Expose set_cursor_hittest() from winit (#6664)

# Objective

- Bevy should be usable to create 'overlay' type apps, where the input is not captured by Bevy, but passed down/into a target app, or to allow passive displays/widgets etc.
 
## Solution

- the `winit:🪟:Window` already has a `set_cursor_hittest()` which basically does this for mouse input events, so I've exposed it (trying to copy the style laid out in the existing wrappings, and added a simple demo.

---

## Changelog

- Added `hittest` to `WindowAttributes`
- Added the `hittest`'s setters/getters
- Modified the `WindowBuilder`
- Modifed the `WindowDescriptor`'s `Default` impl.
- Added an example `cargo run --example fallthrough`

Cargo.toml

@@ -1442,6 +1442,16 @@ description = "Illustrates creating and updating a button"
 category = "UI (User Interface)"
 wasm = true
 
+[[example]]
+name = "window_fallthrough"
+path = "examples/ui/window_fallthrough.rs"
+
+[package.metadata.example.window_fallthrough]
+name = "Window Fallthrough"
+description = "Illustrates how to access `winit::window::Window`'s `hittest` functionality."
+category = "UI (User Interface)"
+wasm = false
+
 [[example]]
 name = "font_atlas_debug"
 path = "examples/ui/font_atlas_debug.rs"

crates/bevy_window/src/window.rs

@@ -286,6 +286,7 @@ pub struct Window {
     cursor_icon: CursorIcon,
     cursor_visible: bool,
     cursor_grab_mode: CursorGrabMode,
+    hittest: bool,
     physical_cursor_position: Option<DVec2>,
     raw_handle: Option<RawHandleWrapper>,
     focused: bool,
@@ -351,6 +352,10 @@ pub enum WindowCommand {
     SetCursorPosition {
         position: Vec2,
     },
+    /// Set whether or not mouse events within *this* window are captured, or fall through to the Window below.
+    SetCursorHitTest {
+        hittest: bool,
+    },
     /// Set whether or not the window is maximized.
     SetMaximized {
         maximized: bool,
@@ -435,6 +440,7 @@ impl Window {
             cursor_visible: window_descriptor.cursor_visible,
             cursor_grab_mode: window_descriptor.cursor_grab_mode,
             cursor_icon: CursorIcon::Default,
+            hittest: true,
             physical_cursor_position: None,
             raw_handle,
             focused: false,
@@ -777,7 +783,20 @@ impl Window {
         self.command_queue
             .push(WindowCommand::SetCursorPosition { position });
     }
-
+    /// Modifies whether the window catches cursor events.
+    ///
+    /// If true, the window will catch the cursor events.
+    /// If false, events are passed through the window such that any other window behind it receives them. By default hittest is enabled.
+    pub fn set_cursor_hittest(&mut self, hittest: bool) {
+        self.hittest = hittest;
+        self.command_queue
+            .push(WindowCommand::SetCursorHitTest { hittest });
+    }
+    /// Get whether or not the hittest is active.
+    #[inline]
+    pub fn hittest(&self) -> bool {
+        self.hittest
+    }
     #[allow(missing_docs)]
     #[inline]
     pub fn update_focused_status_from_backend(&mut self, focused: bool) {
@@ -961,6 +980,8 @@ pub struct WindowDescriptor {
     pub cursor_visible: bool,
     /// Sets whether and how the window grabs the cursor.
     pub cursor_grab_mode: CursorGrabMode,
+    /// Sets whether or not the window listens for 'hits' of mouse activity over _this_ window.
+    pub hittest: bool,
     /// Sets the [`WindowMode`](crate::WindowMode).
     ///
     /// The monitor to go fullscreen on can be selected with the `monitor` field.
@@ -1013,6 +1034,7 @@ impl Default for WindowDescriptor {
             decorations: true,
             cursor_grab_mode: CursorGrabMode::None,
             cursor_visible: true,
+            hittest: true,
             mode: WindowMode::Windowed,
             transparent: false,
             canvas: None,

crates/bevy_winit/src/lib.rs

@@ -230,6 +230,10 @@ fn change_window(
                     let window = winit_windows.get_window(id).unwrap();
                     window.set_always_on_top(always_on_top);
                 }
+                bevy_window::WindowCommand::SetCursorHitTest { hittest } => {
+                    let window = winit_windows.get_window(id).unwrap();
+                    window.set_cursor_hittest(hittest).unwrap();
+                }
                 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

examples/README.md

@@ -319,6 +319,7 @@ Example | Description
 [UI](../examples/ui/ui.rs) | Illustrates various features of Bevy UI
 [UI Scaling](../examples/ui/ui_scaling.rs) | Illustrates how to scale the UI
 [UI Z-Index](../examples/ui/z_index.rs) | Demonstrates how to control the relative depth (z-position) of UI elements
+[Window Fallthrough](../examples/ui/window_fallthrough.rs) | Illustrates how to access `winit::window::Window`'s `hittest` functionality.
 
 ## Window
 

examples/ui/window_fallthrough.rs

@@ -0,0 +1,60 @@
+//! This example illustrates how have a mouse's clicks/wheel/movement etc fall through the spawned transparent window to a window below.
+//! If you build this, and hit 'P' it should toggle on/off the mouse's passthrough.
+
+use bevy::prelude::*;
+
+fn main() {
+    // Set the window's parameters, note we're setting always_on_top to be true.
+    let window_desc = WindowDescriptor {
+        transparent: true,
+        decorations: true,
+        always_on_top: true,
+        ..default()
+    };
+
+    App::new()
+        .insert_resource(ClearColor(Color::NONE)) // Use a transparent window, to make effects obvious.
+        .add_plugins(DefaultPlugins.set(WindowPlugin {
+            window: window_desc,
+            ..default()
+        }))
+        .add_startup_system(setup)
+        .add_system(toggle_mouse_passthrough) // This allows us to hit 'P' to toggle on/off the mouse's passthrough
+        .run();
+}
+
+fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {
+    // UI camera
+    commands.spawn(Camera2dBundle::default());
+    // Text with one section
+    commands.spawn((
+        // Create a TextBundle that has a Text with a single section.
+        TextBundle::from_section(
+            // Accepts a `String` or any type that converts into a `String`, such as `&str`
+            "Hit 'P' then scroll/click around!",
+            TextStyle {
+                font: asset_server.load("fonts/FiraSans-Bold.ttf"),
+                font_size: 100.0, // Nice and big so you can see it!
+                color: Color::WHITE,
+            },
+        )
+        // Set the style of the TextBundle itself.
+        .with_style(Style {
+            position_type: PositionType::Absolute,
+            position: UiRect {
+                bottom: Val::Px(5.),
+                right: Val::Px(10.),
+                ..default()
+            },
+            ..default()
+        }),
+    ));
+}
+// A simple system to handle some keyboard input and toggle on/off the hittest.
+fn toggle_mouse_passthrough(keyboard_input: Res<Input<KeyCode>>, mut windows: ResMut<Windows>) {
+    if keyboard_input.just_pressed(KeyCode::P) {
+        let window = windows.primary_mut();
+        let hittest: bool = window.hittest();
+        window.set_cursor_hittest(!hittest);
+    }
+}