[KBM] Fix modifier key remapped to non-modifier delivering WM_SYSKEYDOWN

[oMatheusmol]

Summary of the Pull Request

When a modifier key (Ctrl/Alt/Shift) is remapped to a non-modifier key using
Keyboard Manager, the injected key event is delivered to applications as
WM_SYSKEYDOWN instead of WM_KEYDOWN. This causes unexpected behavior — for
example, remapping Left Alt to Backspace results in whole words being deleted
instead of single characters, because applications interpret WM_SYSKEYDOWN +
VK_BACK as Alt+Backspace.

The fix resets the modifier state with a suppress-flag key-up event before
injecting the target key, consistent with the existing approach used for the
Caps Lock remapping scenario.

PR Checklist

• Closes: [KBM] Remapping modifier key (Alt/Ctrl/Shift) to non-modifier key causes target key to be delivered as WM_SYSKEYDOWN #47191
• Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end-user-facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places

Detailed Description of the Pull Request / Additional comments

The root cause is that SendInput is called inside the low-level keyboard hook
callback before the original modifier event is suppressed. At that point the
modifier state is still active, so the OS delivers the injected key as
WM_SYSKEYDOWN (system key with Alt context) rather than WM_KEYDOWN.

This is the same mechanism that was already fixed for the Ctrl/Alt/Shift ↔
Caps Lock case. This PR extends the fix to cover modifier → non-modifier remaps.

Validation Steps Performed

1. Remapped Left Alt → Backspace in Keyboard Manager
2. Opened a text editor, typed text, pressed the remapped key
3. Confirmed single characters are deleted instead of whole words
4. All 93 unit tests pass (KeyboardManager.Engine.UnitTests)

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[oMatheusmol]

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[Copilot]

Pull request overview

Fixes an issue in Keyboard Manager where remapping a modifier key (Ctrl/Alt/Shift) to a non-modifier can cause the injected target key to be interpreted with an unintended modifier context (e.g., Alt causing WM_SYSKEYDOWN semantics). The change aligns modifier→non-modifier remaps with the existing “reset modifier state via suppress-flag key-up” approach already used for Caps Lock-related cases.

Changes:

• Add a suppress-flag key-up injection for the source modifier key before injecting the remapped target key when modifier → non-modifier remaps occur.
• Add a unit test to validate that the suppress-flag injection happens for a representative modifier→non-modifier remap (LAlt → Backspace).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/modules/keyboardmanager/KeyboardManagerEngineLibrary/KeyboardEventHandlers.cpp	Injects a suppress-flag key-up for the remapped modifier on keydown to clear modifier state before sending the target key.
src/modules/keyboardmanager/KeyboardManagerEngineTest/SingleKeyRemappingTests.cpp	Adds a unit test asserting exactly one suppress-flag event is issued for a modifier→non-modifier remap.

[oMatheusmol]

Hi @MuyuanMS , just following up on this PR.

Could you please take a look when you have time? Let me know if any changes are needed. Thanks!

[oMatheusmol]

Hi @niels9001 ,

Just following up on this PR since it has been open for a little over a month. The issue is still reproducible and the proposed fix is ready on my side.

Could you please let me know if there's anything else needed from me, or if any changes are required before review?

Thanks!

[MuyuanMS]

/azp run

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[MuyuanMS]

Interestingly I was not able to reproduce the original bug - I remapped left alt to backspace and notepad shows WM_KEYDOWN even without the fix.
My guess is on issue is only reproducible on windows 10, and the fix itself has low risks and looks good to me