Fix stale keysym entries when modifier changes while key is held

[Copilot]

KeyboardStateTracker used the keysym from the key-up event to erase from pressed_keysyms, but that keysym can differ from what was stored at key-down when a modifier changes mid-hold (e.g. 1 down → Shift down → key-up reports XKB_KEY_exclam, leaving XKB_KEY_1 stranded).

Related: #4746

What's new?

• scancode_to_keysym map replaces the two separate sets (pressed_keysyms / pressed_scancodes). Scancode is the stable identity of a physical key; the map stores the keysym recorded at key-down against it.
• Key-up erases by scancode (scancode_to_keysym.erase(scancode)), so mismatched key-up keysyms caused by held modifiers never leave stale entries.
• Shift transitions mutate map values in-place via a range loop applying xkb_keysym_to_upper/xkb_keysym_to_lower, replacing the previous set-rebuild approach.
• keysym_is_pressed uses std::ranges::any_of over map values; scancode_is_pressed uses map.contains. The linear scan for keysyms is inconsequential given the physical bound on simultaneously held keys (≤ ~20).
• New test key_up_clears_key_when_modifier_changed_while_held exercises the exact failure scenario: 1 down → Shift down → ! up → neither XKB_KEY_1 nor XKB_KEY_exclam remains pressed.

How to test

Run the KeyboardStateTrackerTest suite:

./bin/mir_unit_tests.bin --gtest_filter="KeyboardStateTrackerTest.*"

All 17 tests should pass, including the new regression test.

Checklist

• Tests added and pass
• Adequate documentation added
• (optional) Added Screenshots or videos

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

[tarek-y-ismail]

Makes sense. @AlanGriffiths What do you think?

[tarek-y-ismail]

@copilot Rebase this branch on main and fix any conflicts.

[tarek-y-ismail]

Fine, I'll do it myself 🤦

Later...

[github-actions]

TICS Quality Gate

✔️ Passed

mir

Coding Standards: ✔️ Passed

✔️ Condition “No new Coding Standard Violations for level 1, 2, 3 with respect to Previous analysis” passed.

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The following files have been checked for this project

• src/server/frontend_wayland/keyboard_state_tracker.cpp
• src/server/frontend_wayland/keyboard_state_tracker.h

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