Fix window spawning triggering ButtonInput<KeyCode>::just_pressed/just_released (#12372)

# Objective

Fix #12273

## Solution

– Only emit `KeyboardFocusLost` when the keyboard focus is lost
– ignore synthetic key releases too, not just key presses (as they're
already covered by `KeyboardFocusLost`)

---

## Changelog

### Fixed

- Don't trigger `ButtonInput<KeyCode>::just_pressed`/`just_released`
when spawning a window/focus moving between Bevy windows

crates/bevy_input/src/button_input.rs

@@ -62,8 +62,7 @@ use {
 ///
 /// `ButtonInput<KeyCode>` is tied to window focus. For example, if the user holds a button
 /// while the window loses focus, [`ButtonInput::just_released`] will be triggered. Similarly if the window
-/// regains focus, [`ButtonInput::just_pressed`] will be triggered. Currently this happens even if the
-/// focus switches from one Bevy window to another (for example because a new window was just spawned).
+/// regains focus, [`ButtonInput::just_pressed`] will be triggered.
 ///
 /// `ButtonInput<GamepadButton>` is independent of window focus.
 ///

crates/bevy_input/src/keyboard.rs

@@ -134,7 +134,7 @@ pub struct KeyboardFocusLost;
 /// ## Differences
 ///
 /// The main difference between the [`KeyboardInput`] event and the [`ButtonInput<KeyCode>`] resources is that
-/// the latter have convenient functions such as [`ButtonInput::pressed`], [`ButtonInput::just_pressed`] and [`ButtonInput::just_released`].
+/// the latter has convenient functions such as [`ButtonInput::pressed`], [`ButtonInput::just_pressed`] and [`ButtonInput::just_released`] and is window id agnostic.
 pub fn keyboard_input_system(
     mut key_input: ResMut<ButtonInput<KeyCode>>,
     mut keyboard_input_events: EventReader<KeyboardInput>,

crates/bevy_winit/src/lib.rs

@@ -27,7 +27,7 @@ use bevy_ecs::prelude::*;
 use bevy_window::{exit_on_all_closed, Window, WindowCreated};
 pub use converters::convert_system_cursor_icon;
 pub use state::{CursorSource, CustomCursorCache, CustomCursorCacheKey, PendingCursor};
-use system::{changed_windows, despawn_windows};
+use system::{changed_windows, check_keyboard_focus_lost, despawn_windows};
 pub use system::{create_monitors, create_windows};
 #[cfg(all(target_family = "wasm", target_os = "unknown"))]
 pub use winit::platform::web::CustomCursorExtWebSys;
@@ -133,6 +133,7 @@ impl<T: Event> Plugin for WinitPlugin<T> {
                     // so we don't need to care about its ordering relative to `changed_windows`
                     changed_windows.ambiguous_with(exit_on_all_closed),
                     despawn_windows,
+                    check_keyboard_focus_lost,
                 )
                     .chain(),
             );

crates/bevy_winit/src/state.rs

@@ -10,7 +10,6 @@ use bevy_ecs::{
 };
 use bevy_input::{
     gestures::*,
-    keyboard::KeyboardFocusLost,
     mouse::{MouseButtonInput, MouseMotion, MouseScrollUnit, MouseWheel},
 };
 use bevy_log::{error, trace, warn};
@@ -267,18 +266,14 @@ impl<T: Event> ApplicationHandler<T> for WinitAppRunnerState<T> {
                 .send(WindowCloseRequested { window }),
             WindowEvent::KeyboardInput {
                 ref event,
-                is_synthetic,
+                // On some platforms, winit sends "synthetic" key press events when the window
+                // gains or loses focus. These should not be handled, so we only process key
+                // events if they are not synthetic key presses.
+                is_synthetic: false,
                 ..
             } => {
-                // Winit sends "synthetic" key press events when the window gains focus. These
-                // should not be handled, so we only process key events if they are not synthetic
-                // key presses. "synthetic" key release events should still be handled though, for
-                // properly releasing keys when the window loses focus.
-                if !(is_synthetic && event.state.is_pressed()) {
-                    // Process the keyboard input event, as long as it's not a synthetic key press.
-                    self.bevy_window_events
-                        .send(converters::convert_keyboard_input(event, window));
-                }
+                self.bevy_window_events
+                    .send(converters::convert_keyboard_input(event, window));
             }
             WindowEvent::CursorMoved { position, .. } => {
                 let physical_position = DVec2::new(position.x, position.y);
@@ -354,9 +349,6 @@ impl<T: Event> ApplicationHandler<T> for WinitAppRunnerState<T> {
                 win.focused = focused;
                 self.bevy_window_events
                     .send(WindowFocused { window, focused });
-                if !focused {
-                    self.bevy_window_events.send(KeyboardFocusLost);
-                }
             }
             WindowEvent::Occluded(occluded) => {
                 self.bevy_window_events

crates/bevy_winit/src/system.rs

@@ -6,10 +6,11 @@ use bevy_ecs::{
     removal_detection::RemovedComponents,
     system::{Local, NonSendMut, Query, SystemParamItem},
 };
+use bevy_input::keyboard::KeyboardFocusLost;
 use bevy_utils::tracing::{error, info, warn};
 use bevy_window::{
     ClosingWindow, Monitor, PrimaryMonitor, RawHandleWrapper, VideoMode, Window, WindowClosed,
-    WindowClosing, WindowCreated, WindowMode, WindowResized, WindowWrapper,
+    WindowClosing, WindowCreated, WindowFocused, WindowMode, WindowResized, WindowWrapper,
 };
 
 use winit::{
@@ -122,6 +123,26 @@ pub fn create_windows<F: QueryFilter + 'static>(
     }
 }
 
+/// Check whether keyboard focus was lost. This is different from window
+/// focus in that swapping between Bevy windows keeps window focus.
+pub(crate) fn check_keyboard_focus_lost(
+    mut focus_events: EventReader<WindowFocused>,
+    mut keyboard_focus: EventWriter<KeyboardFocusLost>,
+) {
+    let mut focus_lost = false;
+    let mut focus_gained = false;
+    for e in focus_events.read() {
+        if e.focused {
+            focus_gained = true;
+        } else {
+            focus_lost = true;
+        }
+    }
+    if focus_lost & !focus_gained {
+        keyboard_focus.send(KeyboardFocusLost);
+    }
+}
+
 /// Synchronize available monitors as reported by [`winit`] with [`Monitor`] entities in the world.
 pub fn create_monitors(
     event_loop: &ActiveEventLoop,