Input Language Popup

A tiny background (tray) application for Windows that shows the currently active keyboard layout next to the text caret whenever you switch layouts from the keyboard — with the toggle hotkey configured in Windows (Ctrl+Shift or Alt+Shift, read live from the system settings so no restart is needed after changing it) as well as the hardwired Win+Space switcher.

• It does not switch the language itself.
• It does not reserve, intercept, or block any of these combinations — Windows keeps handling the switch normally.
• It shows the layout that is actually active afterwards. If Windows decided not to switch, it shows the unchanged layout (it never assumes a change happened).
• When CapsLock is on, the code is suffixed with CAPS (e.g. EN CAPS).

The indicator appears:

1. next to the text caret in the active application, or
2. next to the mouse cursor when the caret cannot be located.

The caret is located with a cascade of strategies for broad app coverage: the system caret (GetGUIThreadInfo) → MSAA (oleacc) → UI Automation (TextPattern2) → the mouse cursor.

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Requirements

• Windows 11 x64. Windows 10 22H2 x64 works on a best-effort basis (the binary is fully self-contained), but note that regular Windows 10 Home/Pro is no longer in Microsoft's supported-OS matrix for .NET 10 — only certain LTSC/Enterprise editions are — so it is tested but not officially supported.
• To build: .NET 10 SDK (dotnet --version → 10.x). To publish, also Visual Studio or the Build Tools with the Desktop development with C++ workload — Native AOT needs the MSVC linker.
• To run the published binary: nothing — it is a ~3 MB native executable with no .NET runtime dependency.

The app runs entirely as a normal user; administrator rights are not required.

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Build

dotnet build InputLanguagePopup.sln -c Release

Run (from source)

dotnet run --project src\InputLanguagePopup\InputLanguagePopup.csproj -c Release

A tray icon (AЯ) appears. Right-click it for the menu:

Item	Action
Enabled	Turn the indicator on/off (hook stays installed either way)
Show test popup	Show a test popup next to the mouse cursor
Start with Windows	Add/remove an autostart entry (per-user, no admin)
Exit	Cleanly remove the hook and quit

Run the tests

dotnet test InputLanguagePopup.sln -c Release

The unit tests cover the gesture-recognition state machine (Ctrl+Shift / Alt+Shift / Win+Space, cancellation, staleness), the system-hotkey interpretation, popup positioning (DPI, multi-monitor, negative coordinates), and settings normalization.

Publish (Native AOT, x64)

.\publish.ps1

or directly:

dotnet publish src\InputLanguagePopup\InputLanguagePopup.csproj -c Release -r win-x64

(PublishAot is set in the project file.) Verify a build with:

.\publish\InputLanguagePopup.exe --selftest

which exercises settings, layout detection, the MSAA/UI-Automation caret chain, the DPI probe and the layered popup, then writes %LocalAppData%\InputLanguagePopup\selftest.log.

The result is a single native InputLanguagePopup.exe (~3 MB) in .\publish\ that runs on any Windows 10/11 x64 machine without a pre-installed .NET runtime.

The UI is hand-rolled Win32 (window class, message loop, Shell_NotifyIcon tray, TrackPopupMenu, SetTimer) rather than WinForms, because WinForms is not AOT/trim-compatible. Rendering still uses System.Drawing, and the COM calls (UI Automation, MSAA) go straight through the object vtables with function pointers, since Native AOT has no built-in COM interop.

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Continuous integration & releases

Two GitHub Actions workflows (.github/workflows/):

• CI (ci.yml) — builds and runs the unit tests on windows-latest for every push and pull request to main.
• Release (release.yml) — triggered by pushing a v* tag. It runs the tests, publishes the single-file self-contained x64 executable (with the version stamped from the tag), and attaches it to an automatically-created GitHub Release with generated release notes.

To cut a release:

git tag v1.0.0
git push origin v1.0.0

A tag containing a hyphen (e.g. v1.1.0-beta) is published as a pre-release. The downloadable asset is named InputLanguagePopup-<tag>-win-x64.exe.

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Settings

Stored as JSON at:

%LocalAppData%\InputLanguagePopup\settings.json

If the file is missing or corrupt, defaults are used (and a fresh file is written). There is no settings UI in this version — edit the JSON by hand and restart the app.

{
  "enabled": true,
  "popupDurationMs": 850,
  "firstLayoutCheckDelayMs": 50,
  "secondLayoutCheckDelayMs": 140,
  "caretOffsetX": 8,
  "caretOffsetY": 8,
  "cursorOffsetX": 16,
  "cursorOffsetY": 20,
  "useUiAutomation": true,
  "handleWinSpace": true,
  "startWithWindows": false
}

Field	Meaning
enabled	Master on/off (mirrors the tray Enabled item)
popupDurationMs	How long the popup stays fully visible before fading
firstLayoutCheckDelayMs	Delay after the chord before the first layout read
secondLayoutCheckDelayMs	Delay for the second read (catches slow switches)
caretOffsetX/Y	Offset (logical px) of the popup from the caret
cursorOffsetX/Y	Offset (logical px) of the popup from the mouse cursor
useUiAutomation	Enable the UI Automation caret fallback
handleWinSpace	Also show the indicator after Win+Space
startWithWindows	Autostart entry (mirrors the tray item)

Values are clamped to sane ranges on load. If the file cannot be parsed it is moved aside to settings.corrupt.<timestamp>.json and a fresh default file is written.

Logs

Stored at:

%LocalAppData%\InputLanguagePopup\logs\app.log

Rotated at ~1 MB, keeping at most 5 files (app.log, app.log.1 … app.log.4).

Logged: startup/shutdown, hook install/remove, Win32/UI-Automation errors, cases where the layout could not be resolved, and unhandled exceptions. Individual keystrokes are never logged — this is not a keylogger.

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Known limitations

• Does not run on the Secure Desktop (UAC prompt) or the Windows lock/login screen — a normal-privilege hook is inactive there.
• The text caret cannot be located in every application. The cascade (system caret → MSAA → UI Automation) covers most apps — including Chromium/Electron and Qt/Telegram, which are special-cased — but apps that render text themselves and expose no caret or accessibility provider (some games, terminals with custom rendering, certain custom controls) fall back to the mouse cursor position. Unlike some tools, this app never injects code into other processes to read the caret, so a handful of hardened apps will always use the cursor fallback.
• Layouts are shown as a two-letter code. RU, EN, LT are handled explicitly; everything else uses the two-letter ISO code from CultureInfo. Distinguishing several layouts of the same language (e.g. English US vs English UK) is possible in the design but not surfaced in this version.
• The rare grave-accent (`) layout hotkey (registry code 4, used for Thai and a few other languages) is not supported. Per-layout direct hotkeys (e.g. a Ctrl+Shift+0 assignment under HKCU\Control Panel\Input Method\Hot Keys) are intentionally ignored — a chord with an extra key never triggers the indicator.
• Synthetic (injected) key events are ignored. A layout switch triggered by AutoHotkey, key-remapping tools, or some accessibility software will not show the indicator, even if Windows actually changed the layout — the hook filters LLKHF_INJECTED so that programmatic input cannot trigger a popup.
• One instance runs per user session (guarded by a session-local mutex), so Fast User Switching / RDP sessions each get their own indicator.

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Documentation

• ARCHITECTURE.md — component overview, threading model, the full list of Win32 / COM APIs used, and the acceptance-criteria verification report.