Fix Alt modifier stuck after Alt-Tab on Windows

[HuaGu-Dragon]

Context

Fixes a bug on Windows where the Alt modifier key becomes stuck in the pressed state after using Alt-Tab to switch away from and back to the Zed application.

Root Cause

In crates/gpui_windows/src/events.rs, when the window loses focus during Alt-Tab, the WM_KEYUP event for the Alt key is never delivered to the application. The pending modifier state in GPUI remains as alt=true. When the window regains focus, there is no synchronization point to reset this stale state.

How to Review

In handle_activate_msg() (WM_ACTIVATE handler), when the window is activated:

1. Reset the cached modifier tracking state (last_reported_modifiers and last_reported_capslock set to None)
2. Query the actual current modifier state from Windows using GetKeyState() APIs
3. Dispatch a ModifiersChanged event with the true state to synchronize GPUI

Self-Review Checklist

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

I think it's difficult to add tests for this because it's platform-dependent, but I recorded a video for comparison — sorry I only have an anime‑style app to show the key presses.

Video

previous.mp4

after.mp4

Closes #45485

Release Notes:

• Fixed Alt modifier key stuck after Alt-Tab on Windows. Modifier state is now synchronized when the window regains focus, ensuring correct key interpretation after window switching.

[zed-industries-bot]

	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

Generated by 🚫 dangerJS against a9d1588

[osiewicz]

Thank you!

[HuaGu-Dragon]

Hi @osiewicz

I noticed the CI failure was due to a git merge-base error after auto-merge. I've rebased the branch onto the latest main and force-pushed to ensure the common ancestor is clean.

Would you mind taking another look? I think the CI should pass now or let me know if any further action is needed on my side? Thanks!