Add the functions start_drag_move and start_drag_resize to Window (#15674)

# Objective
Expose the `winit` functions
[drag_window](https://docs.rs/winit/latest/winit/window/struct.Window.html#method.drag_window)
and
[resize_window](https://docs.rs/winit/latest/winit/window/struct.Window.html#method.drag_resize_window).

Which allows implementing move & resize for windows without decorations.

## Solution

Add the functions `start_drag_move` and `start_drag_resize` to
`bevy_window::Window`, which are then assigned to fields in
`InternalWindowState`, and propagated to `winit` in the
`changed_windows` system.

## Testing

I've tested that both functions works on x11 and wayland. Not sure if
someone needs to test on windows/mac?

---

## Showcase

[Screencast from 2024-10-06 11-49-58
(trimmed).webm](https://github.com/user-attachments/assets/1cdee7b1-22bd-41d3-8a0a-6872a6ebf62c)

(The flickering in the video is some issue with resizing without
decorations on x11)

<details>
  <summary>Click to view showcase</summary>

Not the same code used in the video, but simple way to test moving a
window without decorations.
```rust
use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins.set(WindowPlugin {
            primary_window: Some(Window {
                decorations: false,
                ..default()
            }),
            ..default()
        }))
        .add_systems(Update, move_windows)
        .run();
}

fn move_windows(mut windows: Query<&mut Window>, input: Res<ButtonInput<MouseButton>>) {
    if input.pressed(MouseButton::Left) {
        for mut window in windows.iter_mut() {
            window.start_drag_move();
        }
    }
}
```

</details>

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_window/src/window.rs

@@ -367,6 +367,22 @@ impl Window {
         self.internal.minimize_request = Some(minimized);
     }
 
+    /// Calling this will attempt to start a drag-move of the window.
+    ///
+    /// There is no guarantee that this will work unless the left mouse button was
+    /// pressed immediately before this function was called.
+    pub fn start_drag_move(&mut self) {
+        self.internal.drag_move_request = true;
+    }
+
+    /// Calling this will attempt to start a drag-resize of the window.
+    ///
+    /// There is no guarantee that this will work unless the left mouse button was
+    /// pressed immediately before this function was called.
+    pub fn start_drag_resize(&mut self, direction: ResizeDirection) {
+        self.internal.drag_resize_request = Some(direction);
+    }
+
     /// The window's client area width in logical pixels.
     ///
     /// See [`WindowResolution`] for an explanation about logical/physical sizes.
@@ -897,6 +913,32 @@ pub enum CursorGrabMode {
     Locked,
 }
 
+/// Defines the orientation in which a window resize will be performed.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Reflect)]
+#[cfg_attr(
+    feature = "serialize",
+    derive(serde::Serialize, serde::Deserialize),
+    reflect(Serialize, Deserialize)
+)]
+pub enum ResizeDirection {
+    /// Resize the window to the west.
+    West,
+    /// Resize the window to the north.
+    North,
+    /// Resize the window to the east.
+    East,
+    /// Resize the window to the south.
+    South,
+    /// Resize the window to the northwest.
+    Northwest,
+    /// Resize the window to the northeast.
+    Northeast,
+    /// Resize the window to the southwest.
+    Southwest,
+    /// Resize the window to the southeast.
+    Southeast,
+}
+
 /// Stores internal [`Window`] state that isn't directly accessible.
 #[derive(Default, Debug, Copy, Clone, PartialEq, Reflect)]
 #[cfg_attr(
@@ -910,6 +952,10 @@ pub struct InternalWindowState {
     minimize_request: Option<bool>,
     /// If this is true then next frame we will ask to maximize/un-maximize the window depending on `maximized`.
     maximize_request: Option<bool>,
+    /// If this is true then next frame we will ask to drag-move the window.
+    drag_move_request: bool,
+    /// If this is `Some` then the next frame we will ask to drag-resize the window.
+    drag_resize_request: Option<ResizeDirection>,
     /// Unscaled cursor position.
     physical_cursor_position: Option<DVec2>,
 }
@@ -924,6 +970,16 @@ impl InternalWindowState {
     pub fn take_minimize_request(&mut self) -> Option<bool> {
         self.minimize_request.take()
     }
+
+    /// Consumes the current move request, if it exists. This should only be called by window backends.
+    pub fn take_move_request(&mut self) -> bool {
+        core::mem::take(&mut self.drag_move_request)
+    }
+
+    /// Consumes the current resize request, if it exists. This should only be called by window backends.
+    pub fn take_resize_request(&mut self) -> Option<ResizeDirection> {
+        self.drag_resize_request.take()
+    }
 }
 
 /// References a screen monitor.

crates/bevy_winit/src/converters.rs

@@ -8,7 +8,7 @@ use bevy_input::{
 use bevy_math::Vec2;
 #[cfg(feature = "custom_cursor")]
 use bevy_window::SystemCursorIcon;
-use bevy_window::{EnabledButtons, WindowLevel, WindowTheme};
+use bevy_window::{EnabledButtons, ResizeDirection, WindowLevel, WindowTheme};
 use winit::keyboard::{Key, NamedKey, NativeKey};
 
 pub fn convert_keyboard_input(
@@ -706,3 +706,18 @@ pub fn convert_enabled_buttons(enabled_buttons: EnabledButtons) -> winit::window
     }
     window_buttons
 }
+
+pub fn convert_resize_direction(
+    resize_direction: ResizeDirection,
+) -> winit::window::ResizeDirection {
+    match resize_direction {
+        ResizeDirection::West => winit::window::ResizeDirection::West,
+        ResizeDirection::North => winit::window::ResizeDirection::North,
+        ResizeDirection::East => winit::window::ResizeDirection::East,
+        ResizeDirection::South => winit::window::ResizeDirection::South,
+        ResizeDirection::Northwest => winit::window::ResizeDirection::NorthWest,
+        ResizeDirection::Northeast => winit::window::ResizeDirection::NorthEast,
+        ResizeDirection::Southwest => winit::window::ResizeDirection::SouthWest,
+        ResizeDirection::Southeast => winit::window::ResizeDirection::SouthEast,
+    }
+}

crates/bevy_winit/src/system.rs

@@ -28,7 +28,8 @@ use winit::platform::web::WindowExtWebSys;
 
 use crate::{
     converters::{
-        convert_enabled_buttons, convert_window_level, convert_window_theme, convert_winit_theme,
+        convert_enabled_buttons, convert_resize_direction, convert_window_level,
+        convert_window_theme, convert_winit_theme,
     },
     get_best_videomode, get_fitting_videomode, select_monitor,
     state::react_to_resize,
@@ -462,6 +463,20 @@ pub(crate) fn changed_windows(
             winit_window.set_minimized(minimized);
         }
 
+        if window.internal.take_move_request() {
+            if let Err(e) = winit_window.drag_window() {
+                warn!("Winit returned an error while attempting to drag the window: {e}");
+            }
+        }
+
+        if let Some(resize_direction) = window.internal.take_resize_request() {
+            if let Err(e) =
+                winit_window.drag_resize_window(convert_resize_direction(resize_direction))
+            {
+                warn!("Winit returned an error while attempting to drag resize the window: {e}");
+            }
+        }
+
         if window.focused != cache.window.focused && window.focused {
             winit_window.focus_window();
         }