fix(app): carry keyboard modifiers on mouse and wheel events

[samyfodil]

Every MouseEvent and WheelEvent the bridge builds is given event.ModNone, so MouseEvent.Modifiers() is always empty. ⌥click, ⇧click, ⌃click and shift-to-extend-a-selection cannot be expressed by an application at all.

The platform's mouse callbacks report a button and a position and nothing else, so the modifier state has to come from the key events, which do carry it. The bridge now tracks it there and stamps it onto the mouse and wheel events it builds.

The two details that are the whole difficulty

• A key event reports the modifiers held before it, so the event that presses Alt reports no Alt. Holding a modifier and clicking, with no other key in between, is exactly the gesture — so the key's own bit is folded in on press and taken out on release.
• Focus loss clears the state. A modifier released while another window had focus was never seen here, and believing it is still held turns the next ordinary click into a modified one.

Found by

A terminal emulator on this toolkit, where ⌥click reveals a masked secret. The click arrived with no modifier, and the app had to reimplement this tracking on top of the key events to tell an ⌥click from a plain one. Doing it in the bridge means every application gets it rather than each one rediscovering it.

Tests

Three, covering held-over-click, cleared-on-release and cleared-on-focus-loss. The first fails on main (click carried None, want Alt). Full suite green (61 packages), golangci-lint clean.

A deeper fix would put the modifiers on gpucontext.EventSource's mouse callbacks — X11 already carries them in the button event — but that changes an interface across repos. This is the non-breaking version and is correct for every current platform backend.

[codecov]

Codecov Report

❌ Patch coverage is 73.91304% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
app/event_bridge.go	73.91%	6 Missing ⚠️

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[kolkov]

Validated: matches winit/SDL3 Model B (tracked modifier state stamped onto pointer events). Focus-loss clearing matches winit exactly. modifierForKey OR-folding is safe across all 4 platforms (X11, Windows, Wayland, macOS) due to idempotency. Thread-safe by design (single-threaded event loop). Three tests cover held/released/focus-loss cases.