Expose winit always_on_top (bevyengine#6527)

# Objective

I needed a window which is always on top, to create a overlay app.

## Solution

expose the `always_on_top` property of winit in bevy's `WindowDescriptor` as a boolean flag

---

## Changelog

### Added
- add `WindowDescriptor.always_on_top` which configures a window to stay on top.

crates/bevy_window/src/window.rs

@@ -294,6 +294,7 @@ pub struct Window {
     fit_canvas_to_parent: bool,
     command_queue: Vec<WindowCommand>,
     alpha_mode: CompositeAlphaMode,
+    always_on_top: bool,
 }
 /// A command to be sent to a window.
 ///
@@ -369,6 +370,10 @@ pub enum WindowCommand {
     SetResizeConstraints {
         resize_constraints: WindowResizeConstraints,
     },
+    /// Set whether the window is always on top.
+    SetAlwaysOnTop {
+        always_on_top: bool,
+    },
     Close,
 }
 
@@ -438,6 +443,7 @@ impl Window {
             fit_canvas_to_parent: window_descriptor.fit_canvas_to_parent,
             command_queue: Vec::new(),
             alpha_mode: window_descriptor.alpha_mode,
+            always_on_top: window_descriptor.always_on_top,
         }
     }
     /// Get the window's [`WindowId`].
@@ -796,6 +802,18 @@ impl Window {
             resolution: UVec2::new(self.physical_width, self.physical_height),
         });
     }
+    /// Get whether or not the window is always on top.
+    #[inline]
+    pub fn always_on_top(&self) -> bool {
+        self.always_on_top
+    }
+
+    /// Set whether of not the window is always on top.
+    pub fn set_always_on_top(&mut self, always_on_top: bool) {
+        self.always_on_top = always_on_top;
+        self.command_queue
+            .push(WindowCommand::SetAlwaysOnTop { always_on_top });
+    }
     /// Close the operating system window corresponding to this [`Window`].
     ///  
     /// This will also lead to this [`Window`] being removed from the
@@ -972,6 +990,12 @@ pub struct WindowDescriptor {
     pub fit_canvas_to_parent: bool,
     /// Specifies how the alpha channel of the textures should be handled during compositing.
     pub alpha_mode: CompositeAlphaMode,
+    /// Sets the window to always be on top of other windows.
+    ///
+    /// ## Platform-specific
+    /// - iOS / Android / Web: Unsupported.
+    /// - Linux (Wayland): Unsupported.
+    pub always_on_top: bool,
 }
 
 impl Default for WindowDescriptor {
@@ -994,6 +1018,7 @@ impl Default for WindowDescriptor {
             canvas: None,
             fit_canvas_to_parent: false,
             alpha_mode: CompositeAlphaMode::Auto,
+            always_on_top: false,
         }
     }
 }

crates/bevy_winit/src/lib.rs

@@ -226,6 +226,10 @@ fn change_window(
                         window.set_max_inner_size(Some(max_inner_size));
                     }
                 }
+                bevy_window::WindowCommand::SetAlwaysOnTop { always_on_top } => {
+                    let window = winit_windows.get_window(id).unwrap();
+                    window.set_always_on_top(always_on_top);
+                }
                 bevy_window::WindowCommand::Close => {
                     // Since we have borrowed `windows` to iterate through them, we can't remove the window from it.
                     // Add the removal requests to a queue to solve this

crates/bevy_winit/src/winit_windows.rs

@@ -72,7 +72,8 @@ impl WinitWindows {
             }
             .with_resizable(window_descriptor.resizable)
             .with_decorations(window_descriptor.decorations)
-            .with_transparent(window_descriptor.transparent),
+            .with_transparent(window_descriptor.transparent)
+            .with_always_on_top(window_descriptor.always_on_top),
         };
 
         let constraints = window_descriptor.resize_constraints.check_constraints();

examples/window/window_settings.rs

@@ -15,6 +15,7 @@ fn main() {
                 width: 500.,
                 height: 300.,
                 present_mode: PresentMode::AutoVsync,
+                always_on_top: true,
                 ..default()
             },
             ..default()
@@ -25,6 +26,7 @@ fn main() {
         .add_system(toggle_cursor)
         .add_system(toggle_vsync)
         .add_system(cycle_cursor_icon)
+        .add_system(toggle_always_on_top)
         .run();
 }
 
@@ -43,6 +45,28 @@ fn toggle_vsync(input: Res<Input<KeyCode>>, mut windows: ResMut<Windows>) {
     }
 }
 
+/// This system toggles whether the window is always on top when pressing the T button
+/// You'll notice it won't be covered by other windows.
+///
+/// This feature only works on some platforms. Please check the
+/// [documentation](https://docs.rs/bevy/latest/bevy/prelude/struct.WindowDescriptor.html#structfield.always_on_top)
+/// for more details.
+fn toggle_always_on_top(input: Res<Input<KeyCode>>, mut windows: ResMut<Windows>) {
+    if input.just_pressed(KeyCode::T) {
+        let window = windows.primary_mut();
+
+        let on_top: bool = window.always_on_top();
+
+        if on_top {
+            info!("UNLOCKING WINDOW");
+        } else {
+            info!("LOCKING WINDOW ON TOP");
+        }
+
+        window.set_always_on_top(!on_top);
+    }
+}
+
 /// This system will then change the title during execution
 fn change_title(time: Res<Time>, mut windows: ResMut<Windows>) {
     let window = windows.primary_mut();