UpdateViewModel viewModel = Create(new FakeUpdateService(), settings);
+
+        viewModel.CheckAutomatically = false;
+
+        Assert.False(settings.CheckForUpdates);
+    }
+
+    [Fact]
+    public async Task A_package_from_Store_is_left_to_Store()
+    {
+        var updates = new FakeUpdateService { IsSupported = false, Release = Release("2.0.0.0") };
+        using UpdateViewModel viewModel = Create(updates);
+
+        await viewModel.StartAsync();
+        await viewModel.CheckCommand.ExecuteAsync(null);
+
+        Assert.False(viewModel.IsSupported);
+        Assert.Equal(0, updates.CheckCalls);
+    }
+
+    [Fact]
+    public async Task A_downloaded_package_is_offered_for_installation()
+    {
+        using TempFolder folder = new();
+        string package = folder.File("CursorLang-2.0.0.0.msixbundle");
+        await File.WriteAllTextAsync(package, "package", TestContext.Current.CancellationToken);
+
+        var updates = new FakeUpdateService { Release = Release("2.0.0.0"), PackagePath = package };
+        using UpdateViewModel viewModel = Create(updates);
+
+        await viewModel.CheckCommand.ExecuteAsync(null);
+        await viewModel.DownloadCommand.ExecuteAsync(null);
+
+        Assert.Equal(UpdateStatus.Ready, viewModel.Status);
+        Assert.True(viewModel.IsInstallOffered);
+        Assert.False(viewModel.IsDownloadOffered);
+
+        viewModel.InstallCommand.Execute(null);
+
+        Assert.Equal([package], updates.Installed);
+    }
+
+    [Fact]
+    public async Task While_the_package_is_downloading_the_section_shows_it()
+    {
+        var updates = new FakeUpdateService
+        {
+            Release = Release("2.0.0.0"),
+            DownloadGate = new TaskCompletionSource(),
+        };
+
+        using UpdateViewModel viewModel = Create(updates);
+        await viewModel.CheckCommand.ExecuteAsync(null);
+
+        Task download = viewModel.DownloadCommand.ExecuteAsync(null);
+
+        Assert.Equal(UpdateStatus.Downloading, viewModel.Status);
+        Assert.True(viewModel.IsProgressShown);
+        Assert.True(viewModel.IsBusy);
+        Assert.False(viewModel.CanCheck);
+
+        updates.DownloadGate.SetResult();
+        await download;
+    }
+
+    [Fact]
+    public async Task A_download_that_did_not_work_out_is_told_about()
+    {
+        var updates = new FakeUpdateService { Release = Release("2.0.0.0") };
+        using UpdateViewModel viewModel = Create(updates);
+        await viewModel.CheckCommand.ExecuteAsync(null);
+
+        updates.Failure = new HttpRequestException("the connection dropped");
+        await viewModel.DownloadCommand.ExecuteAsync(null);
+
+        Assert.Equal(UpdateStatus.Failed, viewModel.Status);
+    }
+
+    // The temp folder is cleared by Windows as it sees fit, and the app has no
+    // business handing a file that is gone to the installer
+    [Fact]
+    public async Task A_package_gone_from_the_disk_is_offered_for_download_again()
+    {
+        var updates = new FakeUpdateService
+        {
+            Release = Release("2.0.0.0"),
+            PackagePath = Path.Combine(Path.GetTempPath(), "CursorLang.Tests", "never-written.msixbundle"),
+        };
+
+        using UpdateViewModel viewModel = Create(updates);
+        await viewModel.CheckCommand.ExecuteAsync(null);
+        await viewModel.DownloadCommand.ExecuteAsync(null);
+
+        viewModel.InstallCommand.Execute(null);
+
+        Assert.Empty(updates.Installed);
+        Assert.Equal(UpdateStatus.Available, viewModel.Status);
+    }
+
+    [Fact]
+    public async Task The_status_is_written_in_the_chosen_language()
+    {
+        var localization = new FakeLocalizationService();
+        using UpdateViewModel viewModel = Create(new FakeUpdateService(), localization: localization);
+
+        await viewModel.CheckCommand.ExecuteAsync(null);
+        Assert.Equal("en:UpdateUpToDate", viewModel.StatusText);
+
+        localization.CurrentLanguage = "ru";
+        Assert.Equal("ru:UpdateUpToDate", viewModel.StatusText);
+    }
+
+    [Fact]
+    public async Task Closing_unsubscribes_from_the_language()
+    {
+        var localization = new FakeLocalizationService();
+        UpdateViewModel viewModel = Create(new FakeUpdateService(), localization: localization);
+        await viewModel.CheckCommand.ExecuteAsync(null);
+
+        List<string?> changed = [];
+        viewModel.PropertyChanged += (_, e) => changed.Add(e.PropertyName);
+
+        viewModel.Dispose();
+        localization.CurrentLanguage = "ru";
+
+        Assert.Empty(changed);
+    }
+
+    private static ReleaseInfo Release(string version) => new(
+        Version.Parse(version),
+        $"v{version}",
+        new Uri("https://host/releases/tag"),
+        new ReleaseAsset($"CursorLang-{version}.msixbundle", new Uri("https://host/package"), 0));
+
+    private static UpdateViewModel Create(
+        IUpdateService updates,
+        AppSettings? settings = null,
+        ILocalizationService? localization = null) =>
+        new(updates,
+            localization ?? new FakeLocalizationService(),
+            settings ?? new AppSettings(),
+            new UpdateOptions());
+}

CursorLang.Tests/Views/ConvertersTests.cs

@@ -0,0 +1,168 @@
+using System.Globalization;
+using System.Windows;
+using System.Windows.Data;
+using System.Windows.Media;
+using CursorLang.Models;
+using CursorLang.Views;
+
+namespace CursorLang.Tests.Views;
+
+/// <summary>
+/// The binding converters: they decide what the settings window shows and what
+/// it keeps out of sight.
+/// </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(Color.FromRgb(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(Color.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(Color), null, Culture));
+    }
+
+    [Fact]
+    public void A_colour_turns_into_a_brush()
+    {
+        var converter = new ColorToBrushConverter();
+        Color color = Color.FromRgb(0x10, 0x20, 0x30);
+
+        var brush = Assert.IsType<SolidColorBrush>(converter.Convert(color, typeof(Brush), null, Culture));
+
+        Assert.Equal(color, 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();
+        Color color = Color.FromRgb(0x10, 0x20, 0x30);
+
+        Assert.Equal(color, converter.ConvertBack(new SolidColorBrush(color), typeof(Color), 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(Color), 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());
+    }
+}

CursorLang.Tests/Views/LayoutPopupWindowTests.cs

@@ -0,0 +1,354 @@
+using System.Runtime.InteropServices;
+using System.Windows;
+using System.Windows.Interop;
+using CursorLang.Interop;
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+using CursorLang.ViewModels;
+using CursorLang.Views;
+
+namespace CursorLang.Tests.Views;
+
+/// <summary>
+/// The tooltip window itself: where it ends up and how Windows sees it.
+/// </summary>
+public sealed class LayoutPopupWindowTests
+{
+    private const int GwlExstyle = -20;
+    private const int WsExNoactivate = 0x08000000;
+    private const int WsExToolwindow = 0x00000080;
+
+    [Fact]
+    public void The_window_is_created_before_the_first_show()
+    {
+        Sta.Run(() =>
+        {
+            using var popup = Popup.Create(new AppSettings());
+
+            // The handle is needed to set the window bounds before the show:
+            // otherwise the window flashes at its default size for a moment
+            Assert.NotEqual(IntPtr.Zero, popup.Handle);
+            Assert.False(popup.Window.IsVisible);
+        });
+    }
+
+    [Fact]
+    public void The_window_shows_what_the_view_model_gave_it()
+    {
+        Sta.Run(() =>
+        {
+            var settings = new AppSettings();
+            using var popup = Popup.Create(settings);
+
+            Assert.Same(popup.ViewModel, popup.Window.DataContext);
+
+            popup.ViewModel.ShortName = "RU";
+            popup.Window.ShowPopup();
+
+            Assert.True(popup.Window.IsVisible);
+        });
+    }
+
+    [Fact]
+    public void The_window_does_what_the_popup_service_expects_of_it()
+    {
+        Sta.Run(() =>
+        {
+            using var popup = Popup.Create(new AppSettings());
+
+            Assert.IsAssignableFrom<ILayoutPopupWindow>(popup.Window);
+        });
+    }
+
+    // The tooltip pops up over other applications and must neither take the
+    // focus nor turn up in Alt+Tab
+    [Fact]
+    public void The_window_takes_no_focus_and_stays_out_of_the_switcher()
+    {
+        Sta.Run(() =>
+        {
+            using var popup = Popup.Create(new AppSettings());
+
+            int style = GetWindowLong(popup.Handle, GwlExstyle);
+
+            Assert.Equal(WsExNoactivate, style & WsExNoactivate);
+            Assert.Equal(WsExToolwindow, style & WsExToolwindow);
+        });
+    }
+
+    [Fact]
+    public void The_window_stays_on_top_and_out_of_the_mouses_way()
+    {
+        Sta.Run(() =>
+        {
+            using var popup = Popup.Create(new AppSettings());
+
+            Assert.True(popup.Window.Topmost);
+            Assert.False(popup.Window.ShowInTaskbar);
+            Assert.False(popup.Window.ShowActivated);
+            Assert.False(popup.Window.IsHitTestVisible);
+            Assert.False(popup.Window.Focusable);
+            Assert.Equal(WindowStyle.None, popup.Window.WindowStyle);
+            Assert.True(popup.Window.AllowsTransparency);
+        });
+    }
+
+    [Fact]
+    public void The_opacity_comes_from_the_settings()
+    {
+        Sta.Run(() =>
+        {
+            var settings = new AppSettings { Opacity = 0.42 };
+            using var popup = Popup.Create(settings);
+            popup.Window.ShowPopup();
+
+            Assert.Equal(0.42, popup.Window.Opacity, precision: 3);
+
+            settings.Opacity = 0.75;
+            Assert.Equal(0.75, popup.Window.Opacity, precision: 3);
+        });
+    }
+
+    [Theory]
+    [InlineData(ScreenPosition.TopLeft)]
+    [InlineData(ScreenPosition.Top)]
+    [InlineData(ScreenPosition.TopRight)]
+    [InlineData(ScreenPosition.Center)]
+    [InlineData(ScreenPosition.BottomLeft)]
+    [InlineData(ScreenPosition.Bottom)]
+    [InlineData(ScreenPosition.BottomRight)]
+    public void In_the_fixed_point_mode_the_window_lands_where_it_was_computed(ScreenPosition position)
+    {
+        Sta.Run(() =>
+        {
+            var settings = new AppSettings
+            {
+                PlacementMode = PopupPlacementMode.FixedPoint, ScreenPosition = position, ScreenMargin = 24,
+            };
+
+            using var popup = Popup.Create(settings);
+            popup.ViewModel.ShortName = "RU";
+
+            (PopupWindowNative.Rect before, _) = PopupWindowNative.GetActiveMonitorWorkArea();
+            popup.Window.ShowPopup();
+            (PopupWindowNative.Rect work, double scale) = PopupWindowNative.GetActiveMonitorWorkArea();
+
+            if (!before.Equals(work))
+            {
+                Assert.Skip("The active monitor changed while the check was running");
+            }
+
+            PopupWindowNative.Rect bounds = WindowPlacementNative.TryGetBounds(popup.Handle)!.Value;
+
+            int width = bounds.Right - bounds.Left;
+            int height = bounds.Bottom - bounds.Top;
+            PopupWindowNative.Point expected = PopupLayout.OnScreen(
+                work, position, PopupLayout.ToPixels(settings.ScreenMargin, scale), width, height);
+
+            Assert.Equal(expected.X, bounds.Left);
+            Assert.Equal(expected.Y, bounds.Top);
+        });
+    }
+
+    [Fact]
+    public void At_the_cursor_the_window_lands_next_to_it()
+    {
+        Sta.Run(() =>
+        {
+            var settings = new AppSettings
+            {
+                PlacementMode = PopupPlacementMode.AtCursor, CursorSide = AnchorSide.BottomRight, CursorOffset = 16,
+            };
+
+            using var popup = Popup.Create(settings);
+            popup.ViewModel.ShortName = "RU";
+
+            // The place is computed from where the cursor was at the moment of
+            // the show. If it was moving right then, show it once more
+            PopupWindowNative.Point before = default;
+
+            for (int attempt = 0; attempt < 10; attempt++)
+            {
+                before = PopupWindowNative.GetCursorPosition();
+                popup.Window.ShowPopup();
+                PopupWindowNative.Point after = PopupWindowNative.GetCursorPosition();
+
+                if (before.X == after.X && before.Y == after.Y)
+                {
+                    break;
+                }
+
+                if (attempt == 9)
+                {
+                    Assert.Skip("The cursor kept moving the whole time");
+                }
+            }
+
+            PopupWindowNative.Rect bounds = WindowPlacementNative.TryGetBounds(popup.Handle)!.Value;
+            double scale = PopupWindowNative.GetScaleAt(before);
+
+            PopupWindowNative.Point expected = PopupLayout.NearAnchor(
+                PopupLayout.AsAnchor(before),
+                AnchorSide.BottomRight,
+                PopupLayout.ToPixels(settings.CursorOffset, scale),
+                bounds.Right - bounds.Left,
+                bounds.Bottom - bounds.Top);
+
+            Assert.Equal(expected.X, bounds.Left);
+            Assert.Equal(expected.Y, bounds.Top);
+        });
+    }
+
+    // There is no caret in the test environment, and the tooltip has to fall
+    // back to the cursor
+    [Fact]
+    public void Without_a_caret_the_window_lands_at_the_cursor()
+    {
+        Sta.Run(() =>
+        {
+            var settings = new AppSettings
+            {
+                PlacementMode = PopupPlacementMode.AtCaret, CaretSide = AnchorSide.BottomRight, CaretOffset = 8,
+            };
+
+            using var popup = Popup.Create(settings);
+            popup.ViewModel.ShortName = "RU";
+
+            PopupWindowNative.Point before = default;
+
+            for (int attempt = 0; attempt < 10; attempt++)
+            {
+                if (CaretNative.TryGetCaretRect() is not null)
+                {
+                    Assert.Skip("The foreground window reported a caret of its own");
+                }
+
+                before = PopupWindowNative.GetCursorPosition();
+                popup.Window.ShowPopup();
+                PopupWindowNative.Point after = PopupWindowNative.GetCursorPosition();
+
+                if (before.X == after.X && before.Y == after.Y)
+                {
+                    break;
+                }
+
+                if (attempt == 9)
+                {
+                    Assert.Skip("The cursor kept moving the whole time");
+                }
+            }
+
+            PopupWindowNative.Rect bounds = WindowPlacementNative.TryGetBounds(popup.Handle)!.Value;
+            double scale = PopupWindowNative.GetScaleAt(before);
+
+            PopupWindowNative.Point expected = PopupLayout.NearAnchor(
+                PopupLayout.AsAnchor(before),
+                settings.CaretSide,
+                PopupLayout.ToPixels(settings.CaretOffset, scale),
+                bounds.Right - bounds.Left,
+                bounds.Bottom - bounds.Top);
+
+            Assert.Equal(expected.X, bounds.Left);
+            Assert.Equal(expected.Y, bounds.Top);
+        });
+    }
+
+    // The window size equals the size of the text: the tooltip has no frame
+    [Fact]
+    public void The_window_size_follows_the_size_of_the_caption()
+    {
+        Sta.Run(() =>
+        {
+            var small = new AppSettings { FontSize = 12, PlacementMode = PopupPlacementMode.FixedPoint };
+            var large = new AppSettings { FontSize = 48, PlacementMode = PopupPlacementMode.FixedPoint };
+
+            using var smallPopup = Popup.Create(small);
+            using var largePopup = Popup.Create(large);
+
+            smallPopup.ViewModel.ShortName = "RU";
+            largePopup.ViewModel.ShortName = "RU";
+
+            smallPopup.Window.ShowPopup();
+            largePopup.Window.ShowPopup();
+
+            PopupWindowNative.Rect smallBounds = WindowPlacementNative.TryGetBounds(smallPopup.Handle)!.Value;
+            PopupWindowNative.Rect largeBounds = WindowPlacementNative.TryGetBounds(largePopup.Handle)!.Value;
+
+            Assert.True(largeBounds.Right - largeBounds.Left > smallBounds.Right - smallBounds.Left);
+            Assert.True(largeBounds.Bottom - largeBounds.Top > smallBounds.Bottom - smallBounds.Top);
+        });
+    }
+
+    [Fact]
+    public void Showing_again_moves_the_window_to_its_new_place()
+    {
+        Sta.Run(() =>
+        {
+            var settings = new AppSettings
+            {
+                PlacementMode = PopupPlacementMode.FixedPoint, ScreenPosition = ScreenPosition.TopLeft,
+            };
+
+            using var popup = Popup.Create(settings);
+            popup.ViewModel.ShortName = "RU";
+            popup.Window.ShowPopup();
+
+            PopupWindowNative.Rect topLeft = WindowPlacementNative.TryGetBounds(popup.Handle)!.Value;
+
+            settings.ScreenPosition = ScreenPosition.BottomRight;
+            popup.Window.ShowPopup();
+
+            PopupWindowNative.Rect bottomRight = WindowPlacementNative.TryGetBounds(popup.Handle)!.Value;
+
+            Assert.True(bottomRight.Left > topLeft.Left);
+            Assert.True(bottomRight.Top > topLeft.Top);
+        });
+    }
+
+    [Fact]
+    public void A_hidden_window_stays_alive()
+    {
+        Sta.Run(() =>
+        {
+            using var popup = Popup.Create(new AppSettings());
+
+            popup.Window.ShowPopup();
+            popup.Window.Hide();
+
+            Assert.False(popup.Window.IsVisible);
+            Assert.NotEqual(IntPtr.Zero, popup.Handle);
+
+            popup.Window.ShowPopup();
+            Assert.True(popup.Window.IsVisible);
+        });
+    }
+
+    [DllImport("user32.dll", SetLastError = true)]
+    private static extern int GetWindowLong(IntPtr hWnd, int nIndex);
+
+    /// <summary>The tooltip window together with everything it needs to work.</summary>
+    private sealed class Popup : IDisposable
+    {
+        private Popup(LayoutPopupWindow window, LayoutPopupViewModel viewModel)
+        {
+            Window = window;
+            ViewModel = viewModel;
+            Handle = new WindowInteropHelper(window).Handle;
+        }
+
+        internal LayoutPopupWindow Window { get; }
+
+        internal LayoutPopupViewModel ViewModel { get; }
+
+        internal IntPtr Handle { get; }
+
+        internal static Popup Create(AppSettings settings)
+        {
+            var viewModel = new LayoutPopupViewModel(settings);
+            return new Popup(new LayoutPopupWindow(viewModel, settings), viewModel);
+        }
+
+        public void Dispose() => Window.Close();
+    }
+}

