gpui: Allow repositioning macOS traffic lights at runtime (#58169)

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

Right now we can only set the traffic light position when creating a
window. This adds a public API so that we can update the position at
runtime, such as when titlebar height or layout changes dynamically.

crates/gpui/src/platform.rs

@@ -670,6 +670,8 @@ pub trait PlatformWindow: HasWindowHandle + HasDisplayHandle {
     }
     fn set_edited(&mut self, _edited: bool) {}
     fn set_document_path(&self, _path: Option<&std::path::Path>) {}
+    #[cfg(target_os = "macos")]
+    fn set_traffic_light_position(&self, _position: Point<Pixels>) {}
     fn show_character_palette(&self) {}
     fn titlebar_double_click(&self) {}
     fn on_move_tab_to_new_window(&self, _callback: Box<dyn FnMut()>) {}

crates/gpui/src/window.rs

@@ -2311,6 +2311,12 @@ impl Window {
         self.platform_window.set_title(title);
     }
 
+    /// Sets the position of the macOS traffic light buttons.
+    #[cfg(target_os = "macos")]
+    pub fn set_traffic_light_position(&self, position: Point<Pixels>) {
+        self.platform_window.set_traffic_light_position(position);
+    }
+
     /// Sets the application identifier.
     pub fn set_app_id(&mut self, app_id: &str) {
         self.platform_window.set_app_id(app_id);

crates/gpui_macos/src/window.rs

@@ -1188,6 +1188,12 @@ impl PlatformWindow for MacWindow {
         }
     }
 
+    fn set_traffic_light_position(&self, position: Point<Pixels>) {
+        let mut state = self.0.lock();
+        state.traffic_light_position = Some(position);
+        state.move_traffic_light();
+    }
+
     fn scale_factor(&self) -> f32 {
         self.0.as_ref().lock().scale_factor()
     }