Add delta to CursorMoved event (#11710)

# Objective

- Fixes #11695

## Solution

- Added `delta: Option<Vec2>` to `bevy_window::CursorMoved`. `delta` is
an `Option` because the `CursorMoved` event does get fired when the
cursor was outside the window area in the last frame. In that case there
is no cursor position from the last frame to compare with the current
cursor position.

---

## Changelog

- Added `delta: Option<Vec2>` to `bevy_window::CursorMoved`. 

## Migration Guide

- You need to add `delta` to any manually created `CursorMoved` struct.

---------

Co-authored-by: Kanabenki <lucien.menassol@gmail.com>
Co-authored-by: James Liu <contact@jamessliu.com>

crates/bevy_window/src/event.rs

@@ -115,10 +115,13 @@ pub struct WindowDestroyed {
 /// An event reporting that the mouse cursor has moved inside a window.
 ///
 /// The event is sent only if the cursor is over one of the application's windows.
-/// It is the translated version of [`WindowEvent::CursorMoved`] from the `winit` crate.
+/// It is the translated version of [`WindowEvent::CursorMoved`] from the `winit` crate with the addition of `delta`.
 ///
 /// Not to be confused with the [`MouseMotion`] event from `bevy_input`.
 ///
+/// Because the range of data is limited by the window area and it may have been transformed by the OS to implement certain effects like acceleration,
+/// you should not use it for non-cursor-like behaviour such as 3D camera control. Please see [`MouseMotion`] instead.
+///
 /// [`WindowEvent::CursorMoved`]: https://docs.rs/winit/latest/winit/event/enum.WindowEvent.html#variant.CursorMoved
 /// [`MouseMotion`]: bevy_input::mouse::MouseMotion
 #[derive(Event, Debug, Clone, PartialEq, Reflect)]
@@ -133,6 +136,13 @@ pub struct CursorMoved {
     pub window: Entity,
     /// The cursor position in logical pixels.
     pub position: Vec2,
+    /// The change in the position of the cursor since the last event was sent.
+    /// This value is `None` if the cursor was outside the window area during the last frame.
+    //
+    // Because the range of this data is limited by the display area and it may have been
+    //  transformed by the OS to implement effects such as cursor acceleration, it should
+    // not be used to implement non-cursor-like interactions such as 3D camera control.
+    pub delta: Option<Vec2>,
 }
 
 /// An event that is sent whenever the user's cursor enters a window.

crates/bevy_winit/src/lib.rs

@@ -444,10 +444,20 @@ fn handle_winit_event(
                 }
                 WindowEvent::CursorMoved { position, .. } => {
                     let physical_position = DVec2::new(position.x, position.y);
+
+                    let last_position = win.physical_cursor_position();
+                    let delta = last_position.map(|last_pos| {
+                        (physical_position.as_vec2() - last_pos) / win.resolution.scale_factor()
+                    });
+
                     win.set_physical_cursor_position(Some(physical_position));
                     let position =
                         (physical_position / win.resolution.scale_factor() as f64).as_vec2();
-                    app.send_event(CursorMoved { window, position });
+                    app.send_event(CursorMoved {
+                        window,
+                        position,
+                        delta,
+                    });
                 }
                 WindowEvent::CursorEntered { .. } => {
                     app.send_event(CursorEntered { window });