CursorLang.Tests/Views/MainWindowTests.cs

@@ -0,0 +1,365 @@
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Controls.Primitives;
+using System.Windows.Data;
+using System.Windows.Media;
+using CursorLang.Models;
+using CursorLang.Services;
+using CursorLang.Tests.Infrastructure;
+using CursorLang.ViewModels;
+using CursorLang.Views;
+
+namespace CursorLang.Tests.Views;
+
+/// <summary>
+/// The settings window as a whole: the markup, the bindings and the hook-up
+/// to the theme.
+/// </summary>
+/// <remarks>
+/// The window has to be shown for real: before the show WPF builds no element
+/// tree and computes no bindings. Full transparency keeps it out of sight.
+/// </remarks>
+public sealed class MainWindowTests
+{
+    [Fact]
+    public void The_window_is_built_and_takes_its_data_from_the_view_model()
+    {
+        Sta.Run(() =>
+        {
+            using SettingsViewModel viewModel = CreateViewModel();
+            var theme = new FakeThemeService();
+
+            Open(viewModel, theme, window =>
+            {
+                Assert.Same(viewModel, window.DataContext);
+                Assert.Equal([window], theme.Registered);
+            });
+        });
+    }
+
+    [Fact]
+    public void The_window_title_comes_from_the_interface_strings()
+    {
+        Sta.Run(() =>
+        {
+            var localization = new FakeLocalizationService();
+            using SettingsViewModel viewModel = CreateViewModel(localization: localization);
+
+            Open(viewModel, window =>
+            {
+                Assert.Equal("en:SettingsTitle", window.Title);
+
+                // A language change goes over every binding to a string
+                localization.CurrentLanguage = "ru";
+                Assert.Equal("ru:SettingsTitle", window.Title);
+            });
+        });
+    }
+
+    [Fact]
+    public void The_window_fits_its_height_to_its_content()
+    {
+        Sta.Run(() =>
+        {
+            using SettingsViewModel viewModel = CreateViewModel();
+
+            Open(viewModel, window =>
+            {
+                Assert.Equal(SizeToContent.Height, window.SizeToContent);
+                Assert.Equal(ResizeMode.CanMinimize, window.ResizeMode);
+                Assert.True(window.ActualHeight > 0);
+                Assert.True(window.ActualWidth > 0);
+            });
+        });
+    }
+
+    // The markup asks the view model for lists and palettes: if a name drifts
+    // apart from the model, the binding silently shows an empty list
+    [Fact]
+    public void The_lists_in_the_window_are_filled()
+    {
+        Sta.Run(() =>
+        {
+            using SettingsViewModel viewModel = CreateViewModel();
+
+            Open(viewModel, window =>
+            {
+                List<ComboBox> boxes = [.. FindAll<ComboBox>(window)];
+
+                Assert.NotEmpty(boxes);
+                Assert.All(boxes, box => Assert.NotEmpty(box.Items));
+            });
+        });
+    }
+
+    [Fact]
+    public void The_lists_show_captions_in_the_chosen_language()
+    {
+        Sta.Run(() =>
+        {
+            using SettingsViewModel viewModel = CreateViewModel();
+
+            Open(viewModel, window =>
+            {
+                // Languages are named in themselves, while the enum options are
+                // named by strings from the resources: the latter are checked
+                List<string> displays =
+                [
+                    .. FindAll<ComboBox>(window)
+                        .SelectMany(box => box.Items.OfType<object>())
+                        .Where(item => item.GetType().Name.StartsWith("EnumOption", StringComparison.Ordinal))
+                        .Select(item => item.ToString() ?? string.Empty),
+                ];
+
+                Assert.NotEmpty(displays);
+                Assert.All(displays, display => Assert.StartsWith("en:", display, StringComparison.Ordinal));
+            });
+        });
+    }
+
+    [Fact]
+    public void The_window_shows_a_preview_of_the_tooltip()
+    {
+        Sta.Run(() =>
+        {
+            var settings = new AppSettings { FontSize = 33 };
+            using SettingsViewModel viewModel = CreateViewModel(settings);
+
+            Open(viewModel, window =>
+                // The font size from the settings is visible right in the window
+                Assert.Contains(FindAll<TextBlock>(window), text => Math.Abs(text.FontSize - 33) < 0.001));
+        });
+    }
+
+    [Fact]
+    public void The_tooltip_colours_are_shown_as_swatches()
+    {
+        Sta.Run(() =>
+        {
+            var settings = new AppSettings();
+            using SettingsViewModel viewModel = CreateViewModel(settings);
+
+            // The chosen colour is shown as a swatch with a caption — in the
+            // same notation the settings file uses
+            Color chosen = viewModel.BackgroundPalette[2];
+            settings.BackgroundColor = chosen;
+
+            string expected = $"#{chosen.R:X2}{chosen.G:X2}{chosen.B:X2}";
+
+            Open(viewModel, window => Assert.Contains(
+                FindAll<TextBlock>(window),
+                text => text.Text.Equals(expected, StringComparison.OrdinalIgnoreCase)));
+        });
+    }
+
+    [Fact]
+    public void The_startup_setting_hides_until_Windows_answers()
+    {
+        Sta.Run(() =>
+        {
+            using SettingsViewModel viewModel = CreateViewModel();
+
+            Open(viewModel, window =>
+            {
+                Assert.False(viewModel.IsStartupAvailable);
+                Assert.All(FindStartupCheckBoxes(window), box => Assert.False(box.IsVisible));
+            });
+        });
+    }
+
+    [Fact]
+    public void An_allowed_startup_shows_up_in_the_window()
+    {
+        Sta.Run(() =>
+        {
+            var startup = new FakeStartupService { State = StartupState.Disabled };
+            using SettingsViewModel viewModel = CreateViewModel(startup: startup);
+
+            Open(viewModel, window =>
+            {
+                viewModel.InitializeAsync().GetAwaiter().GetResult();
+                window.UpdateLayout();
+
+                CheckBox box = Assert.Single(FindStartupCheckBoxes(window));
+                Assert.True(box.IsVisible);
+                Assert.True(box.IsEnabled);
+                Assert.False(box.IsChecked);
+            });
+        });
+    }
+
+    // A ban by the user is not for the app to argue with: the tick is shown,
+    // but it cannot be moved
+    [Fact]
+    public void A_startup_banned_by_Windows_is_shown_as_unavailable()
+    {
+        Sta.Run(() =>
+        {
+            var startup = new FakeStartupService { State = StartupState.DisabledByUser };
+            using SettingsViewModel viewModel = CreateViewModel(startup: startup);
+
+            Open(viewModel, window =>
+            {
+                viewModel.InitializeAsync().GetAwaiter().GetResult();
+                window.UpdateLayout();
+
+                CheckBox box = Assert.Single(FindStartupCheckBoxes(window));
+                Assert.True(box.IsVisible);
+                Assert.False(box.IsEnabled);
+            });
+        });
+    }
+
+    [Fact]
+    public void The_Caps_Lock_interception_is_toggled_by_a_tick()
+    {
+        Sta.Run(() =>
+        {
+            var settings = new AppSettings { UseCapsLockHotkey = false };
+            using SettingsViewModel viewModel = CreateViewModel(settings);
+
+            Open(viewModel, window =>
+            {
+                CheckBox box = Assert.Single(FindCheckBoxesBoundTo(window, "Settings.UseCapsLockHotkey"));
+
+                box.IsChecked = true;
+
+                Assert.True(settings.UseCapsLockHotkey);
+            });
+        });
+    }
+
+    [Fact]
+    public void The_updates_are_checked_by_the_button_in_the_window()
+    {
+        Sta.Run(() =>
+        {
+            var updates = new FakeUpdateService();
+            using UpdateViewModel section = CreateUpdates(updates);
+            using SettingsViewModel viewModel = CreateViewModel(updates: section);
+
+            Open(viewModel, window =>
+            {
+                Button check = Assert.Single(FindButtonsBoundTo(window, "Updates.CheckCommand"));
+
+                Assert.True(check.IsVisible);
+                check.Command.Execute(null);
+
+                Assert.Equal(1, updates.CheckCalls);
+            });
+        });
+    }
+
+    // An app installed from the Store is updated by the Store
+    [Fact]
+    public void An_app_that_updates_itself_elsewhere_shows_no_updates_section()
+    {
+        Sta.Run(() =>
+        {
+            using UpdateViewModel section = CreateUpdates(new FakeUpdateService { IsSupported = false });
+            using SettingsViewModel viewModel = CreateViewModel(updates: section);
+
+            Open(viewModel, window =>
+                Assert.All(FindButtonsBoundTo(window, "Updates.CheckCommand"), button =>
+                    Assert.False(button.IsVisible)));
+        });
+    }
+
+    [Fact]
+    public void The_window_hooks_up_to_the_placement()
+    {
+        Sta.Run(() =>
+        {
+            using SettingsViewModel viewModel = CreateViewModel();
+            var placement = new MainWindowPlacement();
+
+            Open(viewModel, new FakeThemeService(), placement, window =>
+            {
+                // The placement works off the window creation event: the window
+                // has to end up on a monitor rather than beyond its edges
+                Assert.True(window.Left > -10_000);
+                Assert.True(window.Top > -10_000);
+            });
+        });
+    }
+
+    private static SettingsViewModel CreateViewModel(
+        AppSettings? settings = null,
+        ILocalizationService? localization = null,
+        IStartupService? startup = null,
+        UpdateViewModel? updates = null) =>
+        new(settings ?? new AppSettings(),
+            localization ?? new FakeLocalizationService(),
+            startup ?? new FakeStartupService(),
+            updates ?? Fake.Updates());
+
+    private static void Open(SettingsViewModel viewModel, Action<MainWindow> check) =>
+        Open(viewModel, new FakeThemeService(), new MainWindowPlacement(), check);
+
+    private static void Open(SettingsViewModel viewModel, IThemeService theme, Action<MainWindow> check) =>
+        Open(viewModel, theme, new MainWindowPlacement(), check);
+
+    private static void Open(
+        SettingsViewModel viewModel,
+        IThemeService theme,
+        MainWindowPlacement placement,
+        Action<MainWindow> check)
+    {
+        var window = new MainWindow(viewModel, theme, placement)
+        {
+            // The window is needed alive, but not in sight
+            Opacity = 0,
+            ShowInTaskbar = false,
+            ShowActivated = false,
+        };
+
+        try
+        {
+            window.Show();
+            window.UpdateLayout();
+
+            check(window);
+        }
+        finally
+        {
+            window.Close();
+        }
+    }
+
+    private static UpdateViewModel CreateUpdates(IUpdateService updates) =>
+        new(updates, new FakeLocalizationService(), new AppSettings(), new UpdateOptions());
+
+    private static IEnumerable<Button> FindButtonsBoundTo(DependencyObject root, string path) =>
+        FindAll<Button>(root).Where(button =>
+            BindingOperations.GetBinding(button, ButtonBase.CommandProperty)?.Path.Path == path);
+
+    private static IEnumerable<CheckBox> FindStartupCheckBoxes(DependencyObject root) =>
+        FindCheckBoxesBoundTo(root, nameof(SettingsViewModel.RunAtStartup));
+
+    // An element is found by what it is bound to: the markup gives them no names
+    private static IEnumerable<CheckBox> FindCheckBoxesBoundTo(DependencyObject root, string path) =>
+        FindAll<CheckBox>(root).Where(box =>
+            BindingOperations.GetBinding(box, ToggleButton.IsCheckedProperty)?.Path.Path == path);
+
+    // Walking the element tree: the window markup is large, and things have to be searched for
+    private static IEnumerable<TElement> FindAll<TElement>(DependencyObject root)
+        where TElement : DependencyObject
+    {
+        int count = VisualTreeHelper.GetChildrenCount(root);
+
+        for (int i = 0; i < count; i++)
+        {
+            DependencyObject child = VisualTreeHelper.GetChild(root, i);
+
+            if (child is TElement found)
+            {
+                yield return found;
+            }
+
+            foreach (TElement nested in FindAll<TElement>(child))
+            {
+                yield return nested;
+            }
+        }
+    }
+}

CursorLang.Tests/coverage.runsettings

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="utf-8"?>
+<RunSettings>
+    <RunConfiguration>
+        <ResultsDirectory>..\TestResults</ResultsDirectory>
+    </RunConfiguration>
+
+    <DataCollectionRunSettings>
+        <DataCollectors>
+            <DataCollector friendlyName="XPlat code coverage">
+                <Configuration>
+                    <!-- Coverage is measured for the application alone -->
+                    <Include>[CursorLang]*</Include>
+                    <Format>cobertura</Format>
+                    <SingleHit>false</SingleHit>
+                    <UseSourceLink>false</UseSourceLink>
+                    <IncludeTestAssembly>false</IncludeTestAssembly>
+                    <ExcludeByFile>**/*.g.cs,**/*.g.i.cs</ExcludeByFile>
+                </Configuration>
+            </DataCollector>
+        </DataCollectors>
+    </DataCollectionRunSettings>
+</RunSettings>

CursorLang.Tests/xunit.runner.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
+  "methodDisplay": "method",
+  "parallelizeTestCollections": false
+}

CursorLang.sln

@@ -2,15 +2,68 @@
 Microsoft Visual Studio Solution File, Format Version 12.00
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "CursorLang", "CursorLang\CursorLang.csproj", "{4729F06C-53D8-4871-8750-A0632F0A6B07}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "CursorLang.Tests", "CursorLang.Tests\CursorLang.Tests.csproj", "{558F7646-AC3F-4E5E-853D-9F2A09E89C06}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Packaging", "Packaging", "{E927F87B-A585-A586-52B2-1371051E189F}"
+	ProjectSection(SolutionItems) = preProject
+		Packaging\AppxManifest.xml = Packaging\AppxManifest.xml
+		Packaging\build-msix.ps1 = Packaging\build-msix.ps1
+		Packaging\New-Assets.ps1 = Packaging\New-Assets.ps1
+	EndProjectSection
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "CI", "CI", "{3F1B9C24-7A0E-4C55-9E2D-6B41A8D5E713}"
+	ProjectSection(SolutionItems) = preProject
+		.gitea\workflows\pull-request.yml = .gitea\workflows\pull-request.yml
+		.gitea\workflows\release.yml = .gitea\workflows\release.yml
+	EndProjectSection
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "SdkTools", "Packaging\Tools\SdkTools.csproj", "{788339D3-F0C3-4F1A-9216-501814C7BF78}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
+		Debug|x64 = Debug|x64
+		Debug|x86 = Debug|x86
 		Release|Any CPU = Release|Any CPU
+		Release|x64 = Release|x64
+		Release|x86 = Release|x86
 	EndGlobalSection
 	GlobalSection(ProjectConfigurationPlatforms) = postSolution
 		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Debug|x64.Build.0 = Debug|Any CPU
+		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Debug|x86.Build.0 = Debug|Any CPU
 		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Release|Any CPU.Build.0 = Release|Any CPU
+		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Release|x64.ActiveCfg = Release|Any CPU
+		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Release|x64.Build.0 = Release|Any CPU
+		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Release|x86.ActiveCfg = Release|Any CPU
+		{4729F06C-53D8-4871-8750-A0632F0A6B07}.Release|x86.Build.0 = Release|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Debug|x64.Build.0 = Debug|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Debug|x86.Build.0 = Debug|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Release|Any CPU.Build.0 = Release|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Release|x64.ActiveCfg = Release|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Release|x64.Build.0 = Release|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Release|x86.ActiveCfg = Release|Any CPU
+		{558F7646-AC3F-4E5E-853D-9F2A09E89C06}.Release|x86.Build.0 = Release|Any CPU
+		{788339D3-F0C3-4F1A-9216-501814C7BF78}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{788339D3-F0C3-4F1A-9216-501814C7BF78}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{788339D3-F0C3-4F1A-9216-501814C7BF78}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{788339D3-F0C3-4F1A-9216-501814C7BF78}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{788339D3-F0C3-4F1A-9216-501814C7BF78}.Release|x64.ActiveCfg = Release|Any CPU
+		{788339D3-F0C3-4F1A-9216-501814C7BF78}.Release|x86.ActiveCfg = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(NestedProjects) = preSolution
+		{788339D3-F0C3-4F1A-9216-501814C7BF78} = {E927F87B-A585-A586-52B2-1371051E189F}
 	EndGlobalSection
 EndGlobal

