Replace winit's synthetic events by our own key tracking

[Azorlogh]

Objective

Defocusing a window while a key is held (such as when Alt+tabbing), will currently send a key release on X11 and Windows. This is likely the behavior that most people expect.
However it's synthetic events from winit are unimplemented for WASM and some other platforms. (See rust-windowing/winit#1272).
While we can implement it upstream, there is also some doubt about the synthetic events API as a whole (rust-windowing/winit#3543), so I propose to do it in bevy directly for now.

Solution

This PR implements key tracking in bevy directly so we can synthesize our own key release events across all platforms.

Note regarding X11 specifically:

• On main, pressing a keyboard shortcut to unfocus the window (Ctrl + Super + ArrowRight in my case) will yield the following events:

Pressed Control
Pressed Super
Released Control
Released ArrowRight
Released Super

• On this branch, it will yield the following sequence:

Pressed Control
Pressed Super
Released Control
Released Super

To me the behavior of this branch is more expected than main, because main produces an ArrowRight release without producing a press first.

Testing

Tested in WASM and X11 with

App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Update, |mut keys: EventReader<KeyboardInput>| {
            for ev in keys.read() {
                error!("received {ev:?}");
            }
        })
        .run();

[RobWalt]

Code looks good to me!

[NEON725]

Will this fix the issue of the Alt key "sticking" when alt-tabbing out of the browser running a WASM build? Logic seems like it should so long as WASM builds correctly track window focus.

[Azorlogh]

Will this fix the issue of the Alt key "sticking" when alt-tabbing out of the browser running a WASM build? Logic seems like it should so long as WASM builds correctly track window focus.

Yep, this should fix it.

[alice-i-cecile]

@richchurcher @SpecificProtagonist can I have your review over here please?

[richchurcher]

@Azorlogh I think because #12372 got merged and touched some of the same areas, this is gonna have a few conflicts. Would you mind taking another look and updating the branch?

[Azorlogh]

I updated it for #12372, though I have a couple concerns on reliability:

• As I wrote in Fix window spawning triggering ButtonInput<KeyCode>::just_pressed/just_released #12372 (comment), there are false positives when switching between bevy windows (the check_keyboard_focus_lost sometimes erroneously thinks we switched to a non-bevy window). After my changes this will mean extra release events will be emitted (though maybe that's not too a huge problem).
  • I worried it might reintroduce the bug in Opening a Window on keyboard_input.just_pressed makes it trigger again #12273, but at the same time I didn't manage to reproduce it so I'm not sure.
• Also, check_keyboard_focus_lost doesn't emit events in Events<bevy_window::WindowEvent>, so if the user listens to those events they will miss them. (I guess we can just emit them in both streams as a quick fix though)

[alice-i-cecile]

• As I wrote in Fix window spawning triggering ButtonInput::just_pressed/just_released #12372 (comment), there are false positives when switching between bevy windows (the check_keyboard_focus_lost sometimes erroneously thinks we switched to a non-bevy window). After my changes this will mean extra release events will be emitted (though maybe that's not too a huge problem).

Extra release events should generally be safe enough.

• I worried it might reintroduce the bug in Opening a Window on keyboard_input.just_pressed makes it trigger again #12273, but at the same time I didn't manage to reproduce it so I'm not sure.

I think this is broadly fine.

• Also, check_keyboard_focus_lost doesn't emit events in Events<bevy_window::WindowEvent>, so if the user listens to those events they will miss them. (I guess we can just emit them in both streams as a quick fix though)

Emitting it in both would be good.

[Azorlogh]

Extra release events should generally be safe enough

Fair. They are now emitted in both streams 👍