Add Context::set_cursor_image for OS-level custom cursors (#8155)

## What

Adds a way for apps to push an RGBA bitmap as the OS cursor — the
missing companion to `Context::set_cursor_icon`. The integration
translates it into a real `winit::CustomCursor`, so the cursor is drawn
by the compositor and can extend past the egui window edge like any
native cursor.

## Why

Apps with custom-shaped windows (Winamp-style skins, themed launchers,
kiosk apps) currently have no clean way to display a custom cursor:
- `CursorIcon` is limited to the standard system enum.
- Painting the cursor sprite via `egui::Painter` works inside the canvas
but gets clipped at the window edge — the bottom/right of the cursor
disappears the moment the pointer is near the boundary, and there's no
way to render onto the desktop area exposed by a
transparent/region-shaped window.

`winit` 0.30+ already supports `CustomCursor::from_rgba` +
`ActiveEventLoop::create_custom_cursor`, but `egui-winit` doesn't
surface it. This PR exposes it through egui.

### Visual demonstration

Driving use case: a Winamp WSZ skin player
([all3f0r1/oneamp](https://github.com/all3f0r1/oneamp)) with a
transparent + region-shaped window where the skin ships its own `.cur`
files. The bottom-right corner of the playlist exposes the resize cursor
— notice how it gets clipped at the window edge in the painter-based
approach.

| Before (cursor painted via `egui::Painter`) | After (cursor pushed via
`set_cursor_image`) |
| --- | --- |
| ![cursor clipped at the bottom-right of the playlist
window](https://raw.githubusercontent.com/all3f0r1/egui/pr-assets/cursor-clipping-before.png)
| ![cursor extends cleanly past the window edge onto the
desktop](https://raw.githubusercontent.com/all3f0r1/egui/pr-assets/cursor-clipping-after.png)
|

## API

```rust
// new in egui::data::output
pub struct CustomCursorImage {
    pub rgba: std::sync::Arc<[u8]>,
    pub size: [u16; 2],     // matches winit's u16 to avoid lossy casts
    pub hotspot: [u16; 2],
}

// new field on PlatformOutput (skipped from serde — ephemeral)
pub cursor_image: Option<CustomCursorImage>,

// new method on Context
ctx.set_cursor_image(Some(image)); // overrides cursor_icon for this frame
ctx.set_cursor_image(None);        // revert to cursor_icon
```

`Arc<[u8]>` is intentional: the integration dedupes by `Arc::as_ptr`, so
reusing the same Arc across frames means the bitmap is only uploaded to
the OS once per skin, not once per frame.

## Integration changes

- `egui_winit::State::handle_platform_output_with_event_loop(window,
Option<&ActiveEventLoop>, ...)` is a new method that threads the active
event loop so it can call `event_loop.create_custom_cursor(...)`.
- The legacy `handle_platform_output(window, ...)` delegates with `None`
and silently drops `cursor_image`. **No existing callers break.**
- The icon and bitmap paths are unified in a private `apply_cursor`. The
no-flicker dedupe of the old `set_cursor_icon` is preserved on both
paths.
- If `CustomCursor::from_rgba` rejects the bitmap (bad dimensions,
hotspot OOB, etc.), we log a warning and fall back to the icon path.
- eframe's wgpu + glow integrations thread `&ActiveEventLoop` through
`run_ui_and_paint` (glow already had it; wgpu needed one extra
parameter) and call the new method.
- Immediate viewports keep the old path because they're invoked from a
`Context` callback that doesn't have an event loop reference. Custom
cursors are a no-op in immediate viewports — acceptable since they're a
niche path.

## Fallback semantics

| backend / context              | what happens                  |
|--------------------------------|-------------------------------|
| eframe wgpu/glow main viewport | bitmap displayed via OS       |
| eframe immediate viewport      | falls back to `cursor_icon`   |
| eframe web                     | falls back to `cursor_icon`   |
| custom integrations not opted in | falls back to `cursor_icon` |
| `from_rgba` returns `BadImage` | warning + falls back to icon  |

## Verification

- `cargo fmt --all -- --check` ✅
- `cargo clippy -p egui -p egui-winit -p eframe --all-targets
--all-features -- -D warnings` ✅
- `cargo doc --lib --no-deps -p egui -p egui-winit -p eframe
--all-features` ✅
- `cargo check -p egui --no-default-features --features serde` ✅
(validates the `serde(skip)` on `cursor_image`)
- Interactive validation on Linux/Wayland with the OneAmp WSZ skin
player — see screenshots above.

I haven't run the full snapshot test suite (`scripts/check.sh`) because
we're on Linux and the snapshots are macOS-rendered — happy to run it if
you'd like.

## Notes

Drafted per the contributing guide ("open a draft PR, you may get
helpful feedback early"). Open to design feedback on:
1. Whether `CustomCursorImage` should live in `egui::viewport` rather
than `egui::data::output`.
2. Whether the legacy `handle_platform_output` should grow `event_loop`
directly (breaking) instead of getting a sibling method (non-breaking,
what I did).
3. Whether to also wire it through eframe-web (probably not —
`wasm-bindgen-cursor` would need its own path).