CursorLang/App.xaml

@@ -3,6 +3,10 @@
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              ShutdownMode="OnMainWindowClose">
     <Application.Resources>
-
+        <ResourceDictionary>
+            <ResourceDictionary.MergedDictionaries>
+                <ResourceDictionary Source="Themes/Controls.xaml" />
+            </ResourceDictionary.MergedDictionaries>
+        </ResourceDictionary>
     </Application.Resources>
 </Application>

CursorLang/App.xaml.cs

@@ -1,5 +1,4 @@
 using System.Windows;
-using CursorLang.Models;
 using CursorLang.Services;
 using CursorLang.ViewModels;
 using CursorLang.Views;
@@ -7,51 +6,102 @@ using Microsoft.Extensions.DependencyInjection;
 
 namespace CursorLang;
 
-/// <summary>
-/// Композиционный корень: собирает контейнер и запускает окно настроек.
-/// </summary>
+// ReSharper disable once RedundantExtendsListEntry
 public partial class App : Application
 {
     private ServiceProvider? _services;
+    private SingleInstanceGate? _instanceGate;
+    private MainWindowPlacement? _placement;
 
     protected override void OnStartup(StartupEventArgs e)
     {
         base.OnStartup(e);
 
+        var gate = new SingleInstanceGate();
+        if (!gate.TryAcquire())
+        {
+            gate.Dispose();
+            Shutdown();
+            return;
+        }
+
+        _instanceGate = gate;
+        _instanceGate.ActivationRequested += OnActivationRequested;
+
         var services = new ServiceCollection();
         ConfigureServices(services);
         _services = services.BuildServiceProvider();
 
+        _services.GetRequiredService<ThemeService>();
+
+        _placement = _services.GetRequiredService<MainWindowPlacement>();
         MainWindow = _services.GetRequiredService<MainWindow>();
         MainWindow.Show();
 
+        _ = _services.GetRequiredService<SettingsViewModel>().InitializeAsync();
+        _ = _services.GetRequiredService<UpdateViewModel>().StartAsync();
         _services.GetRequiredService<LayoutNotificationCoordinator>().Start();
+        _services.GetRequiredService<CapsLockSwitchCoordinator>().Start();
     }
 
     protected override void OnExit(ExitEventArgs e)
     {
-        // Контейнер сам остановит таймеры, сохранит настройки
-        // и закроет окно подсказки
         _services?.Dispose();
+
+        if (_instanceGate is not null)
+        {
+            _instanceGate.ActivationRequested -= OnActivationRequested;
+            _instanceGate.Dispose();
+        }
+
         base.OnExit(e);
     }
 
-    private static void ConfigureServices(IServiceCollection services)
+    private void OnActivationRequested(object? sender, EventArgs e)
+    {
+        if (MainWindow is not { IsVisible: true })
+        {
+            return;
+        }
+
+        if (MainWindow.WindowState == WindowState.Minimized)
+        {
+            MainWindow.WindowState = WindowState.Normal;
+        }
+
+        _placement?.Apply(MainWindow);
+
+        MainWindow.Activate();
+    }
+
+    internal static void ConfigureServices(IServiceCollection services)
     {
         services.AddSingleton(new KeyboardLayoutOptions());
+        services.AddSingleton(new UpdateOptions());
 
         services.AddSingleton<SettingsService>();
         services.AddSingleton(provider => provider.GetRequiredService<SettingsService>().Load());
 
+        services.AddSingleton<ThemeService>();
+        services.AddSingleton<IThemeService>(provider => provider.GetRequiredService<ThemeService>());
+
+        services.AddSingleton<MainWindowPlacement>();
+
         services.AddSingleton<ILocalizationService, LocalizationService>();
+        services.AddSingleton<IStartupService, StartupService>();
+        services.AddSingleton<IUpdateService, UpdateService>();
         services.AddSingleton<IKeyboardLayoutService, KeyboardLayoutService>();
         services.AddSingleton<ILayoutPopupService, LayoutPopupService>();
+        services.AddSingleton<ICapsLockHotkeyService, CapsLockHotkeyService>();
         services.AddSingleton<LayoutNotificationCoordinator>();
+        services.AddSingleton<CapsLockSwitchCoordinator>();
 
         services.AddSingleton<LayoutPopupViewModel>();
+        services.AddSingleton<UpdateViewModel>();
         services.AddSingleton<SettingsViewModel>();
 
         services.AddSingleton<LayoutPopupWindow>();
+        services.AddSingleton<ILayoutPopupWindow>(provider => provider.GetRequiredService<LayoutPopupWindow>());
         services.AddSingleton<MainWindow>();
     }
 }

CursorLang/AssemblyInfo.cs

@@ -1,10 +1,5 @@
+using System.Runtime.CompilerServices;
 using System.Windows;
 
-[assembly: ThemeInfo(
-    ResourceDictionaryLocation.None, //where theme specific resource dictionaries are located
-    //(used if a resource is not found in the page,
-    // or application resource dictionaries)
-    ResourceDictionaryLocation.SourceAssembly //where the generic resource dictionary is located
-    //(used if a resource is not found in the page,
-    // app, or any theme specific resource dictionaries)
-)]
+[assembly: ThemeInfo(ResourceDictionaryLocation.None, ResourceDictionaryLocation.SourceAssembly)]
+[assembly: InternalsVisibleTo("CursorLang.Tests")]

CursorLang/CursorLang.csproj

@@ -1,11 +1,30 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
         <OutputType>WinExe</OutputType>
-        <TargetFramework>net10.0-windows</TargetFramework>
+        <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>$(NoWarn);CS1591</NoWarn>
         <UseWPF>true</UseWPF>
+        <ApplicationManifest>app.manifest</ApplicationManifest>
+        <ApplicationIcon>Resources\CursorLang.ico</ApplicationIcon>
+        <RuntimeIdentifiers>win-x64;win-arm64</RuntimeIdentifiers>
+        <PlatformTarget>AnyCPU</PlatformTarget>
+        <SelfContained Condition="'$(RuntimeIdentifier)' != ''">true</SelfContained>
+        <PublishTrimmed>false</PublishTrimmed>
+        <PublishReadyToRun Condition="'$(RuntimeIdentifier)' == 'win-x64'">true</PublishReadyToRun>
+        <Version>1.0.0</Version>
+        <AssemblyVersion>1.0.0.0</AssemblyVersion>
+        <FileVersion>1.0.0.0</FileVersion>
+        <Product>CursorLang</Product>
+        <Company>Aleksandr Neychev</Company>
+        <Description>Shows the keyboard layout at the cursor</Description>
+        <Copyright>Copyright (c) 2026</Copyright>
     </PropertyGroup>
 
     <ItemGroup>

CursorLang/Interop/CaretNative.cs

@@ -6,12 +6,12 @@ using Accessibility;
 namespace CursorLang.Interop;
 
 /// <summary>
-/// Определяет положение каретки в активном поле ввода — в том числе в чужом приложении.
+/// Locates the caret in the active input field — including one in another application.
 /// </summary>
 /// <remarks>
-/// Единого способа нет: классические Win32-приложения заводят системную каретку,
-/// а Chrome, Electron и прочие рисуют её сами и сообщают положение только через
-/// средства доступности. Поэтому сначала спрашиваем систему, затем — приложение.
+/// 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.
 /// </remarks>
 internal static class CaretNative
 {
@@ -32,8 +32,8 @@ internal static class CaretNative
     private const int CHILDID_SELF = 0;
 
     /// <summary>
-    /// Прямоугольник каретки в пикселях экрана или <c>null</c>, если активное
-    /// приложение не сообщает её положение.
+    /// 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()
     {
@@ -50,13 +50,13 @@ internal static class CaretNative
     }
 
     /// <summary>
-    /// Отсеивает заведомо неверные координаты.
+    /// Filters out obviously wrong coordinates.
     /// </summary>
     /// <remarks>
-    /// Часть приложений отдаёт положение каретки в своей системе координат либо
-    /// без учёта масштаба экрана, и подсказка уезжает далеко от поля ввода.
-    /// Каретка обязана находиться внутри окна ввода — это и проверяем, а перед
-    /// отказом пробуем истолковать координаты как немасштабированные.
+    /// 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)
     {
@@ -65,12 +65,21 @@ internal static class CaretNative
             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;
         }
 
-        double scale = GetDpiForWindow(hwndFocus) / 96.0;
         var scaled = new PopupWindowNative.Rect
         {
             Left = (int)(caret.Left * scale),
@@ -82,20 +91,20 @@ internal static class CaretNative
         return IsInside(scaled, window) ? scaled : null;
     }
 
-    private static bool IsInside(PopupWindowNative.Rect inner, PopupWindowNative.Rect outer) =>
+    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;
 
-    /// <summary>Сколько ждём ответа от чужого приложения по UI Automation.</summary>
+    /// <summary>How long we wait for another application to answer over UI Automation.</summary>
     private static readonly TimeSpan AutomationTimeout = TimeSpan.FromMilliseconds(150);
 
-    // Браузеры и другие приложения на своих движках рисуют каретку сами и
-    // сообщают её положение только через UI Automation. Запрос идёт в чужой
-    // процесс, поэтому он самый медленный и стоит последним
+    // 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()
     {
-        // Зависшее приложение не должно подвешивать подсказку вместе с собой:
-        // ждём ответ ограниченное время, иначе показываем у курсора
+        // 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;
     }
@@ -117,8 +126,8 @@ internal static class CaretNative
                 return null;
             }
 
-            // У каретки выделение пустое и прямоугольника не имеет,
-            // поэтому расширяем его до ближайшего символа
+            // 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);
 
@@ -141,12 +150,12 @@ internal static class CaretNative
                                    or InvalidOperationException
                                    or COMException)
         {
-            // Приложение закрылось или не отвечает — подсказку это ронять не должно
+            // The application closed or stopped responding — that must not take the popup down
             return null;
         }
     }
 
-    // Системная каретка: координаты приходят относительно окна, которому она принадлежит
+    // 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))
@@ -170,7 +179,7 @@ internal static class CaretNative
         };
     }
 
-    // Каретка через средства доступности: сюда попадают браузеры и Electron
+    // The caret through accessibility interfaces: this is where browsers and Electron land
     private static PopupWindowNative.Rect? TryGetAccessibleCaret(IntPtr hwndFocus)
     {
         if (hwndFocus == IntPtr.Zero)
@@ -199,7 +208,7 @@ internal static class CaretNative
         }
         catch (COMException)
         {
-            // Приложение объявило поддержку, но положение не отдало
+            // The application declared support but did not report the position
             return null;
         }
         finally
@@ -208,7 +217,7 @@ internal static class CaretNative
         }
     }
 
-    // Когда каретки нет, её прямоугольник приходит нулевой высоты.
-    // Судим только по высоте: нулевые координаты — это обычное начало пустого поля
-    private static bool IsEmpty(PopupWindowNative.Rect rect) => rect.Bottom - rect.Top <= 0;
+    // 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/Interop/ForegroundInputNative.cs

@@ -3,8 +3,8 @@ using System.Runtime.InteropServices;
 namespace CursorLang.Interop;
 
 /// <summary>
-/// Сведения о вводе в активном приложении: какое окно держит фокус клавиатуры
-/// и где находится каретка.
+/// Input details of the active application: which window holds keyboard focus
+/// and where the caret is.
 /// </summary>
 internal static class ForegroundInputNative
 {
@@ -26,12 +26,12 @@ internal static class ForegroundInputNative
     private static extern bool GetGUIThreadInfo(uint idThread, ref GuiThreadInfo lpgui);
 
     /// <summary>
-    /// Состояние ввода потока переднего плана.
+    /// The input state of the foreground thread.
     /// </summary>
     /// <remarks>
-    /// Нулевой идентификатор потока не случаен: у современных приложений окно
-    /// верхнего уровня и окно ввода живут в разных потоках, и спрашивать нужно
-    /// именно про передний план целиком.
+    /// 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)
     {

CursorLang/Interop/ForegroundPermissionNative.cs

@@ -0,0 +1,26 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.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/Interop/KeyboardLayoutNative.cs

@@ -3,10 +3,18 @@ using System.Runtime.InteropServices;
 namespace CursorLang.Interop;
 
 /// <summary>
-/// Win32 API для определения раскладки активного приложения.
+/// 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();
 
@@ -16,29 +24,58 @@ internal static class KeyboardLayoutNative
     [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>
-    /// Раскладка, которой сейчас печатает пользователь.
+    /// 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>
-    /// Спрашиваем окно с фокусом клавиатуры, а не окно верхнего плана: у Блокнота
-    /// Windows 11, меню «Пуск» и прочих приложений на WinUI поле ввода живёт в
-    /// отдельном потоке, и раскладка меняется только у него. У потока главного
-    /// окна она остаётся прежней, и переключение проходит незамеченным.
+    /// 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 int GetActiveLocaleId()
+    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 GetLocaleIdOf(info.hwndFocus);
+            return info.hwndFocus;
         }
 
-        return GetLocaleIdOf(GetForegroundWindow());
+        return GetForegroundWindow();
     }
 
     /// <summary>
-    /// Идентификатор локали для окна. Раскладка в Windows привязана к потоку,
-    /// поэтому так её видно у любого приложения, а не только у своего.
+    /// 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)
     {

CursorLang/Interop/LowLevelKeyboardHook.cs

@@ -0,0 +1,130 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.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/Interop/PackageIdentityNative.cs

@@ -0,0 +1,38 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.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/Interop/PopupWindowNative.cs

@@ -3,8 +3,8 @@ using System.Runtime.InteropServices;
 namespace CursorLang.Interop;
 
 /// <summary>
-/// Win32 API для окна-подсказки: стили, позиционирование у курсора
-/// и масштаб монитора, на котором курсор находится.
+/// 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
 {
@@ -62,9 +62,9 @@ internal static class PopupWindowNative
     }
 
     private const int GWL_EXSTYLE = -20;
-    // Окно не забирает фокус у активного приложения
+    // The window does not take focus away from the active application
     private const int WS_EX_NOACTIVATE = 0x08000000;
-    // И не попадает в Alt+Tab
+    // And does not show up in Alt+Tab
     private const int WS_EX_TOOLWINDOW = 0x00000080;
 
     private const uint SWP_NOSIZE = 0x0001;
@@ -81,8 +81,8 @@ internal static class PopupWindowNative
     }
 
     /// <summary>
-    /// Подсказка всплывает поверх чужих приложений, поэтому она не должна
-    /// ни активироваться сама, ни отбирать фокус ввода у активного окна.
+    /// The popup shows up on top of other applications, so it must neither
+    /// activate itself nor steal input focus from the active window.
     /// </summary>
     internal static void MakePassive(IntPtr hWnd)
     {
@@ -91,10 +91,10 @@ internal static class PopupWindowNative
     }
 
     /// <summary>
-    /// Двигает окно в точку экрана, не меняя размер и порядок окон.
-    /// Координаты — физические пиксели: у мониторов разный масштаб, а
-    /// Window.Left/Top пересчитываются по DPI того монитора, где окно сейчас,
-    /// и на соседнем мониторе дают промах.
+    /// 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)
     {
@@ -102,26 +102,27 @@ internal static class PopupWindowNative
     }
 
     /// <summary>
-    /// Задаёт положение и размер окна в физических пикселях.
+    /// Sets the window position and size in physical pixels.
     /// </summary>
     /// <remarks>
-    /// Размер выставляется именно так, а не через Width/Height: при первом показе
-    /// окно ещё подчиняется системному минимальному размеру окна (SM_CXMIN×SM_CYMIN)
-    /// и подсказка выходит заметно крупнее текста. К моменту вызова окно уже
-    /// показано и стало popup-окном, на которое это ограничение не действует.
+    /// The size is set this way rather than through Width/Height: on the first show the
+    /// window is still subject to the system minimum window size (SM_CXMIN×SM_CYMIN)
+    /// and the popup comes out noticeably larger than its text. By the time this is
+    /// called the window is already shown and has become a popup window, which that
+    /// restriction does not apply to.
     /// </remarks>
     internal static void SetBounds(IntPtr hWnd, int x, int y, int width, int height)
     {
         SetWindowPos(hWnd, IntPtr.Zero, x, y, width, height, SWP_NOZORDER | SWP_NOACTIVATE);
     }
 
-    /// <summary>Масштаб монитора, на котором находится точка (1.0 при 96 DPI).</summary>
+    /// <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>
-    /// Рабочая область монитора с активным окном — без панели задач — и его масштаб.
-    /// Именно на этом мониторе пользователь сейчас работает.
+    /// 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()
     {

