flush key_input cache when Bevy loses focus

This helps avoiding stuck key presses after switching from and then back to Bevy

crates/bevy_input/src/keyboard.rs

@@ -105,6 +105,20 @@ pub struct KeyboardInput {
     pub window: Entity,
 }
 
+/// Gets generated from `bevy_winit::winit_runner`
+///
+/// Used for clearing all cached states to avoid having 'stuck' key presses
+/// when, for example, switching between windows with 'Alt-Tab' or using any other
+/// OS specific key combination that leads to Bevy window losing focus and not receiving any
+/// input events
+#[derive(Event, Debug, Clone, PartialEq, Eq, Reflect)]
+#[cfg_attr(
+    feature = "serialize",
+    derive(serde::Serialize, serde::Deserialize),
+    reflect(Serialize, Deserialize)
+)]
+pub struct KeyboardFocusLost;
+
 /// Updates the [`ButtonInput<KeyCode>`] resource with the latest [`KeyboardInput`] events.
 ///
 /// ## Differences
@@ -114,6 +128,7 @@ pub struct KeyboardInput {
 pub fn keyboard_input_system(
     mut key_input: ResMut<ButtonInput<KeyCode>>,
     mut keyboard_input_events: EventReader<KeyboardInput>,
+    mut focus_events: EventReader<KeyboardFocusLost>,
 ) {
     // Avoid clearing if it's not empty to ensure change detection is not triggered.
     key_input.bypass_change_detection().clear();
@@ -126,6 +141,12 @@ pub fn keyboard_input_system(
             ButtonState::Released => key_input.release(*key_code),
         }
     }
+
+    // Release all cached input to avoid having stuck input when switching between windows in os
+    if !focus_events.is_empty() {
+        key_input.release_all();
+        focus_events.clear();
+    }
 }
 
 /// Contains the platform-native physical key identifier

crates/bevy_input/src/lib.rs

@@ -41,7 +41,7 @@ pub mod prelude {
 use bevy_app::prelude::*;
 use bevy_ecs::prelude::*;
 use bevy_reflect::Reflect;
-use keyboard::{keyboard_input_system, KeyCode, KeyboardInput};
+use keyboard::{keyboard_input_system, KeyCode, KeyboardFocusLost, KeyboardInput};
 use mouse::{mouse_button_input_system, MouseButton, MouseButtonInput, MouseMotion, MouseWheel};
 use touch::{touch_screen_input_system, TouchInput, Touches};
 use touchpad::{TouchpadMagnify, TouchpadRotate};
@@ -69,6 +69,7 @@ impl Plugin for InputPlugin {
         app
             // keyboard
             .add_event::<KeyboardInput>()
+            .add_event::<KeyboardFocusLost>()
             .init_resource::<ButtonInput<KeyCode>>()
             .add_systems(PreUpdate, keyboard_input_system.in_set(InputSystem))
             // mouse

crates/bevy_winit/src/lib.rs

@@ -34,6 +34,7 @@ use bevy_ecs::event::ManualEventReader;
 use bevy_ecs::prelude::*;
 use bevy_ecs::system::SystemState;
 use bevy_input::{
+    keyboard::KeyboardFocusLost,
     mouse::{MouseButtonInput, MouseMotion, MouseScrollUnit, MouseWheel},
     touchpad::{TouchpadMagnify, TouchpadRotate},
 };
@@ -583,6 +584,9 @@ fn handle_winit_event(
                 WindowEvent::Focused(focused) => {
                     win.focused = focused;
                     winit_events.send(WindowFocused { window, focused });
+                    if !focused {
+                        winit_events.send(KeyboardFocusLost);
+                    }
                 }
                 WindowEvent::Occluded(occluded) => {
                     winit_events.send(WindowOccluded { window, occluded });

crates/bevy_winit/src/winit_event.rs

@@ -5,6 +5,7 @@ use bevy_ecs::prelude::*;
 use bevy_input::keyboard::KeyboardInput;
 use bevy_input::touch::TouchInput;
 use bevy_input::{
+    keyboard::KeyboardFocusLost,
     mouse::{MouseButtonInput, MouseMotion, MouseWheel},
     touchpad::{TouchpadMagnify, TouchpadRotate},
 };
@@ -61,6 +62,7 @@ pub enum WinitEvent {
     TouchInput(TouchInput),
 
     KeyboardInput(KeyboardInput),
+    KeyboardFocusLost(KeyboardFocusLost),
 }
 
 impl From<ApplicationLifetime> for WinitEvent {
@@ -188,6 +190,11 @@ impl From<KeyboardInput> for WinitEvent {
         Self::KeyboardInput(e)
     }
 }
+impl From<KeyboardFocusLost> for WinitEvent {
+    fn from(e: KeyboardFocusLost) -> Self {
+        Self::KeyboardFocusLost(e)
+    }
+}
 
 /// Forwards buffered [`WinitEvent`] events to the app.
 pub(crate) fn forward_winit_events(buffered_events: &mut Vec<WinitEvent>, app: &mut App) {
@@ -271,6 +278,9 @@ pub(crate) fn forward_winit_events(buffered_events: &mut Vec<WinitEvent>, app: &
             WinitEvent::KeyboardInput(e) => {
                 app.world_mut().send_event(e);
             }
+            WinitEvent::KeyboardFocusLost(e) => {
+                app.world_mut().send_event(e);
+            }
         }
     }
     app.world_mut()