added tests to project

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/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));
+}