CursorLang/Interop/WindowPlacementNative.cs

@@ -0,0 +1,52 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Interop;
+
+/// <summary>
+/// Win32 API for placing the settings window: its own bounds and the work area
+/// of the monitor the window is asked to be put on.
+/// </summary>
+/// <remarks>
+/// The bounds are taken from the system rather than from <c>Window.Left/Top/Width/Height</c>:
+/// the window height adapts to its content, and WPF converts those properties using the
+/// monitor DPI, while the monitor work area comes in pixels. Computing the centre in a
+/// single unit is simpler than converting back and forth.
+/// </remarks>
+internal static class WindowPlacementNative
+{
+    [DllImport("user32.dll")]
+    private static extern bool GetWindowRect(IntPtr hWnd, out PopupWindowNative.Rect lpRect);
+
+    [DllImport("user32.dll")]
+    private static extern IntPtr MonitorFromRect(ref PopupWindowNative.Rect lprc, uint dwFlags);
+
+    [DllImport("user32.dll")]
+    private static extern bool GetMonitorInfo(IntPtr hMonitor, ref MonitorInfo lpmi);
+
+    [StructLayout(LayoutKind.Sequential)]
+    private struct MonitorInfo
+    {
+        public int cbSize;
+        public PopupWindowNative.Rect rcMonitor;
+        public PopupWindowNative.Rect rcWork;
+        public uint dwFlags;
+    }
+
+    private const uint MONITOR_DEFAULTTONEAREST = 2;
+
+    /// <summary>The window bounds in screen pixels — including the frame and the title bar.</summary>
+    internal static PopupWindowNative.Rect? TryGetBounds(IntPtr hWnd) =>
+        GetWindowRect(hWnd, out PopupWindowNative.Rect bounds) ? bounds : null;
+
+    /// <summary>
+    /// The work area — without the taskbar — of the monitor that holds the
+    /// rectangle entirely, or at least most of it.
+    /// </summary>
+    internal static PopupWindowNative.Rect? TryGetWorkAreaNear(PopupWindowNative.Rect rect)
+    {
+        IntPtr monitor = MonitorFromRect(ref rect, MONITOR_DEFAULTTONEAREST);
+
+        var info = new MonitorInfo { cbSize = Marshal.SizeOf<MonitorInfo>() };
+        return GetMonitorInfo(monitor, ref info) ? info.rcWork : null;
+    }
+}

CursorLang/Interop/WindowThemeNative.cs

@@ -0,0 +1,30 @@
+using System.Runtime.InteropServices;
+
+namespace CursorLang.Interop;
+
+/// <summary>
+/// Window frame styling by the system: the title bar is drawn by Windows,
+/// and in the dark theme it has to be switched separately from the window content.
+/// </summary>
+internal static class WindowThemeNative
+{
+    [DllImport("dwmapi.dll")]
+    private static extern int DwmSetWindowAttribute(IntPtr hWnd, int attribute, ref int value, int size);
+
+    private const int DWMWA_USE_IMMERSIVE_DARK_MODE = 20;
+
+    /// <summary>
+    /// Recolours the window title bar. On Windows 10 builds before 2004 the attribute
+    /// is not supported — the title bar simply stays light.
+    /// </summary>
+    internal static void SetDarkTitleBar(IntPtr hWnd, bool isDark)
+    {
+        if (hWnd == IntPtr.Zero)
+        {
+            return;
+        }
+
+        int value = isDark ? 1 : 0;
+        DwmSetWindowAttribute(hWnd, DWMWA_USE_IMMERSIVE_DARK_MODE, ref value, sizeof(int));
+    }
+}

CursorLang/Models/AppSettings.cs

@@ -5,46 +5,106 @@ using CommunityToolkit.Mvvm.ComponentModel;
 namespace CursorLang.Models;
 
 /// <summary>
-/// Настройки приложения. Все изменения применяются на лету: подсказка и окно
-/// настроек привязаны к этим свойствам, а <c>SettingsService</c> сохраняет их на диск.
+/// The application settings. Every change applies on the fly: the popup and the
+/// settings window are bound to these properties, and <c>SettingsService</c> saves
+/// them to disk.
 /// </summary>
 public sealed partial class AppSettings : ObservableObject
 {
-    /// <summary>Язык интерфейса в виде кода культуры: «ru», «en».</summary>
+    /// <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;
 
-    [ObservableProperty]
-    private CursorCorner _cursorCorner = CursorCorner.BottomRight;
+    // The side and the offset are stored per mode: the cursor and the caret call for
+    // different settings, and switching the mode does not reset them
 
-    /// <summary>Отступ от курсора в единицах WPF.</summary>
+    /// <summary>The side of the cursor the popup is put on.</summary>
+    [ObservableProperty]
+    private AnchorSide _cursorSide = AnchorSide.BottomRight;
+
+    /// <summary>The offset from the cursor in WPF units.</summary>
     [ObservableProperty]
     private double _cursorOffset = 16;
 
+    /// <summary>The side of the caret the popup is put on.</summary>
+    [ObservableProperty]
+    private AnchorSide _caretSide = AnchorSide.BottomRight;
+
+    /// <summary>The offset from the caret in WPF units.</summary>
+    [ObservableProperty]
+    private double _caretOffset = 16;
+
+    /// <summary>
+    /// The place on the monitor for the <see cref="PopupPlacementMode.FixedPoint"/> mode.
+    /// </summary>
     [ObservableProperty]
     private ScreenPosition _screenPosition = ScreenPosition.BottomRight;
 
-    /// <summary>Отступ от края монитора в единицах WPF.</summary>
+    /// <summary>The offset from the monitor edge in WPF units.</summary>
     [ObservableProperty]
     private double _screenMargin = 24;
 
+    /// <summary>The size of the layout name in the popup, in WPF units.</summary>
     [ObservableProperty]
     private double _fontSize = 20;
 
-    /// <summary>Непрозрачность подсказки: 1.0 — полностью непрозрачная.</summary>
+    /// <summary>The popup opacity: 1.0 is fully opaque.</summary>
     [ObservableProperty]
     private double _opacity = 0.9;
 
-    /// <summary>Сколько подсказка держится на экране, в миллисекундах.</summary>
+    /// <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;
+
+    /// <summary>
+    /// Ask the repository about new versions on startup. A check can always be
+    /// started by hand — this setting turns off only the automatic one.
+    /// </summary>
+    [ObservableProperty]
+    private bool _checkForUpdates = true;
+
+    /// <summary>
+    /// When the application last asked about new versions successfully.
+    /// Stored so as not to go to the network on every startup.
+    /// </summary>
+    [ObservableProperty]
+    private DateTimeOffset? _lastUpdateCheck;
+
+    /// <summary>The fill of the popup. The opacity is set by <see cref="Opacity"/>.</summary>
     [ObservableProperty]
     private Color _backgroundColor = Color.FromRgb(0x20, 0x20, 0x20);
 
+    /// <summary>The colour of the layout name in the popup.</summary>
+    [ObservableProperty]
+    private Color _foregroundColor = Color.FromRgb(0xFF, 0xFF, 0xFF);
+
+    /// <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);
 }

CursorLang/Models/AppTheme.cs

@@ -0,0 +1,12 @@
+namespace CursorLang.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/Models/KeyboardLayout.cs

@@ -3,16 +3,16 @@ using System.Globalization;
 namespace CursorLang.Models;
 
 /// <summary>
-/// Раскладка клавиатуры в удобном для отображения виде.
+/// A keyboard layout in a form convenient for display.
 /// </summary>
-/// <param name="LocaleId">Идентификатор локали (младшее слово HKL).</param>
-/// <param name="ShortName">Короткое имя для подсказки у курсора, например «RU».</param>
-/// <param name="DisplayName">Полное имя, например «RU — русский (Россия)».</param>
+/// <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>
-    /// Строит модель по идентификатору локали. Неизвестные локали не являются
-    /// ошибкой: для них показываем сам идентификатор.
+    /// 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)
     {

CursorLang/Models/LayoutChangedEventArgs.cs

@@ -1,19 +1,19 @@
 namespace CursorLang.Models;
 
 /// <summary>
-/// Почему изменилась текущая раскладка.
+/// Why the current layout has changed.
 /// </summary>
 public enum LayoutChangeReason
 {
-    /// <summary>Пользователь переключил раскладку в активном приложении.</summary>
+    /// <summary>The user switched the layout in the active application.</summary>
     UserSwitched,
 
-    /// <summary>Пользователь перешёл в другое приложение, где своя раскладка.</summary>
+    /// <summary>The user moved to another application that has a layout of its own.</summary>
     ApplicationSwitched,
 }
 
 /// <summary>
-/// Данные события смены раскладки.
+/// The data of a layout change event.
 /// </summary>
 public sealed class LayoutChangedEventArgs(KeyboardLayout layout, LayoutChangeReason reason) : EventArgs
 {

CursorLang/Models/PopupPlacement.cs

@@ -1,43 +1,46 @@
 namespace CursorLang.Models;
 
 /// <summary>
-/// Способ выбора места для подсказки.
+/// How the place for the popup is chosen.
 /// </summary>
 public enum PopupPlacementMode
 {
-    /// <summary>Рядом с курсором мыши.</summary>
+    /// <summary>Next to the mouse cursor.</summary>
     AtCursor,
 
     /// <summary>
-    /// Рядом с кареткой в активном поле ввода. Если приложение не сообщает
-    /// её положение, подсказка показывается у курсора мыши.
+    /// 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>В заданной точке монитора с активным окном.</summary>
+    /// <summary>At a fixed point of the monitor holding the active window.</summary>
     FixedPoint,
 }
 
 /// <summary>
-/// С какой стороны от курсора или каретки показывать подсказку.
+/// Which side of the cursor or the caret to show the popup on.
 /// </summary>
-
-public enum CursorCorner
+public enum AnchorSide
 {
-    BottomRight,
-    BottomLeft,
-    TopRight,
     TopLeft,
+    TopRight,
+    Left,
+    Right,
+    BottomLeft,
+    BottomRight,
 }
 
 /// <summary>
-/// Место на мониторе для режима <see cref="PopupPlacementMode.FixedPoint"/>.
+/// The place on the monitor for the <see cref="PopupPlacementMode.FixedPoint"/> mode.
 /// </summary>
 public enum ScreenPosition
 {
     TopLeft,
+    Top,
     TopRight,
-    BottomLeft,
-    BottomRight,
     Center,
+    BottomLeft,
+    Bottom,
+    BottomRight,
 }

CursorLang/Models/ReleaseInfo.cs

@@ -0,0 +1,18 @@
+namespace CursorLang.Models;
+
+/// <summary>
+/// A file attached to a release.
+/// </summary>
+/// <param name="FileName">The name the file is saved to disk under.</param>
+/// <param name="Url">A direct link to the content.</param>
+/// <param name="Size">The size in bytes; zero when the hosting did not report it.</param>
+public sealed record ReleaseAsset(string FileName, Uri Url, long Size);
+
+/// <summary>
+/// A release found in the repository.
+/// </summary>
+/// <param name="Version">The version parsed from the tag.</param>
+/// <param name="Tag">The tag as is — that is what the interface shows.</param>
+/// <param name="PageUrl">The release page: the release notes live there too.</param>
+/// <param name="Package">The MSIX package the application updates itself with.</param>
+public sealed record ReleaseInfo(Version Version, string Tag, Uri? PageUrl, ReleaseAsset Package);

CursorLang/Models/StartupState.cs

@@ -0,0 +1,30 @@
+namespace CursorLang.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/Models/UpdateStatus.cs

@@ -0,0 +1,29 @@
+namespace CursorLang.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/Resources/Strings.resx

@@ -67,6 +67,18 @@
   <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="SectionPlacement" xml:space="preserve">
     <value>Placement</value>
   </data>
@@ -85,18 +97,24 @@
   <data name="CursorCornerLabel" xml:space="preserve">
     <value>Side</value>
   </data>
-  <data name="CursorCorner_BottomRight" xml:space="preserve">
+  <data name="AnchorSide_BottomRight" xml:space="preserve">
     <value>Bottom right</value>
   </data>
-  <data name="CursorCorner_BottomLeft" xml:space="preserve">
+  <data name="AnchorSide_BottomLeft" xml:space="preserve">
     <value>Bottom left</value>
   </data>
-  <data name="CursorCorner_TopRight" xml:space="preserve">
+  <data name="AnchorSide_TopRight" xml:space="preserve">
     <value>Top right</value>
   </data>
-  <data name="CursorCorner_TopLeft" xml:space="preserve">
+  <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="CursorOffsetLabel" xml:space="preserve">
     <value>Offset</value>
   </data>
@@ -106,6 +124,12 @@
   <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>
@@ -133,6 +157,9 @@
   <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>
@@ -145,4 +172,73 @@
   <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>
+  <data name="SectionUpdates" xml:space="preserve">
+    <value>Updates</value>
+  </data>
+  <data name="CurrentVersionLabel" xml:space="preserve">
+    <value>Installed version</value>
+  </data>
+  <data name="CheckUpdatesButton" xml:space="preserve">
+    <value>Check for updates</value>
+  </data>
+  <data name="UpdateAutoCheck" xml:space="preserve">
+    <value>Check for updates at startup</value>
+  </data>
+  <data name="UpdateChecking" xml:space="preserve">
+    <value>Checking for updates…</value>
+  </data>
+  <data name="UpdateUpToDate" xml:space="preserve">
+    <value>The installed version is the latest one.</value>
+  </data>
+  <data name="UpdateAvailable" xml:space="preserve">
+    <value>Version {0} is available.</value>
+  </data>
+  <data name="UpdateDownloading" xml:space="preserve">
+    <value>Downloading the package…</value>
+  </data>
+  <data name="UpdateReady" xml:space="preserve">
+    <value>The package has been downloaded.</value>
+  </data>
+  <data name="UpdateFailed" xml:space="preserve">
+    <value>Could not reach the releases. Check the connection and try again.</value>
+  </data>
+  <data name="DownloadUpdateButton" xml:space="preserve">
+    <value>Download</value>
+  </data>
+  <data name="InstallUpdateButton" xml:space="preserve">
+    <value>Install</value>
+  </data>
+  <data name="ReleasePageLink" xml:space="preserve">
+    <value>Release page</value>
+  </data>
+  <data name="UpdateInstallHint" xml:space="preserve">
+    <value>Windows will show the package and ask to confirm the installation. The new version takes over once the app is restarted.</value>
+  </data>
 </root>

CursorLang/Resources/Strings.ru.resx

@@ -67,6 +67,18 @@
   <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="SectionPlacement" xml:space="preserve">
     <value>Расположение</value>
   </data>
@@ -85,18 +97,24 @@
   <data name="CursorCornerLabel" xml:space="preserve">
     <value>Сторона</value>
   </data>
-  <data name="CursorCorner_BottomRight" xml:space="preserve">
+  <data name="AnchorSide_BottomRight" xml:space="preserve">
     <value>Справа снизу</value>
   </data>
-  <data name="CursorCorner_BottomLeft" xml:space="preserve">
+  <data name="AnchorSide_BottomLeft" xml:space="preserve">
     <value>Слева снизу</value>
   </data>
-  <data name="CursorCorner_TopRight" xml:space="preserve">
+  <data name="AnchorSide_TopRight" xml:space="preserve">
     <value>Справа сверху</value>
   </data>
-  <data name="CursorCorner_TopLeft" xml:space="preserve">
+  <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="CursorOffsetLabel" xml:space="preserve">
     <value>Отступ</value>
   </data>
@@ -106,6 +124,12 @@
   <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>
@@ -133,6 +157,9 @@
   <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>
@@ -145,4 +172,73 @@
   <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>
+  <data name="SectionUpdates" xml:space="preserve">
+    <value>Обновления</value>
+  </data>
+  <data name="CurrentVersionLabel" xml:space="preserve">
+    <value>Установленная версия</value>
+  </data>
+  <data name="CheckUpdatesButton" xml:space="preserve">
+    <value>Проверить обновления</value>
+  </data>
+  <data name="UpdateAutoCheck" xml:space="preserve">
+    <value>Проверять обновления при запуске</value>
+  </data>
+  <data name="UpdateChecking" xml:space="preserve">
+    <value>Идёт проверка обновлений…</value>
+  </data>
+  <data name="UpdateUpToDate" xml:space="preserve">
+    <value>Установлена последняя версия.</value>
+  </data>
+  <data name="UpdateAvailable" xml:space="preserve">
+    <value>Доступна версия {0}.</value>
+  </data>
+  <data name="UpdateDownloading" xml:space="preserve">
+    <value>Идёт загрузка пакета…</value>
+  </data>
+  <data name="UpdateReady" xml:space="preserve">
+    <value>Пакет скачан.</value>
+  </data>
+  <data name="UpdateFailed" xml:space="preserve">
+    <value>Не удалось обратиться к выпускам. Проверьте подключение и повторите попытку.</value>
+  </data>
+  <data name="DownloadUpdateButton" xml:space="preserve">
+    <value>Скачать</value>
+  </data>
+  <data name="InstallUpdateButton" xml:space="preserve">
+    <value>Установить</value>
+  </data>
+  <data name="ReleasePageLink" xml:space="preserve">
+    <value>Страница выпуска</value>
+  </data>
+  <data name="UpdateInstallHint" xml:space="preserve">
+    <value>Windows покажет пакет и попросит подтвердить установку. Новая версия начнёт работать после перезапуска приложения.</value>
+  </data>
 </root>

