release cursor when alt-tabbing out of the game

src/game.rs

@@ -619,7 +619,9 @@ fn check_achievements(
 }
 
 fn handle_window_focus(
+    mut local_paused: Local<bool>,
     mut settings: ResMut<Settings>,
+    mut q_window: Query<&mut Window, With<PrimaryWindow>>,
     mut er_focus: EventReader<bevy::window::WindowFocused>,
     mut physicstime: ResMut<Time<Physics>>,
 ) {
@@ -627,9 +629,23 @@ fn handle_window_focus(
         settings.window_focused = event.focused;
     }
 
-    if settings.is_game_running() {
-        physicstime.unpause();
-    } else {
-        physicstime.pause();
+    let paused = !settings.is_game_running();
+
+    if paused != *local_paused {
+        if paused {
+            physicstime.pause();
+        } else {
+            physicstime.unpause();
+        }
+
+        for mut window in &mut q_window {
+            window.cursor.grab_mode = if paused {
+                bevy::window::CursorGrabMode::None
+            } else {
+                bevy::window::CursorGrabMode::Locked
+            };
+            window.cursor.visible = paused;
+        }
+        *local_paused = paused;
     }
 }