gpui

Implement the Element trait with RequestLayoutState and PrepaintState associated types, plus request_layout, prepaint, and paint methods. Create hitboxes in prepaint, pass state through the associated types, and avoid allocations in the paint phase.

Use cx.spawn for foreground tasks that update entities and cx.background_spawn for CPU-intensive work on background threads. Chain background results back to the foreground with .then(cx.spawn(...)) since entities can only be updated on the UI thread.

Define actions with the actions! macro or #[derive(Action)], bind keys with cx.bind_keys using KeyBinding::new, and handle them with .on_action(cx.listener(...)) on elements. Use key_context to scope bindings so the same key can trigger different actions per context.

This happens when entity.update is called on the same entity inside a defer_in callback, re-entering its lock. Use the &mut reference provided directly by the defer_in callback instead of calling update on that entity.

An ElementId is required for elements that handle mouse events, store keyed state via window.use_keyed_state, or need interaction tracking. Call .id() on a div to make it stateful; IDs only need uniqueness within the same stateful parent scope.

Use the #[gpui::test] attribute with TestAppContext for logic tests and VisualTestContext for rendering tests. You can open windows, dispatch mouse events, and assert on entity state within the test context.