CursorLang/Services/CapsLockHotkeyService.cs

@@ -0,0 +1,137 @@
+using System.Windows.Threading;
+using CursorLang.Interop;
+using CursorLang.Models;
+
+namespace CursorLang.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.
+/// </remarks>
+public sealed class CapsLockHotkeyService : ICapsLockHotkeyService, IDisposable
+{
+    private const int VirtualKeyCapsLock = 0x14;
+
+    private readonly AppSettings _settings;
+    private readonly LowLevelKeyboardHook _hook;
+    private readonly DispatcherTimer _holdTimer = new();
+    private readonly Dispatcher _dispatcher = Dispatcher.CurrentDispatcher;
+
+    private bool _isPressed;
+    private bool _isHolding;
+
+    public CapsLockHotkeyService(AppSettings settings)
+    {
+        _settings = settings;
+        _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 dispatcher queue may be shut
+    // down by that moment
+    public void Dispose()
+    {
+        _holdTimer.Tick -= OnHoldTimerTick;
+        _holdTimer.Stop();
+        _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)
+    {
+        _holdTimer.Stop();
+        _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)
+        {
+            _dispatcher.BeginInvoke(() => 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/Services/CapsLockSwitchCoordinator.cs

@@ -0,0 +1,77 @@
+using System.ComponentModel;
+using CursorLang.Models;
+
+namespace CursorLang.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/Services/GiteaReleaseFeed.cs

@@ -0,0 +1,229 @@
+using System.IO;
+using System.Net.Http;
+using System.Net.Http.Headers;
+using System.Runtime.InteropServices;
+using System.Text.Json;
+using CursorLang.Models;
+
+namespace CursorLang.Services;
+
+/// <summary>
+/// Gitea releases.
+/// </summary>
+/// <remarks>
+/// The service address is the address of the server itself — "https://git.example.com/":
+/// the Gitea API lives on the same host as the repository pages.
+/// </remarks>
+internal sealed class GiteaReleaseFeed : IReleaseFeed
+{
+    /// <summary>
+    /// How many releases to ask the server for. It returns them newest first, but the
+    /// newest one may turn out to have no package — when the build has not been
+    /// published yet, for instance — so a small reserve is taken.
+    /// </summary>
+    private const int PageSize = 10;
+
+    /// <summary>
+    /// A response with the release list is a few kilobytes of text. There is no point
+    /// waiting longer: the check runs in the background, and a failed one bothers nobody.
+    /// </summary>
+    private static readonly TimeSpan RequestTimeout = TimeSpan.FromSeconds(20);
+
+    private readonly HttpClient _client;
+    private readonly UpdateOptions _options;
+
+    public GiteaReleaseFeed(HttpClient client, UpdateOptions options)
+    {
+        _client = client;
+        _options = options;
+    }
+
+    /// <summary>
+    /// How the architecture is spelled in the file names built by
+    /// <c>build-msix.ps1</c>: <c>CursorLang-1.0.0.0-x64.msix</c>.
+    /// </summary>
+    private static string ArchitectureName => RuntimeInformation.ProcessArchitecture == Architecture.Arm64
+        ? "arm64"
+        : "x64";
+
+    public async Task<ReleaseInfo?> GetLatestAsync(CancellationToken cancellationToken)
+    {
+        using var timeout = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+        timeout.CancelAfter(RequestTimeout);
+
+        using var request = new HttpRequestMessage(HttpMethod.Get, BuildReleasesUri());
+        Authorize(request);
+
+        using HttpResponseMessage response = await _client.SendAsync(
+            request, HttpCompletionOption.ResponseHeadersRead, timeout.Token);
+        response.EnsureSuccessStatusCode();
+
+        await using Stream stream = await response.Content.ReadAsStreamAsync(timeout.Token);
+        using JsonDocument document = await JsonDocument.ParseAsync(stream, cancellationToken: timeout.Token);
+
+        if (document.RootElement.ValueKind != JsonValueKind.Array)
+        {
+            return null;
+        }
+
+        // The order of the releases is up to the server, while what we need is the
+        // highest version number: a fix released for an old branch may well be the newest one
+        return document.RootElement.EnumerateArray()
+            .Select(Read)
+            .OfType<ReleaseInfo>()
+            .MaxBy(release => release.Version);
+    }
+
+    public void Authorize(HttpRequestMessage request)
+    {
+        if (!string.IsNullOrWhiteSpace(_options.AccessToken))
+        {
+            // "token" is the Gitea scheme of its own for access keys; "Bearer" is not
+            // understood by every version, while this one has been there since the API appeared
+            request.Headers.Authorization = new AuthenticationHeaderValue("token", _options.AccessToken);
+        }
+    }
+
+    /// <summary>
+    /// Parses a single release. <c>null</c> means the release will not do: a draft,
+    /// a prerelease or a release without a package.
+    /// </summary>
+    private static ReleaseInfo? Read(JsonElement release)
+    {
+        // A draft is visible only to whoever created it, and the application does not
+        // offer a prerelease: those are sought out deliberately
+        if (ReadFlag(release, "draft") || ReadFlag(release, "prerelease"))
+        {
+            return null;
+        }
+
+        Version? version = ParseTag(ReadString(release, "tag_name"));
+        if (version is null)
+        {
+            return null;
+        }
+
+        if (!release.TryGetProperty("assets", out JsonElement assets) || assets.ValueKind != JsonValueKind.Array)
+        {
+            return null;
+        }
+
+        ReleaseAsset? package = PickPackage(assets.EnumerateArray().Select(ReadAsset).OfType<ReleaseAsset>());
+        if (package is null)
+        {
+            return null;
+        }
+
+        return new ReleaseInfo(
+            version,
+            ReadString(release, "tag_name") ?? version.ToString(),
+            ReadUri(release, "html_url"),
+            package);
+    }
+
+    private static ReleaseAsset? ReadAsset(JsonElement asset)
+    {
+        string? name = ReadString(asset, "name");
+        Uri? url = ReadUri(asset, "browser_download_url");
+
+        if (string.IsNullOrWhiteSpace(name) || url is null)
+        {
+            return null;
+        }
+
+        long size = asset.TryGetProperty("size", out JsonElement value) && value.TryGetInt64(out long bytes)
+            ? bytes
+            : 0;
+
+        return new ReleaseAsset(name, url, size);
+    }
+
+    /// <summary>
+    /// The version from a tag. "1.2.3" and "v1.2.3" are understood; a tag with
+    /// anything besides numbers — "v1.2.3-beta" — counts as a prerelease and is
+    /// skipped: the application does not offer such versions on its own.
+    /// </summary>
+    private static Version? ParseTag(string? tag)
+    {
+        if (string.IsNullOrWhiteSpace(tag))
+        {
+            return null;
+        }
+
+        ReadOnlySpan<char> numbers = tag.AsSpan().Trim().TrimStart("vV");
+
+        foreach (char symbol in numbers)
+        {
+            if (!char.IsAsciiDigit(symbol) && symbol != '.')
+            {
+                return null;
+            }
+        }
+
+        if (!Version.TryParse(numbers, out Version? version))
+        {
+            return null;
+        }
+
+        // In a tag such as "v1.2" the lower parts are not set at all, yet comparing
+        // them with the version of the installed package calls for zeros
+        return new Version(
+            version.Major,
+            version.Minor,
+            Math.Max(version.Build, 0),
+            Math.Max(version.Revision, 0));
+    }
+
+    /// <summary>
+    /// Picks the attached file the application updates itself with.
+    /// </summary>
+    private static ReleaseAsset? PickPackage(IEnumerable<ReleaseAsset> assets)
+    {
+        // An unencrypted connection is out right away: Windows will check the package
+        // signature by itself, but a substituted file is not even worth downloading
+        ReleaseAsset[] packages = [.. assets.Where(asset => asset.Url.Scheme == Uri.UriSchemeHttps)];
+
+        ReleaseAsset? bundle = packages.FirstOrDefault(
+            asset => asset.FileName.EndsWith(".msixbundle", StringComparison.OrdinalIgnoreCase));
+
+        if (bundle is not null)
+        {
+            // A bundle carries both architectures, so there is nothing to choose between
+            return bundle;
+        }
+
+        ReleaseAsset[] single = [.. packages.Where(
+            asset => asset.FileName.EndsWith(".msix", StringComparison.OrdinalIgnoreCase))];
+
+        ReleaseAsset? matching = single.FirstOrDefault(
+            asset => asset.FileName.Contains(ArchitectureName, StringComparison.OrdinalIgnoreCase));
+
+        // A package without an architecture in its name will do only when it is the
+        // only one: otherwise it is unclear which of them is for this machine
+        return matching ?? (single.Length == 1 ? single[0] : null);
+    }
+
+    private static string? ReadString(JsonElement element, string name) =>
+        element.TryGetProperty(name, out JsonElement value) && value.ValueKind == JsonValueKind.String
+            ? value.GetString()
+            : null;
+
+    private static Uri? ReadUri(JsonElement element, string name) =>
+        Uri.TryCreate(ReadString(element, name), UriKind.Absolute, out Uri? uri) ? uri : null;
+
+    private static bool ReadFlag(JsonElement element, string name) =>
+        element.TryGetProperty(name, out JsonElement value) && value.ValueKind == JsonValueKind.True;
+
+    /// <summary>
+    /// The address the server returns the release list at. The trailing slash matters:
+    /// without it <c>Uri</c> drops the last part of the address, and
+    /// "https://host/gitea" would have turned into "https://host/api/...".
+    /// </summary>
+    private Uri BuildReleasesUri()
+    {
+        string service = _options.ServiceUri.AbsoluteUri;
+        string path = $"api/v1/repos/{_options.Project.Trim('/')}/releases?limit={PageSize}";
+
+        return new Uri(service.EndsWith('/') ? service + path : $"{service}/{path}");
+    }
+}

CursorLang/Services/ICapsLockHotkeyService.cs

@@ -0,0 +1,29 @@
+namespace CursorLang.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/Services/IKeyboardLayoutService.cs

@@ -3,11 +3,12 @@ using CursorLang.Models;
 namespace CursorLang.Services;
 
 /// <summary>
-/// Следит за раскладкой активного окна — в том числе в чужих приложениях.
+/// Tracks the layout of the active window — including in other applications —
+/// and can switch it.
 /// </summary>
 public interface IKeyboardLayoutService
 {
-    /// <summary>Раскладка активного окна на текущий момент.</summary>
+    /// <summary>The layout of the active window at the moment.</summary>
     KeyboardLayout Current { get; }
 
     event EventHandler<LayoutChangedEventArgs>? LayoutChanged;
@@ -15,4 +16,6 @@ public interface IKeyboardLayoutService
     void Start();
 
     void Stop();
+
+    void SwitchToNext();
 }

CursorLang/Services/ILayoutPopupService.cs

@@ -3,9 +3,18 @@ using CursorLang.Models;
 namespace CursorLang.Services;
 
 /// <summary>
-/// Показывает подсказку с раскладкой у курсора.
+/// 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/Services/ILayoutPopupWindow.cs

@@ -0,0 +1,21 @@
+namespace CursorLang.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 at the place set by the settings.</summary>
+    void ShowPopup();
+
+    /// <summary>Takes the window off the screen without destroying it.</summary>
+    void Hide();
+
+    /// <summary>Closes the window for good.</summary>
+    void Close();
+}

CursorLang/Services/ILocalizationService.cs

@@ -2,21 +2,21 @@ using System.ComponentModel;
 
 namespace CursorLang.Services;
 
-/// <summary>Язык интерфейса для выбора в настройках.</summary>
-/// <param name="Code">Код культуры: «ru», «en».</param>
-/// <param name="DisplayName">Название на самом этом языке.</param>
+/// <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)
 {
-    // Средства доступности берут имя элемента списка отсюда
+    // Accessibility tools take the name of the list item from here
     public override string ToString() => DisplayName;
 }
 
 /// <summary>
-/// Даёт строки интерфейса и умеет менять язык без перезапуска.
+/// Provides the interface strings and can change the language without a restart.
 /// </summary>
 public interface ILocalizationService : INotifyPropertyChanged
 {
-    /// <summary>Строка по ключу ресурса. Привязки обновляются при смене языка.</summary>
+    /// <summary>The string for a resource key. Bindings update when the language changes.</summary>
     string this[string key] { get; }
 
     IReadOnlyList<LanguageOption> AvailableLanguages { get; }

CursorLang/Services/IReleaseFeed.cs

@@ -0,0 +1,22 @@
+using System.Net.Http;
+using CursorLang.Models;
+
+namespace CursorLang.Services;
+
+/// <summary>
+/// The release list of the repository.
+/// </summary>
+public interface IReleaseFeed
+{
+    /// <summary>
+    /// Returns the newest release carrying an MSIX package, or <c>null</c>
+    /// when there is no suitable release.
+    /// </summary>
+    Task<ReleaseInfo?> GetLatestAsync(CancellationToken cancellationToken);
+
+    /// <summary>
+    /// Adds to the request whatever a private repository needs. The package is
+    /// downloaded not by the list itself, but access to it is closed just the same.
+    /// </summary>
+    void Authorize(HttpRequestMessage request);
+}

CursorLang/Services/IStartupService.cs

@@ -0,0 +1,18 @@
+using CursorLang.Models;
+
+namespace CursorLang.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/Services/IThemeService.cs

@@ -0,0 +1,19 @@
+using System.Windows;
+using CursorLang.Models;
+
+namespace CursorLang.Services;
+
+/// <summary>
+/// Applies the light or the dark look to the windows of the application.
+/// </summary>
+public interface IThemeService
+{
+    /// <summary>The theme in effect right now.</summary>
+    AppTheme CurrentTheme { get; }
+
+    /// <summary>
+    /// Hooks a window up to theme changes: the window title bar is drawn by Windows,
+    /// and its colour has to be switched for each window separately.
+    /// </summary>
+    void Register(Window window);
+}

CursorLang/Services/IUpdateService.cs

@@ -0,0 +1,40 @@
+using CursorLang.Models;
+
+namespace CursorLang.Services;
+
+/// <summary>
+/// Checking for and installing new versions of the application.
+/// </summary>
+public interface IUpdateService
+{
+    /// <summary>
+    /// It makes sense for this installation to update itself.
+    /// </summary>
+    /// <remarks>
+    /// An application installed from the Store is updated by the Store itself:
+    /// offering a package from elsewhere on top of it will not do — Windows would
+    /// not accept it anyway.
+    /// </remarks>
+    bool IsSupported { get; }
+
+    /// <summary>The version of the running application.</summary>
+    Version CurrentVersion { get; }
+
+    /// <summary>
+    /// Looks for a release newer than the installed one. <c>null</c> means the latest
+    /// version is installed or there is no suitable release in the repository.
+    /// </summary>
+    Task<ReleaseInfo?> CheckAsync(CancellationToken cancellationToken);
+
+    /// <summary>
+    /// Downloads the release package and returns the path to it.
+    /// </summary>
+    /// <remarks>
+    /// <c>progress</c> receives the downloaded fraction from 0 to 1. While the file
+    /// size is unknown — not every hosting reports it — there will be no calls at all.
+    /// </remarks>
+    Task<string> DownloadAsync(ReleaseInfo release, IProgress<double>? progress, CancellationToken cancellationToken);
+
+    /// <summary>Hands the downloaded package over to the Windows app installer.</summary>
+    void Install(string packagePath);
+}

CursorLang/Services/KeyboardLayoutService.cs

@@ -5,58 +5,91 @@ using CursorLang.Models;
 namespace CursorLang.Services;
 
 /// <summary>
-/// Настройки слежения за раскладкой.
+/// The settings of layout tracking.
 /// </summary>
 public sealed class KeyboardLayoutOptions
 {
-    /// <summary>Как часто проверять раскладку активного окна.</summary>
+    /// <summary>How often to check the layout of the active window.</summary>
     public TimeSpan PollInterval { get; init; } = TimeSpan.FromMilliseconds(150);
 }
 
 /// <summary>
-/// Опрашивает активное окно по таймеру.
+/// Polls the active window on a timer.
 /// </summary>
 /// <remarks>
-/// Опрос выбран не от простоты: событийных способов узнать о смене раскладки
-/// в чужом процессе из managed-кода нет. HSHELL_LANGUAGE от RegisterShellHookWindow
-/// в Windows 10/11 не приходит, а уведомления TSF (ITfLanguageProfileNotifySink)
-/// сообщают только о смене языка внутри своего процесса — оба варианта проверены
-/// и не сработали. Один тик — три вызова Win32, читающих данные из памяти ядра.
+/// 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.
 /// </remarks>
 public sealed class KeyboardLayoutService : IKeyboardLayoutService, IDisposable
 {
     private readonly DispatcherTimer _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 DispatcherTimer { Interval = options.PollInterval };
         _pollTimer.Tick += OnTick;
     }
 
     public event EventHandler<LayoutChangedEventArgs>? LayoutChanged;
 
-    public KeyboardLayout Current => KeyboardLayout.FromLocaleId(KeyboardLayoutNative.GetActiveLocaleId());
+    public KeyboardLayout Current => KeyboardLayout.FromLocaleId(_getActiveLocaleId());
 
     public void Start()
     {
-        _lastForegroundWindow = KeyboardLayoutNative.GetForegroundWindow();
-        _lastLocaleId = KeyboardLayoutNative.GetActiveLocaleId();
+        _lastForegroundWindow = _getForegroundWindow();
+        _lastLocaleId = _getActiveLocaleId();
         _pollTimer.Start();
     }
 
     public void Stop() => _pollTimer.Stop();
 
