This was adopted from #12878. I rebased the changes resolved the following merge conflicts:

• moved over the changes originally done in bevy_winit/src/lib.rs's handle_winit_event into bevy_winit/src/state.rs's window_event function
• moved WinitEvent::KeyboardFocusLost event forwarding originally done in bevy_winit/src/winit_event.rs to the equivalent in bevy_winit/src/state.rs

Tested this by following the modified keyboard_input example from the original PR.

First, I verified I could reproduce the issue without the changes. Then, after applying the changes, I verified that when I Alt+Tabbed away from the running example that the log showed I released Alt and when I tabbed back it didn't behave like Alt was stuck.

The following is from the original pull request by @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.