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/egui/src/context.rs

@@ -1533,6 +1533,19 @@ impl Context {
         self.output_mut(|o| o.cursor_icon = cursor_icon);
     }
 
+    /// Request that the integration display this RGBA bitmap as the OS
+    /// cursor for the next frame, instead of the standard `cursor_icon`.
+    /// Backends that don't support custom cursors (web, eframe with
+    /// non-winit integrations) silently fall back to the icon.
+    ///
+    /// Pass `None` to clear and revert to `cursor_icon` selection.
+    ///
+    /// The integration is expected to dedupe by `Arc` pointer identity,
+    /// so reusing the same `Arc<[u8]>` across frames is cheap.
+    pub fn set_cursor_image(&self, image: Option<crate::CustomCursorImage>) {
+        self.output_mut(|o| o.cursor_image = image);
+    }
+
     /// Add a command to [`PlatformOutput::commands`],
     /// for the integration to execute at the end of the frame.
     pub fn send_cmd(&self, cmd: crate::OutputCommand) {

crates/egui/src/data/output.rs

@@ -116,6 +116,16 @@ pub struct PlatformOutput {
     /// Set the cursor to this icon.
     pub cursor_icon: CursorIcon,
 
+    /// If set, the integration should display this RGBA image as the OS
+    /// cursor (via e.g. `winit::window::CustomCursor`) instead of the
+    /// standard `cursor_icon`. Set per frame; integrations that don't
+    /// support custom cursors fall back to `cursor_icon`.
+    ///
+    /// Skipped from serde because the bitmap is ephemeral and shouldn't
+    /// roundtrip through persisted state.
+    #[cfg_attr(feature = "serde", serde(skip))]
+    pub cursor_image: Option<CustomCursorImage>,
+
     /// Events that may be useful to e.g. a screen reader.
     pub events: Vec<OutputEvent>,
 
@@ -177,6 +187,7 @@ impl PlatformOutput {
         let Self {
             mut commands,
             cursor_icon,
+            cursor_image,
             mut events,
             mutable_text_under_cursor,
             ime,
@@ -187,6 +198,7 @@ impl PlatformOutput {
 
         self.commands.append(&mut commands);
         self.cursor_icon = cursor_icon;
+        self.cursor_image = cursor_image;
         self.events.append(&mut events);
         self.mutable_text_under_cursor = mutable_text_under_cursor;
         self.ime = ime.or(self.ime);
@@ -198,10 +210,12 @@ impl PlatformOutput {
         self.accesskit_update = accesskit_update;
     }
 
-    /// Take everything ephemeral (everything except `cursor_icon` currently)
+    /// Take everything ephemeral (everything except `cursor_icon` and
+    /// `cursor_image` currently)
     pub fn take(&mut self) -> Self {
         let taken = std::mem::take(self);
-        self.cursor_icon = taken.cursor_icon; // everything else is ephemeral
+        self.cursor_icon = taken.cursor_icon; // sticky between frames
+        self.cursor_image = taken.cursor_image.clone(); // sticky between frames
         taken
     }
 
@@ -261,6 +275,39 @@ pub enum UserAttentionType {
     Reset,
 }
 
+/// A bitmap cursor pushed to the integration via [`PlatformOutput::cursor_image`].
+///
+/// The integration is expected to upload this to the OS as a real cursor
+/// (so the image is not clipped by the egui window — what `egui::Painter`
+/// drawn cursors suffer from). Backends that don't support it should fall
+/// back to [`PlatformOutput::cursor_icon`].
+///
+/// `rgba` is straight (non-premultiplied) RGBA — same encoding as
+/// `winit::window::CustomCursor::from_rgba`. The buffer length must be
+/// exactly `size[0] * size[1] * 4` bytes. `size` and `hotspot` use
+/// `u16` to match winit's native types and avoid a lossy cast in the
+/// integration layer.
+///
+/// `Arc<[u8]>` is used so integrations can dedupe / cache by pointer
+/// identity (`Arc::ptr_eq`) and avoid re-uploading the same bitmap to
+/// the OS every frame.
+#[derive(Clone, PartialEq, Eq)]
+pub struct CustomCursorImage {
+    pub rgba: std::sync::Arc<[u8]>,
+    pub size: [u16; 2],
+    pub hotspot: [u16; 2],
+}
+
+impl std::fmt::Debug for CustomCursorImage {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("CustomCursorImage")
+            .field("size", &self.size)
+            .field("hotspot", &self.hotspot)
+            .field("rgba_len", &self.rgba.len())
+            .finish()
+    }
+}
+
 /// A mouse cursor icon.
 ///
 /// egui emits a [`CursorIcon`] in [`PlatformOutput`] each frame as a request to the integration.

crates/egui/src/lib.rs

@@ -467,8 +467,8 @@ pub use self::{
         Key, UserData,
         input::*,
         output::{
-            self, CursorIcon, FullOutput, OpenUrl, OutputCommand, PlatformOutput,
-            UserAttentionType, WidgetInfo,
+            self, CursorIcon, CustomCursorImage, FullOutput, OpenUrl, OutputCommand,
+            PlatformOutput, UserAttentionType, WidgetInfo,
         },
     },
     drag_and_drop::DragAndDrop,