+    public void SwitchToNext() => _requestNextLayout();
+
     public void Dispose()
     {
         _pollTimer.Stop();
         _pollTimer.Tick -= OnTick;
     }
 
-    private void OnTick(object? sender, EventArgs e)
+    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 = KeyboardLayoutNative.GetForegroundWindow();
+        IntPtr foreground = _getForegroundWindow();
         if (foreground == IntPtr.Zero)
         {
             return;
@@ -65,7 +98,7 @@ public sealed class KeyboardLayoutService : IKeyboardLayoutService, IDisposable
         bool appSwitched = foreground != _lastForegroundWindow;
         _lastForegroundWindow = foreground;
 
-        int localeId = KeyboardLayoutNative.GetActiveLocaleId();
+        int localeId = _getActiveLocaleId();
         if (localeId == _lastLocaleId)
         {
             return;
@@ -73,9 +106,9 @@ public sealed class KeyboardLayoutService : IKeyboardLayoutService, IDisposable
 
         _lastLocaleId = localeId;
 
-        // Переход в другое приложение со своей раскладкой — не то же самое,
-        // что переключение раскладки пользователем, и подписчики вправе
-        // реагировать на эти случаи по-разному
+        // 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;

CursorLang/Services/LayoutNotificationCoordinator.cs

@@ -3,8 +3,8 @@ using CursorLang.Models;
 namespace CursorLang.Services;
 
 /// <summary>
-/// Связывает слежение за раскладкой с показом подсказки.
-/// Живёт всё время работы приложения независимо от открытых окон.
+/// 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
 {
@@ -31,8 +31,8 @@ public sealed class LayoutNotificationCoordinator : IDisposable
 
     private void OnLayoutChanged(object? sender, LayoutChangedEventArgs e)
     {
-        // При переходе в другое приложение раскладка меняется без участия
-        // пользователя, и всплывающая подсказка была бы навязчивой
+        // 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/Services/LayoutPopupService.cs

@@ -1,22 +1,21 @@
 using System.Windows.Threading;
 using CursorLang.Models;
 using CursorLang.ViewModels;
-using CursorLang.Views;
 
 namespace CursorLang.Services;
 
 /// <summary>
-/// Управляет временем жизни подсказки: окно отвечает только за показ,
-/// а решение «когда показать и когда убрать» принимается здесь.
+/// 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>
 public sealed class LayoutPopupService : ILayoutPopupService, IDisposable
 {
-    private readonly LayoutPopupWindow _window;
+    private readonly ILayoutPopupWindow _window;
     private readonly LayoutPopupViewModel _viewModel;
     private readonly AppSettings _settings;
     private readonly DispatcherTimer _hideTimer = new();
 
-    public LayoutPopupService(LayoutPopupWindow window, LayoutPopupViewModel viewModel, AppSettings settings)
+    public LayoutPopupService(ILayoutPopupWindow window, LayoutPopupViewModel viewModel, AppSettings settings)
     {
         _window = window;
         _viewModel = viewModel;
@@ -27,16 +26,28 @@ public sealed class LayoutPopupService : ILayoutPopupService, IDisposable
 
     public void Show(KeyboardLayout layout)
     {
-        _viewModel.ShortName = layout.ShortName;
-        _window.ShowPopup();
+        ShowUntilHidden(layout);
 
-        // Длительность читаем при каждом показе: её меняют в настройках на лету.
-        // Перезапуск таймера заодно продлевает показ при быстрых переключениях
-        _hideTimer.Stop();
+        // 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();
+
+        _viewModel.ShortName = layout.ShortName;
+        _window.ShowPopup();
+    }
+
+    public void Hide()
+    {
+        _hideTimer.Stop();
+        _window.Hide();
+    }
+
     public void Dispose()
     {
         _hideTimer.Stop();

CursorLang/Services/LocalizationService.cs

@@ -6,7 +6,8 @@ using CommunityToolkit.Mvvm.ComponentModel;
 namespace CursorLang.Services;
 
 /// <summary>
-/// Берёт строки из ресурсов и при смене языка просит WPF перечитать все привязки.
+/// Takes the strings from the resources and, when the language changes, asks WPF to
+/// re-read every binding.
 /// </summary>
 public sealed class LocalizationService : ObservableObject, ILocalizationService
 {
@@ -38,8 +39,8 @@ public sealed class LocalizationService : ObservableObject, ILocalizationService
 
             OnPropertyChanged(nameof(CurrentLanguage));
 
-            // Сообщаем об изменении индексатора: так обновляются все привязки
-            // вида {Binding Localization[Key]}, то есть весь текст интерфейса
+            // 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(Binding.IndexerName);
         }
     }

CursorLang/Services/MainWindowPlacement.cs

@@ -0,0 +1,178 @@
+using System.Windows;
+using System.Windows.Interop;
+using CursorLang.Interop;
+
+namespace CursorLang.Services;
+
+/// <summary>
+/// Decides where the settings window shows up: for the first time in a session — in
+/// the centre of the monitor the user is working on, and after that — where they
+/// left that window.
+/// </summary>
+/// <remarks>
+/// The position lives in memory only and is not kept between launches: the set of
+/// monitors may be different by the next launch, while "in the centre of the active
+/// one" is always right.
+/// </remarks>
+public sealed class MainWindowPlacement
+{
+    private PopupWindowNative.Point? _position;
+
+    // The window reports a move when we move it ourselves as well;
+    // what has to be remembered is only what the user chose
+    private bool _isPlacing;
+
+    /// <summary>
+    /// Takes over the placement of the window: puts it in place by the first show
+    /// and follows where the user moves it.
+    /// </summary>
+    public void Attach(Window window)
+    {
+        window.SourceInitialized += OnSourceInitialized;
+        window.LocationChanged += OnLocationChanged;
+    }
+
+    /// <summary>
+    /// Returns the window to the remembered place, and when it has not been shown
+    /// yet during this session — puts it in the centre of the active monitor.
+    /// </summary>
+    public void Apply(Window window)
+    {
+        // A minimized window has no meaningful bounds: it is restored in its former
+        // place, and it can be positioned only after that
+        if (window.WindowState != WindowState.Normal)
+        {
+            return;
+        }
+
+        IntPtr handle = new WindowInteropHelper(window).Handle;
+        if (handle == IntPtr.Zero || WindowPlacementNative.TryGetBounds(handle) is not { } bounds)
+        {
+            return;
+        }
+
+        PopupWindowNative.Point? wanted = _position ?? CenterOnActiveMonitor(bounds);
+        if (wanted is null || KeepOnScreen(wanted.Value, bounds) is not { } target)
+        {
+            return;
+        }
+
+        _isPlacing = true;
+        try
+        {
+            PopupWindowNative.MoveTo(handle, target.X, target.Y);
+        }
+        finally
+        {
+            _isPlacing = false;
+        }
+
+        _position = target;
+    }
+
+    // The window height adapts to its content and is unknown until the first layout
+    // pass — an empty window frame would end up in the centre. So we ask for the
+    // layout to be computed right away: by that moment the window is not shown yet,
+    // so it will not flash in its former place
+    private void OnSourceInitialized(object? sender, EventArgs e)
+    {
+        if (sender is not Window window)
+        {
+            return;
+        }
+
+        window.SourceInitialized -= OnSourceInitialized;
+        window.UpdateLayout();
+        Apply(window);
+    }
+
+    private void OnLocationChanged(object? sender, EventArgs e)
+    {
+        if (_isPlacing || sender is not Window { WindowState: WindowState.Normal } window)
+        {
+            return;
+        }
+
+        IntPtr handle = new WindowInteropHelper(window).Handle;
+        if (handle != IntPtr.Zero && WindowPlacementNative.TryGetBounds(handle) is { } bounds)
+        {
+            _position = new PopupWindowNative.Point { X = bounds.Left, Y = bounds.Top };
+        }
+    }
+
+    private static PopupWindowNative.Point? CenterOnActiveMonitor(PopupWindowNative.Rect bounds)
+    {
+        (PopupWindowNative.Rect work, _) = PopupWindowNative.GetActiveMonitorWorkArea();
+        return IsEmpty(work) ? null : Center(bounds, work);
+    }
+
+    /// <summary>
+    /// The point at which a window with the given bounds ends up in the centre of the work area.
+    /// </summary>
+    internal static PopupWindowNative.Point Center(
+        PopupWindowNative.Rect bounds, PopupWindowNative.Rect work) => new()
+        {
+            X = work.Left + (((work.Right - work.Left) - Width(bounds)) / 2),
+            Y = work.Top + (((work.Bottom - work.Top) - Height(bounds)) / 2),
+        };
+
+    /// <summary>
+    /// Pulls the window into the work area of the nearest monitor.
+    /// </summary>
+    /// <remarks>
+    /// Needed in two cases. The monitor the user put the window on may be disconnected
+    /// during the session — returning the window to its place would then leave the
+    /// user without a window, so the remembered position is a wish here rather than an
+    /// order. And the window height equals the height of its content and may exceed
+    /// the work area on a short monitor — then the title bar of a window placed in the
+    /// centre would go past the top edge.
+    /// </remarks>
+    private static PopupWindowNative.Point? KeepOnScreen(
+        PopupWindowNative.Point position, PopupWindowNative.Rect bounds)
+    {
+        int width = Width(bounds);
+        int height = Height(bounds);
+
+        var wanted = new PopupWindowNative.Rect
+        {
+            Left = position.X,
+            Top = position.Y,
+            Right = position.X + width,
+            Bottom = position.Y + height,
+        };
+
+        if (WindowPlacementNative.TryGetWorkAreaNear(wanted) is not { } work || IsEmpty(work))
+        {
+            return null;
+        }
+
+        return Clamp(position, bounds, work);
+    }
+
+    /// <summary>
+    /// Pulls the point so that a window with the given bounds fits into the work area
+    /// entirely. A window taller than the work area gets its top edge: the title bar
+    /// is needed more than the lower part of the window.
+    /// </summary>
+    internal static PopupWindowNative.Point Clamp(
+        PopupWindowNative.Point position,
+        PopupWindowNative.Rect bounds,
+        PopupWindowNative.Rect work)
+    {
+        int width = Width(bounds);
+        int height = Height(bounds);
+
+        return new PopupWindowNative.Point
+        {
+            X = Math.Clamp(position.X, work.Left, Math.Max(work.Left, work.Right - width)),
+            Y = Math.Clamp(position.Y, work.Top, Math.Max(work.Top, work.Bottom - height)),
+        };
+    }
+
+    private static int Width(PopupWindowNative.Rect rect) => rect.Right - rect.Left;
+
+    private static int Height(PopupWindowNative.Rect rect) => rect.Bottom - rect.Top;
+
+    internal static bool IsEmpty(PopupWindowNative.Rect rect) =>
+        rect.Right <= rect.Left || rect.Bottom <= rect.Top;
+}

CursorLang/Services/PopupLayout.cs

@@ -0,0 +1,95 @@
+using CursorLang.Interop;
+using CursorLang.Models;
+
+namespace CursorLang.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/Services/RegistryStartup.cs

@@ -0,0 +1,120 @@
+using System.IO;
+using System.Security;
+using CursorLang.Models;
+using Microsoft.Win32;
+
+namespace CursorLang.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 path of the running program is
+    /// unknown, and there is nothing to write down.
+    /// </summary>
+    private static string? GetCommand() =>
+        Environment.ProcessPath is { Length: > 0 } path ? $"\"{path}\"" : null;
+}

CursorLang/Services/SettingsService.cs

@@ -4,13 +4,27 @@ using System.Text.Json;
 using System.Text.Json.Serialization;
 using System.Windows.Media;
 using System.Windows.Threading;
+using CursorLang.Interop;
 using CursorLang.Models;
+using Windows.Storage;
 
 namespace CursorLang.Services;
 
 /// <summary>
-/// Хранит настройки в %APPDATA%\CursorLang\settings.json.
+/// Keeps the settings in the settings.json file.
 /// </summary>
+/// <remarks>
+/// The location of the file 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()
@@ -19,31 +33,68 @@ public sealed class SettingsService : IDisposable
         Converters = { new ColorJsonConverter(), new JsonStringEnumConverter() },
     };
 
+    private const string FileName = "settings.json";
+
     private readonly string _filePath;
+    private readonly string _inheritedFilePath;
     private readonly DispatcherTimer _saveTimer;
     private AppSettings? _settings;
 
-    public SettingsService()
-    {
-        string folder = Path.Combine(
-            Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
-            "CursorLang");
-        _filePath = Path.Combine(folder, "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);
 
-        // Ползунки меняют значения непрерывно, поэтому запись на диск
-        // откладывается до паузы в изменениях
-        _saveTimer = new DispatcherTimer { Interval = TimeSpan.FromMilliseconds(500) };
+    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 DispatcherTimer { Interval = saveDelay };
         _saveTimer.Tick += OnSaveTimerTick;
     }
 
     /// <summary>
-    /// Читает настройки с диска либо отдаёт значения по умолчанию,
-    /// и дальше сам сохраняет любые изменения.
+    /// Reads the settings from disk or returns the default values,
+    /// and from then on saves any changes by itself.
     /// </summary>
     public AppSettings Load()
     {
-        _settings = ReadFile() ?? CreateDefault();
+        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();
         _settings.PropertyChanged += OnSettingsChanged;
+
+        // 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;
     }
 
@@ -61,7 +112,7 @@ public sealed class SettingsService : IDisposable
         }
         catch (Exception e) when (e is IOException or UnauthorizedAccessException)
         {
-            // Настройки — не тот случай, ради которого стоит ронять приложение
+            // The settings are not the kind of thing worth bringing the application down for
         }
     }
 
@@ -77,25 +128,46 @@ public sealed class SettingsService : IDisposable
         }
     }
 
-    private AppSettings? ReadFile()
+    /// <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(_filePath))
+            if (!File.Exists(path))
             {
                 return null;
             }
 
-            return JsonSerializer.Deserialize<AppSettings>(File.ReadAllText(_filePath), SerializerOptions);
+            return JsonSerializer.Deserialize<AppSettings>(File.ReadAllText(path), SerializerOptions);
         }
         catch (Exception e) when (e is IOException or UnauthorizedAccessException or JsonException)
         {
-            // Испорченный файл не должен мешать запуску: начнём с умолчаний
             return null;
         }
     }
 
-    // Язык по умолчанию берём системный, если он поддерживается
     private static AppSettings CreateDefault()
     {
         string uiLanguage = System.Globalization.CultureInfo.CurrentUICulture.TwoLetterISOLanguageName;
@@ -114,7 +186,7 @@ public sealed class SettingsService : IDisposable
         Save();
     }
 
-    // Color не сериализуется штатно, а хранить его читаемым в файле удобно
+    // Color is not serialized out of the box, and keeping it readable in the file is handy
     private sealed class ColorJsonConverter : JsonConverter<Color>
     {
         public override Color Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

CursorLang/Services/SingleInstanceGate.cs

@@ -0,0 +1,117 @@
+using System.Windows.Threading;
+using CursorLang.Interop;
+
+namespace CursorLang.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.
+/// </remarks>
+public sealed class SingleInstanceGate : IDisposable
+{
+    private const string MutexName = "CursorLang.SingleInstance";
+    private const string ActivationEventName = "CursorLang.ActivationRequest";
+
+    private readonly Dispatcher _dispatcher = Dispatcher.CurrentDispatcher;
+    private readonly string _mutexName;
+    private readonly string _activationEventName;
+
+    private Mutex? _mutex;
+    private EventWaitHandle? _activationRequest;
+    private RegisteredWaitHandle? _activationWait;
+    private bool _isOwner;
+
+    public SingleInstanceGate()
+        : this(string.Empty)
+    {
+    }
+
+    /// <summary>
+    /// Adds a distinguishing part to the kernel object names. Needed by the tests:
+    /// otherwise they would share the single-instance slot with the running
+    /// application and get in its way.
+    /// </summary>
+    internal SingleInstanceGate(string nameSuffix)
+    {
+        _mutexName = MutexName + nameSuffix;
+        _activationEventName = ActivationEventName + nameSuffix;
+    }
+
+    /// <summary>Another launch asks for the window to be shown.</summary>
+    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()
+    {
+        _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)
+        {
+            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;
+    }
+
+    // The thread pool reports the request from wherever it happens to be, while the
+    // window obeys only its own thread
+    private void OnActivationSignalled(object? state, bool timedOut) =>
+        _dispatcher.BeginInvoke(() => ActivationRequested?.Invoke(this, EventArgs.Empty));
+}

CursorLang/Services/StartupService.cs

@@ -0,0 +1,85 @@
+using System.Runtime.InteropServices;
+using CursorLang.Interop;
+using CursorLang.Models;
+using Windows.ApplicationModel;
+
+namespace CursorLang.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/Services/ThemeService.cs

