flush ButtonInput<KeyCode> cache when Bevy window loses focus

[gavlig]

Objective

This helps avoiding stuck key presses after switching from and back to Bevy window. Key press event gets stuck because window loses focus before receiving a key release event thus we end up with false positive in ButtonInput.

Solution

I saw two ways to fix this:

1. add bevy_window as dependency and read WindowFocus events
2. add a KeyboardFocusLost event specifically for this.

I chose the latter because adding another dependency felt wrong, but if that is more preferable changing this pr won't be a problem. Also if someone sees another way please let me know.

---

To test the bug use this small modification over examples/keyboard_input.rs:
(it will work only if you have Alt-Tab combination for switching between windows in your OS, otherwise change AltLeft accordingly)

//! Demonstrates handling a key press/release.

use bevy::{prelude::*, input::keyboard::KeyboardInput};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Update, keyboard_input_system)
        .run();
}

/// This system prints 'Alt' key state
fn keyboard_input_system(keyboard_input: Res<ButtonInput<KeyCode>>, mut keyboard_input_events: EventReader<KeyboardInput>) {
    for event in keyboard_input_events.read() {
        info!("{:?}", event);
    }

    if keyboard_input.pressed(KeyCode::AltLeft) {
        info!("'Alt' currently pressed");
    }

    if keyboard_input.just_pressed(KeyCode::AltLeft) {
        info!("'Alt' just pressed");
    }
    if keyboard_input.just_released(KeyCode::AltLeft) {
        info!("'Alt' just released");
    }
}

Here i made a quick video with demo of the fix: https://youtu.be/qTvUCk4IHvo
In first part i press Alt and Alt+Tab to switch back and forth from example app, logs will indicate that too. In second part I applied fix and you'll see that Alt will no longer be pressed when window gets unfocused

Migration Guide

WinitEvent has a new enum variant: WinitEvent::KeyboardFocusLost.

[github-actions]

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Please make sure you've read our contributing guide and we look forward to reviewing your pull request shortly ✨

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[alice-i-cecile]

I've run into this bug before and found it quite frustrating. I think this is a good fix that should result in consistent cross-platform behavior.

[dimvoly]

Not sure if this is related, but when I run debug builds, I seem to get random keys starting in the pressed state (e.g. KeyCode::At, or KeyCode::Key6). I haven't pressed these keys when I launch my app, to resolve I have to insert a breakpoint to find out the stuck key, then press it to get it unpressed. Happens sporadically so kind of hard to untangle the reasons for it happening. Am on 0.10.1 on Ubuntu 20.04.

A general purpose flush like this PR I think would be welcome as a catch-all for .pressed() desyncing from the keyboard.

[hymm]

This looks good, but should be merged after the 0.30 pr to avoid conflicting with that pr.

[MiniaczQ]

Unblocked

[mockersf]

@gavlig could you resolve the conflicts or do you need help for that?

Worst case if they're too hard, you can open a new PR with the same changes

[alice-i-cecile]

I'll adopt this tomorrow and resolve the merge conflicts.

[alice-i-cecile]

This has been merged via #13678 :) Thanks a ton for your contribution @gavlig: see you in the 0.14 contributors list!

[gavlig]

Thank you for handling this and for all your work on Bevy! Should I close this pr or will it be closed automatically?