@@ -0,0 +1,161 @@
+using System.ComponentModel;
+using System.Windows;
+using System.Windows.Interop;
+using CursorLang.Interop;
+using CursorLang.Models;
+using Microsoft.Win32;
+
+namespace CursorLang.Services;
+
+/// <summary>
+/// Keeps the palette of the chosen theme in the application resources and swaps it
+/// when the setting changes — the windows are recoloured without a restart.
+/// </summary>
+public sealed class ThemeService : IThemeService, IDisposable
+{
+    private const string PersonalizeKey =
+        @"Software\Microsoft\Windows\CurrentVersion\Themes\Personalize";
+
+    private readonly AppSettings _settings;
+    private readonly Func<AppTheme> _detectSystemTheme;
+    private readonly List<Window> _windows = [];
+    private ResourceDictionary? _palette;
+    private AppTheme _current;
+
+    public ThemeService(AppSettings settings)
+        : this(settings, DetectSystemTheme)
+    {
+    }
+
+    /// <summary>
+    /// Takes the source of the system theme explicitly: in tests it is not the registry
+    /// that provides it.
+    /// </summary>
+    internal ThemeService(AppSettings settings, Func<AppTheme> detectSystemTheme)
+    {
+        _settings = settings;
+        _detectSystemTheme = detectSystemTheme;
+        _settings.PropertyChanged += OnSettingsChanged;
+        SystemEvents.UserPreferenceChanged += OnUserPreferenceChanged;
+        Apply();
+    }
+
+    /// <summary>The theme the user sees: <c>System</c> is already resolved here.</summary>
+    public AppTheme CurrentTheme => _current;
+
+    public void Register(Window window)
+    {
+        _windows.Add(window);
+        window.Closed += OnWindowClosed;
+
+        if (new WindowInteropHelper(window).Handle == IntPtr.Zero)
+        {
+            window.SourceInitialized += OnWindowSourceInitialized;
+            return;
+        }
+
+        ApplyTitleBar(window);
+    }
+
+    public void Dispose()
+    {
+        _settings.PropertyChanged -= OnSettingsChanged;
+        SystemEvents.UserPreferenceChanged -= OnUserPreferenceChanged;
+
+        foreach (Window window in _windows)
+        {
+            window.Closed -= OnWindowClosed;
+            window.SourceInitialized -= OnWindowSourceInitialized;
+        }
+
+        _windows.Clear();
+    }
+
+    /// <summary>The app theme from the Windows settings.</summary>
+    internal static AppTheme DetectSystemTheme()
+    {
+        try
+        {
+            using RegistryKey? key = Registry.CurrentUser.OpenSubKey(PersonalizeKey);
+            return key?.GetValue("AppsUseLightTheme") is int light && light == 0
+                ? AppTheme.Dark
+                : AppTheme.Light;
+        }
+        catch (Exception e) when (e is System.Security.SecurityException or UnauthorizedAccessException)
+        {
+            return AppTheme.Light;
+        }
+    }
+
+    private void OnSettingsChanged(object? sender, PropertyChangedEventArgs e)
+    {
+        if (e.PropertyName == nameof(AppSettings.Theme))
+        {
+            Apply();
+        }
+    }
+
+    // Windows reports a theme change from outside the interface thread
+    private void OnUserPreferenceChanged(object? sender, UserPreferenceChangedEventArgs e) =>
+        Application.Current?.Dispatcher.InvokeAsync(Apply);
+
+    private void Apply()
+    {
+        AppTheme theme = _settings.Theme == AppTheme.System ? _detectSystemTheme() : _settings.Theme;
+        if (_palette is not null && theme == _current)
+        {
+            return;
+        }
+
+        _current = theme;
+
+        var next = new ResourceDictionary
+        {
+            Source = PaletteUri(theme),
+        };
+
+        ICollection<ResourceDictionary> dictionaries = Application.Current.Resources.MergedDictionaries;
+        dictionaries.Add(next);
+
+        if (_palette is not null)
+        {
+            dictionaries.Remove(_palette);
+        }
+
+        _palette = next;
+
+        foreach (Window window in _windows)
+        {
+            ApplyTitleBar(window);
+        }
+    }
+
+    // The assembly name in the address is there on purpose: without it the dictionary
+    // is looked up in the assembly the process started from, which is not always the
+    // application itself
+    internal static Uri PaletteUri(AppTheme theme) =>
+        new($"pack://application:,,,/CursorLang;component/Themes/{theme}.xaml", UriKind.Absolute);
+
+    private void ApplyTitleBar(Window window) =>
+        WindowThemeNative.SetDarkTitleBar(
+            new WindowInteropHelper(window).Handle,
+            _current == AppTheme.Dark);
+
+    private void OnWindowSourceInitialized(object? sender, EventArgs e)
+    {
+        if (sender is Window window)
+        {
+            window.SourceInitialized -= OnWindowSourceInitialized;
+            ApplyTitleBar(window);
+        }
+    }
+
+    private void OnWindowClosed(object? sender, EventArgs e)
+    {
+        if (sender is Window window)
+        {
+            window.Closed -= OnWindowClosed;
+            _windows.Remove(window);
+        }
+    }
+}

CursorLang/Services/UpdateOptions.cs

@@ -0,0 +1,37 @@
+namespace CursorLang.Services;
+
+/// <summary>
+/// Where the application learns about new versions from.
+/// </summary>
+/// <remarks>
+/// These settings belong to the build rather than to the user: the repository is
+/// chosen by whoever releases the application, and these values have no business
+/// being in <c>settings.json</c>. The defaults point at the repository the
+/// application is built from.
+/// </remarks>
+public sealed class UpdateOptions
+{
+    /// <summary>
+    /// The address of the Gitea server. Its API lives on the same host as the
+    /// repository pages, so this is the same address the repository is opened at
+    /// in a browser.
+    /// </summary>
+    public Uri ServiceUri { get; init; } = new("https://git.alrakis.kz/");
+
+    /// <summary>The project: <c>owner/repository</c>.</summary>
+    public string Project { get; init; } = "alrakis/cursor-lang";
+
+    /// <summary>How often the application checks the releases on its own.</summary>
+    public TimeSpan CheckInterval { get; init; } = TimeSpan.FromDays(1);
+
+    /// <summary>
+    /// An access token for a private repository.
+    /// </summary>
+    /// <remarks>
+    /// Taken from an environment variable rather than from a file in the repository:
+    /// a secret that gets into a build gets to everyone who received it as well.
+    /// A public repository needs no token at all.
+    /// </remarks>
+    public string? AccessToken { get; init; } =
+        Environment.GetEnvironmentVariable("CURSORLANG_UPDATE_TOKEN");
+}

CursorLang/Services/UpdateService.cs

@@ -0,0 +1,226 @@
+using System.Diagnostics;
+using System.IO;
+using System.Net.Http;
+using System.Net.Http.Headers;
+using System.Reflection;
+using System.Runtime.InteropServices;
+using CursorLang.Interop;
+using CursorLang.Models;
+using Windows.ApplicationModel;
+
+namespace CursorLang.Services;
+
+/// <summary>
+/// Learns about new versions from the repository and hands the downloaded package
+/// over to the installer.
+/// </summary>
+/// <remarks>
+/// The package is installed by the Windows app installer, not by the application on
+/// its own. Through <c>PackageManager</c> the update would go without a single
+/// window, but then the application would have to explain both an untrusted signature
+/// and a policy ban to the user itself — the installer already knows how to do all
+/// that and shows the package publisher before the installation, not after.
+/// </remarks>
+public sealed class UpdateService : IUpdateService, IDisposable
+{
+    /// <summary>The package is large and the network can be slow: the buffer is taken with room to spare.</summary>
+    private const int BufferSize = 81920;
+
+    /// <summary>The version of the running application — it does not change while it runs.</summary>
+    private static readonly Version Current = DetectCurrentVersion();
+
+    private readonly IReleaseFeed _feed;
+    private readonly HttpClient _client;
+    private readonly bool _ownsClient;
+    private readonly string _downloadFolder;
+
+    public UpdateService(UpdateOptions options)
+    {
+        _client = CreateClient();
+        _ownsClient = true;
+        _downloadFolder = Path.Combine(Path.GetTempPath(), "CursorLang");
+        _feed = new GiteaReleaseFeed(_client, options);
+
+        CurrentVersion = Current;
+    }
+
+    /// <summary>
+    /// Takes the releases, the network and the version explicitly: in tests they are
+    /// not provided by Windows.
+    /// </summary>
+    internal UpdateService(IReleaseFeed feed, HttpClient client, Version current, string downloadFolder)
+    {
+        _feed = feed;
+        _client = client;
+        _ownsClient = false;
+        _downloadFolder = downloadFolder;
+
+        CurrentVersion = current;
+    }
+
+    public bool IsSupported { get; } = DetectSupport();
+
+    public Version CurrentVersion { get; }
+
+    public async Task<ReleaseInfo?> CheckAsync(CancellationToken cancellationToken)
+    {
+        ReleaseInfo? release = await _feed.GetLatestAsync(cancellationToken);
+        return release is not null && release.Version > CurrentVersion ? release : null;
+    }
+
+    public async Task<string> DownloadAsync(
+        ReleaseInfo release,
+        IProgress<double>? progress,
+        CancellationToken cancellationToken)
+    {
+        string path = Path.Combine(_downloadFolder, BuildFileName(release));
+        string partial = path + ".part";
+
+        Directory.CreateDirectory(_downloadFolder);
+        RemoveLeftovers(path);
+
+        using var request = new HttpRequestMessage(HttpMethod.Get, release.Package.Url);
+        _feed.Authorize(request);
+
+        using HttpResponseMessage response = await _client.SendAsync(
+            request, HttpCompletionOption.ResponseHeadersRead, cancellationToken);
+        response.EnsureSuccessStatusCode();
+
+        long total = response.Content.Headers.ContentLength ?? release.Package.Size;
+
+        await using (Stream source = await response.Content.ReadAsStreamAsync(cancellationToken))
+        await using (FileStream target = File.Create(partial))
+        {
+            byte[] buffer = new byte[BufferSize];
+            long copied = 0;
+            int reported = -1;
+            int read;
+
+            while ((read = await source.ReadAsync(buffer, cancellationToken)) > 0)
+            {
+                await target.WriteAsync(buffer.AsMemory(0, read), cancellationToken);
+                copied += read;
+
+                if (total <= 0)
+                {
+                    continue;
+                }
+
+                // The progress bar cannot tell fractions of a percent apart, and
+                // redrawing on every chunk read would cost more than the download itself
+                int percent = (int)(copied * 100 / total);
+                if (percent != reported)
+                {
+                    reported = percent;
+                    progress?.Report(percent / 100d);
+                }
+            }
+        }
+
+        // A file becomes ready only once downloaded in full: an interrupted download
+        // must not stay on disk under the package name
+        File.Move(partial, path, overwrite: true);
+        return path;
+    }
+
+    public void Install(string packagePath) =>
+        Process.Start(new ProcessStartInfo(packagePath) { UseShellExecute = true })?.Dispose();
+
+    public void Dispose()
+    {
+        if (_ownsClient)
+        {
+            _client.Dispose();
+        }
+    }
+
+    private static HttpClient CreateClient()
+    {
+        // The check and the download have different deadlines: seconds are enough for
+        // the first one, while the second one takes minutes on a slow network. So the
+        // client has no shared timeout, and every operation allots time for itself
+        var handler = new SocketsHttpHandler { ConnectTimeout = TimeSpan.FromSeconds(15) };
+        var client = new HttpClient(handler) { Timeout = Timeout.InfiniteTimeSpan };
+
+        // The User-Agent shows who came: a request without one may well be taken
+        // for a robot and rejected by the server
+        client.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("CursorLang", Current.ToString()));
+
+        return client;
+    }
+
+    /// <summary>
+    /// Whether to check for updates at all: a package from the Store gets them from the Store.
+    /// </summary>
+    private static bool DetectSupport()
+    {
+        if (!PackageIdentityNative.IsPackaged)
+        {
+            return true;
+        }
+
+        try
+        {
+            return Package.Current.SignatureKind != PackageSignatureKind.Store;
+        }
+        catch (Exception e) when (e is COMException or InvalidOperationException)
+        {
+            return true;
+        }
+    }
+
+    private static Version DetectCurrentVersion()
+    {
+        if (PackageIdentityNative.IsPackaged)
+        {
+            try
+            {
+                // A package has a version of its own — the one from the manifest. That
+                // is also the one in the release tag, while the assembly version may differ
+                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);
+    }
+
+    /// <summary>
+    /// The file name on disk. Only the extension is taken from the hosting response:
+    /// the name itself comes from the outside, and a file is created with it.
+    /// </summary>
+    private static string BuildFileName(ReleaseInfo release)
+    {
+        string extension = release.Package.FileName.EndsWith(".msixbundle", StringComparison.OrdinalIgnoreCase)
+            ? ".msixbundle"
+            : ".msix";
+
+        return $"CursorLang-{release.Version}{extension}";
+    }
+
+    /// <summary>
+    /// Removes packages downloaded earlier: they take up a noticeable amount of
+    /// space and are needed only until the installation.
+    /// </summary>
+    private void RemoveLeftovers(string keep)
+    {
+        try
+        {
+            foreach (string file in Directory.EnumerateFiles(_downloadFolder))
+            {
+                if (!string.Equals(file, keep, StringComparison.OrdinalIgnoreCase))
+                {
+                    File.Delete(file);
+                }
+            }
+        }
+        catch (Exception e) when (e is IOException or UnauthorizedAccessException)
+        {
+            // The file is held by another installer — that does not get in the way of the update
+        }
+    }
+}

CursorLang/Themes/Controls.xaml

@@ -0,0 +1,480 @@
+<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+
+    <!-- ======================= GroupBox ======================= -->
+
+    <Style TargetType="GroupBox">
+        <Setter Property="Padding" Value="12" />
+        <Setter Property="Margin" Value="0,0,0,12" />
+        <Setter Property="Foreground" Value="{DynamicResource Theme.Foreground}" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="GroupBox">
+                    <Grid>
+                        <Grid.RowDefinitions>
+                            <RowDefinition Height="Auto" />
+                            <RowDefinition Height="*" />
+                        </Grid.RowDefinitions>
+                        <ContentPresenter ContentSource="Header"
+                                          Margin="2,0,0,6"
+                                          TextBlock.FontWeight="SemiBold" />
+                        <Border Grid.Row="1"
+                                Background="{DynamicResource Theme.Surface}"
+                                BorderBrush="{DynamicResource Theme.ControlBorder}"
+                                BorderThickness="1"
+                                CornerRadius="6"
+                                Padding="{TemplateBinding Padding}"
+                                SnapsToDevicePixels="True">
+                            <ContentPresenter />
+                        </Border>
+                    </Grid>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <!-- ======================= Button ======================= -->
+
+    <Style TargetType="Button">
+        <Setter Property="Foreground" Value="{DynamicResource Theme.Foreground}" />
+        <Setter Property="Padding" Value="12,5" />
+        <Setter Property="MinHeight" Value="28" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="Button">
+                    <Border x:Name="Border"
+                            Background="{DynamicResource Theme.ControlBackground}"
+                            BorderBrush="{DynamicResource Theme.ControlBorder}"
+                            BorderThickness="1"
+                            CornerRadius="4"
+                            Padding="{TemplateBinding Padding}"
+                            SnapsToDevicePixels="True">
+                        <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center" />
+                    </Border>
+                    <ControlTemplate.Triggers>
+                        <Trigger Property="IsMouseOver" Value="True">
+                            <Setter TargetName="Border" Property="Background"
+                                    Value="{DynamicResource Theme.ControlHoverBackground}" />
+                        </Trigger>
+                        <Trigger Property="IsKeyboardFocused" Value="True">
+                            <Setter TargetName="Border" Property="BorderBrush"
+                                    Value="{DynamicResource Theme.Accent}" />
+                        </Trigger>
+                        <Trigger Property="IsPressed" Value="True">
+                            <Setter TargetName="Border" Property="Background"
+                                    Value="{DynamicResource Theme.SelectionBackground}" />
+                            <Setter TargetName="Border" Property="BorderBrush"
+                                    Value="{DynamicResource Theme.Accent}" />
+                        </Trigger>
+                        <Trigger Property="IsEnabled" Value="False">
+                            <Setter Property="Opacity" Value="0.5" />
+                        </Trigger>
+                    </ControlTemplate.Triggers>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <!-- ======================= ComboBox ======================= -->
+
+    <Style x:Key="ComboBoxToggleStyle" TargetType="ToggleButton">
+        <Setter Property="OverridesDefaultStyle" Value="True" />
+        <Setter Property="Focusable" Value="False" />
+        <Setter Property="IsTabStop" Value="False" />
+        <Setter Property="ClickMode" Value="Press" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="ToggleButton">
+                    <Border x:Name="Border"
+                            Background="{DynamicResource Theme.ControlBackground}"
+                            BorderBrush="{DynamicResource Theme.ControlBorder}"
+                            BorderThickness="1"
+                            CornerRadius="4"
+                            SnapsToDevicePixels="True">
+                        <Path x:Name="Arrow"
+                              HorizontalAlignment="Right" VerticalAlignment="Center"
+                              Margin="0,0,10,0"
+                              Data="M0,0 L4,4 L8,0"
+                              Stroke="{DynamicResource Theme.SecondaryForeground}"
+                              StrokeThickness="1.4" />
+                    </Border>
+                    <ControlTemplate.Triggers>
+                        <Trigger Property="IsMouseOver" Value="True">
+                            <Setter TargetName="Border" Property="Background"
+                                    Value="{DynamicResource Theme.ControlHoverBackground}" />
+                        </Trigger>
+                        <Trigger Property="IsChecked" Value="True">
+                            <Setter TargetName="Border" Property="BorderBrush"
+                                    Value="{DynamicResource Theme.Accent}" />
+                            <Setter TargetName="Arrow" Property="Stroke"
+                                    Value="{DynamicResource Theme.Accent}" />
+                        </Trigger>
+                        <Trigger Property="IsEnabled" Value="False">
+                            <Setter TargetName="Border" Property="Opacity" Value="0.5" />
+                        </Trigger>
+                    </ControlTemplate.Triggers>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <Style TargetType="ComboBoxItem">
+        <Setter Property="Foreground" Value="{DynamicResource Theme.Foreground}" />
+        <Setter Property="Padding" Value="8,5" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="ComboBoxItem">
+                    <Border x:Name="Border"
+                            Background="Transparent"
+                            CornerRadius="3"
+                            Margin="3,1"
+                            Padding="{TemplateBinding Padding}">
+                        <ContentPresenter />
+                    </Border>
+                    <ControlTemplate.Triggers>
+                        <Trigger Property="IsHighlighted" Value="True">
+                            <Setter TargetName="Border" Property="Background"
+                                    Value="{DynamicResource Theme.SelectionBackground}" />
+                        </Trigger>
+                        <Trigger Property="IsSelected" Value="True">
+                            <Setter TargetName="Border" Property="Background"
+                                    Value="{DynamicResource Theme.SelectionBackground}" />
+                        </Trigger>
+                    </ControlTemplate.Triggers>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <Style TargetType="ComboBox">
+        <Setter Property="VerticalAlignment" Value="Center" />
+        <Setter Property="Foreground" Value="{DynamicResource Theme.Foreground}" />
+        <!-- Right padding leaves space for the arrow -->
+        <Setter Property="Padding" Value="9,5,28,5" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="ComboBox">
+                    <Grid>
+                        <ToggleButton Style="{StaticResource ComboBoxToggleStyle}"
+                                      IsChecked="{Binding IsDropDownOpen, Mode=TwoWay,
+                                          RelativeSource={RelativeSource TemplatedParent}}" />
+                        <ContentPresenter Margin="{TemplateBinding Padding}"
+                                          Content="{TemplateBinding SelectionBoxItem}"
+                                          ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
+                                          ContentStringFormat="{TemplateBinding SelectionBoxItemStringFormat}"
+                                          HorizontalAlignment="Left" VerticalAlignment="Center"
+                                          IsHitTestVisible="False" />
+                        <Popup x:Name="PART_Popup"
+                               Placement="Bottom"
+                               IsOpen="{TemplateBinding IsDropDownOpen}"
+                               AllowsTransparency="True"
+                               Focusable="False"
+                               PopupAnimation="Fade">
+                            <Border Background="{DynamicResource Theme.PopupBackground}"
+                                    BorderBrush="{DynamicResource Theme.ControlBorder}"
+                                    BorderThickness="1"
+                                    CornerRadius="4"
+                                    Margin="0,2,0,0"
+                                    MinWidth="{Binding ActualWidth,
+                                        RelativeSource={RelativeSource TemplatedParent}}"
+                                    MaxHeight="{TemplateBinding MaxDropDownHeight}">
+                                <ScrollViewer>
+                                    <ItemsPresenter KeyboardNavigation.DirectionalNavigation="Contained" />
+                                </ScrollViewer>
+                            </Border>
+                        </Popup>
+                    </Grid>
+                    <ControlTemplate.Triggers>
+                        <Trigger Property="IsEnabled" Value="False">
+                            <Setter Property="Opacity" Value="0.5" />
+                        </Trigger>
+                    </ControlTemplate.Triggers>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <!-- ======================= CheckBox ======================= -->
+
+    <Style TargetType="CheckBox">
+        <Setter Property="VerticalAlignment" Value="Center" />
+        <Setter Property="Foreground" Value="{DynamicResource Theme.Foreground}" />
+        <Setter Property="MinHeight" Value="24" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="CheckBox">
+                    <!-- Transparent background makes clicking on the label register as a click on the checkbox.
+                         The label lies in a column of limited width: long translations wrap by words
+                         instead of being cut off -->
+                    <Grid Background="Transparent">
+                        <Grid.ColumnDefinitions>
+                            <ColumnDefinition Width="Auto" />
+                            <ColumnDefinition Width="*" />
+                        </Grid.ColumnDefinitions>
+                        <Border x:Name="Box"
+                                Width="18" Height="18"
+                                CornerRadius="4"
+                                VerticalAlignment="Center"
+                                Background="{DynamicResource Theme.ControlBackground}"
+                                BorderBrush="{DynamicResource Theme.ControlBorder}"
+                                BorderThickness="1"
+                                SnapsToDevicePixels="True">
+                            <Path x:Name="Check"
+                                  HorizontalAlignment="Center" VerticalAlignment="Center"
+                                  Data="M0,4 L3.5,7.5 L9.5,0.5"
+                                  Stroke="White"
+                                  StrokeThickness="1.8"
+                                  StrokeStartLineCap="Round" StrokeEndLineCap="Round"
+                                  Visibility="Collapsed" />
+                        </Border>
+                        <ContentPresenter Grid.Column="1" Margin="8,0,0,0"
+                                          VerticalAlignment="Center">
+                            <ContentPresenter.Resources>
+                                <Style TargetType="TextBlock">
+                                    <Setter Property="TextWrapping" Value="Wrap" />
+                                </Style>
+                            </ContentPresenter.Resources>
+                        </ContentPresenter>
+                    </Grid>
+                    <ControlTemplate.Triggers>
+                        <Trigger Property="IsMouseOver" Value="True">
+                            <Setter TargetName="Box" Property="Background"
+                                    Value="{DynamicResource Theme.ControlHoverBackground}" />
+                        </Trigger>
+                        <Trigger Property="IsChecked" Value="True">
+                            <Setter TargetName="Box" Property="Background"
+                                    Value="{DynamicResource Theme.Accent}" />
+                            <Setter TargetName="Box" Property="BorderBrush"
+                                    Value="{DynamicResource Theme.Accent}" />
+                            <Setter TargetName="Check" Property="Visibility" Value="Visible" />
+                        </Trigger>
+                        <Trigger Property="IsEnabled" Value="False">
+                            <Setter Property="Opacity" Value="0.5" />
+                        </Trigger>
+                    </ControlTemplate.Triggers>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <!-- ======================= Slider ======================= -->
+
+    <Style x:Key="SliderThumbStyle" TargetType="Thumb">
+        <Setter Property="OverridesDefaultStyle" Value="True" />
+        <Setter Property="Width" Value="14" />
+        <Setter Property="Height" Value="14" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="Thumb">
+                    <Ellipse x:Name="Circle"
+                             Fill="{DynamicResource Theme.Accent}"
+                             Stroke="{DynamicResource Theme.ControlBackground}"
+                             StrokeThickness="2" />
+                    <ControlTemplate.Triggers>
+                        <Trigger Property="IsMouseOver" Value="True">
+                            <Setter TargetName="Circle" Property="StrokeThickness" Value="1" />
+                        </Trigger>
+                    </ControlTemplate.Triggers>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <!-- Completed part of the scale: filled with accent color -->
+    <Style x:Key="SliderDecreaseStyle" TargetType="RepeatButton">
+        <Setter Property="OverridesDefaultStyle" Value="True" />
+        <Setter Property="Focusable" Value="False" />
+        <Setter Property="IsTabStop" Value="False" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="RepeatButton">
+                    <Border Height="4" CornerRadius="2"
+                            VerticalAlignment="Center"
+                            Background="{DynamicResource Theme.Accent}" />
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <Style x:Key="TransparentRepeatStyle" TargetType="RepeatButton">
+        <Setter Property="OverridesDefaultStyle" Value="True" />
+        <Setter Property="Focusable" Value="False" />
+        <Setter Property="IsTabStop" Value="False" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="RepeatButton">
+                    <Border Background="Transparent" />
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <Style TargetType="Slider">
+        <Setter Property="VerticalAlignment" Value="Center" />
+        <Setter Property="IsSnapToTickEnabled" Value="True" />
+        <Setter Property="MinHeight" Value="24" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="Slider">
+                    <Grid Background="Transparent">
+                        <Border Height="4" CornerRadius="2"
+                                VerticalAlignment="Center"
+                                Background="{DynamicResource Theme.SliderTrack}" />
+                        <Track x:Name="PART_Track">
+                            <Track.DecreaseRepeatButton>
+                                <RepeatButton Command="Slider.DecreaseLarge"
+                                              Style="{StaticResource SliderDecreaseStyle}" />
+                            </Track.DecreaseRepeatButton>
+                            <Track.Thumb>
+                                <Thumb Style="{StaticResource SliderThumbStyle}" />
+                            </Track.Thumb>
+                            <Track.IncreaseRepeatButton>
+                                <RepeatButton Command="Slider.IncreaseLarge"
+                                              Style="{StaticResource TransparentRepeatStyle}" />
+                            </Track.IncreaseRepeatButton>
+                        </Track>
+                    </Grid>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <!-- ======================= ProgressBar ======================= -->
+
+    <Style TargetType="ProgressBar">
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="ProgressBar">
+                    <Border Background="{DynamicResource Theme.SliderTrack}"
+                            CornerRadius="2"
+                            SnapsToDevicePixels="True">
+                        <Grid x:Name="PART_Track" ClipToBounds="True">
+                            <Border x:Name="PART_Indicator"
+                                    HorizontalAlignment="Left"
+                                    Background="{DynamicResource Theme.Accent}"
+                                    CornerRadius="2"
+                                    RenderTransformOrigin="0,0">
+                                <Border.RenderTransform>
+                                    <ScaleTransform ScaleX="1" />
+                                </Border.RenderTransform>
+                            </Border>
+                        </Grid>
+                    </Border>
+                    <ControlTemplate.Triggers>
+                        <!-- The share downloaded is unknown: the bar fills over and over instead -->
+                        <Trigger Property="IsIndeterminate" Value="True">
+                            <Trigger.EnterActions>
+                                <BeginStoryboard x:Name="Running">
+                                    <Storyboard>
+                                        <DoubleAnimation
+                                            Storyboard.TargetName="PART_Indicator"
+                                            Storyboard.TargetProperty="(UIElement.RenderTransform).(ScaleTransform.ScaleX)"
+                                            From="0" To="1"
+                                            Duration="0:0:1.2"
+                                            RepeatBehavior="Forever" />
+                                    </Storyboard>
+                                </BeginStoryboard>
+                            </Trigger.EnterActions>
+                            <Trigger.ExitActions>
+                                <StopStoryboard BeginStoryboardName="Running" />
+                            </Trigger.ExitActions>
+                        </Trigger>
+                    </ControlTemplate.Triggers>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <!-- ======================= Hyperlink ======================= -->
+
+    <!-- The underline is left for the pointer: a line under every link is noise -->
+    <Style TargetType="Hyperlink">
+        <Setter Property="Foreground" Value="{DynamicResource Theme.Accent}" />
+        <Setter Property="TextDecorations" Value="{x:Null}" />
+        <Style.Triggers>
+            <Trigger Property="IsMouseOver" Value="True">
+                <Setter Property="TextDecorations" Value="Underline" />
+            </Trigger>
+        </Style.Triggers>
+    </Style>
+
+    <!-- ======================= ScrollBar ======================= -->
+
+    <Style x:Key="ScrollBarThumbStyle" TargetType="Thumb">
+        <Setter Property="OverridesDefaultStyle" Value="True" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="Thumb">
+                    <Border Background="{DynamicResource Theme.ScrollBarThumb}"
+                            CornerRadius="3" Margin="3" />
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+    <Style TargetType="ScrollBar">
+        <Setter Property="Background" Value="Transparent" />
+        <Setter Property="Width" Value="12" />
+        <Setter Property="MinWidth" Value="12" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="ScrollBar">
+                    <Grid Background="{TemplateBinding Background}">
+                        <Track x:Name="PART_Track"
+                               Orientation="{TemplateBinding Orientation}"
+                               IsDirectionReversed="True">
+                            <Track.DecreaseRepeatButton>
+                                <RepeatButton Command="ScrollBar.PageUpCommand"
+                                              Style="{StaticResource TransparentRepeatStyle}" />
+                            </Track.DecreaseRepeatButton>
+                            <Track.Thumb>
+                                <Thumb Style="{StaticResource ScrollBarThumbStyle}" />
+                            </Track.Thumb>
+                            <Track.IncreaseRepeatButton>
+                                <RepeatButton Command="ScrollBar.PageDownCommand"
+                                              Style="{StaticResource TransparentRepeatStyle}" />
+                            </Track.IncreaseRepeatButton>
+                        </Track>
+                    </Grid>
+                    <ControlTemplate.Triggers>
+                        <!-- For the horizontal bar, the track direction is normal -->
+                        <Trigger Property="Orientation" Value="Horizontal">
+                            <Setter TargetName="PART_Track" Property="IsDirectionReversed" Value="False" />
+                        </Trigger>
+                    </ControlTemplate.Triggers>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+        <Style.Triggers>
+            <Trigger Property="Orientation" Value="Horizontal">
+                <Setter Property="Width" Value="Auto" />
+                <Setter Property="MinWidth" Value="0" />
+                <Setter Property="Height" Value="12" />
+                <Setter Property="MinHeight" Value="12" />
+            </Trigger>
+        </Style.Triggers>
+    </Style>
+
+    <!-- ======================= ToolTip ======================= -->
+
+    <Style TargetType="ToolTip">
+        <Setter Property="Foreground" Value="{DynamicResource Theme.Foreground}" />
+        <Setter Property="MaxWidth" Value="360" />
+        <Setter Property="HasDropShadow" Value="False" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="ToolTip">
+                    <Border Background="{DynamicResource Theme.PopupBackground}"
+                            BorderBrush="{DynamicResource Theme.ControlBorder}"
+                            BorderThickness="1"
+                            CornerRadius="4"
+                            Padding="10,6"
+                            SnapsToDevicePixels="True">
+                        <ContentPresenter />
+                    </Border>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
+</ResourceDictionary>

CursorLang/Themes/Dark.xaml

@@ -0,0 +1,21 @@
+<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+
+    <SolidColorBrush x:Key="Theme.WindowBackground" Color="#1E1E21" />
+    <SolidColorBrush x:Key="Theme.Foreground" Color="#E8E8EA" />
+    <SolidColorBrush x:Key="Theme.SecondaryForeground" Color="#9A9AA2" />
+
+    <SolidColorBrush x:Key="Theme.Surface" Color="#26262A" />
+    <SolidColorBrush x:Key="Theme.SurfaceStrong" Color="#303036" />
+
+    <SolidColorBrush x:Key="Theme.ControlBackground" Color="#2C2C31" />
+    <SolidColorBrush x:Key="Theme.ControlHoverBackground" Color="#38383E" />
+    <SolidColorBrush x:Key="Theme.ControlBorder" Color="#46464C" />
+    <SolidColorBrush x:Key="Theme.PopupBackground" Color="#2A2A2F" />
+    <SolidColorBrush x:Key="Theme.SelectionBackground" Color="#3A4665" />
+
+    <SolidColorBrush x:Key="Theme.Accent" Color="#5B8CF7" />
+    <SolidColorBrush x:Key="Theme.SliderTrack" Color="#46464C" />
+    <SolidColorBrush x:Key="Theme.ScrollBarThumb" Color="#55555C" />
+
+</ResourceDictionary>

CursorLang/Themes/Light.xaml

@@ -0,0 +1,21 @@
+<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+
+    <SolidColorBrush x:Key="Theme.WindowBackground" Color="#FFFFFF" />
+    <SolidColorBrush x:Key="Theme.Foreground" Color="#1B1B1F" />
+    <SolidColorBrush x:Key="Theme.SecondaryForeground" Color="#6B6B70" />
+
+    <SolidColorBrush x:Key="Theme.Surface" Color="#FAFAFB" />
+    <SolidColorBrush x:Key="Theme.SurfaceStrong" Color="#EFEFF1" />
+
+    <SolidColorBrush x:Key="Theme.ControlBackground" Color="#FFFFFF" />
+    <SolidColorBrush x:Key="Theme.ControlHoverBackground" Color="#F2F2F4" />
+    <SolidColorBrush x:Key="Theme.ControlBorder" Color="#D0D0D4" />
+    <SolidColorBrush x:Key="Theme.PopupBackground" Color="#FFFFFF" />
+    <SolidColorBrush x:Key="Theme.SelectionBackground" Color="#E4EBFB" />
+
+    <SolidColorBrush x:Key="Theme.Accent" Color="#2563EB" />
+    <SolidColorBrush x:Key="Theme.SliderTrack" Color="#DCDCE0" />
+    <SolidColorBrush x:Key="Theme.ScrollBarThumb" Color="#C2C2C7" />
+
+</ResourceDictionary>

CursorLang/ViewModels/LayoutPopupViewModel.cs

@@ -4,8 +4,8 @@ using CursorLang.Models;
 namespace CursorLang.ViewModels;
 
 /// <summary>
-/// Содержимое подсказки у курсора. Внешний вид берётся прямо из настроек,
-/// поэтому их правка применяется без перезапуска.
+/// The content of the popup at the cursor. The look is taken straight from the
+/// settings, so editing them applies without a restart.
 /// </summary>
 public sealed partial class LayoutPopupViewModel : ObservableObject
 {