gpui_macos: Improve mouse tracking for normal windows (#64234)

To reproduce:

1. Place Safari over part of Zed, then focus Zed.
2. Right-click Safari to open its context menu, then left-click Safari
anywhere in it.
3. Move the pointer over Safari where it covers the Zed. Zed shows hover
effects underneath.

Use a view tracking area and disable `acceptsMouseMovedEvents` for
normal macOS windows. This addresses hover effects on covered content
when the pointer moves over another window.

[Chromium also requests mouse movement through tracking
areas](https://github.com/chromium/chromium/blob/5a43095e95ffe566d1512b9d67d534e3ae05b975/components/remote_cocoa/app_shim/bridged_content_view.mm#L436-L465).
A search of its macOS window/view code found no explicit use of
`acceptsMouseMovedEvents`, which [defaults to
`false`](https://developer.apple.com/documentation/appkit/nswindow/acceptsmousemovedevents).

Tracking stays enabled in unfocused windows while the app is active.

Before:


https://github.com/user-attachments/assets/f21cd3e5-04d5-4d34-b495-02f8d0f68ecf

After:


https://github.com/user-attachments/assets/89a8b9c9-5aa3-4075-aa91-13e200ea3613

Release Notes:

- [GPUI] Fixed mouse tracking on macOS so moving the pointer over
another app’s window no longer triggers hover effects in covered GPUI
windows.
- Fixed an issue on macOS where moving the pointer over another app
could trigger hover effects in a Zed window underneath it.

---------

Co-authored-by: Anthony Eid <hello@anthonyeid.me>

Cargo.toml

@@ -708,6 +708,7 @@ objc2-app-kit = { version = "0.3.2", default-features = false, features = [
     "NSResponder",
     "NSSavePanel",
     "NSScreen",
+    "NSTrackingArea",
     "NSView",
     "NSWindow",
     "NSWorkspace",

crates/gpui_macos/src/window.rs

@@ -56,8 +56,9 @@ use objc2::{
 };
 use objc2_app_kit::{
     NSAlert, NSAlertStyle, NSBeep, NSButton as Objc2NSButton, NSDraggingImageComponent,
-    NSDraggingImageComponentIconKey, NSDraggingItem, NSPasteboardWriting, NSView as Objc2NSView,
-    NSWindow as Objc2NSWindow, NSWindowButton as Objc2NSWindowButton, NSWorkspace,
+    NSDraggingImageComponentIconKey, NSDraggingItem, NSPasteboardWriting, NSTrackingArea,
+    NSTrackingAreaOptions, NSView as Objc2NSView, NSWindow as Objc2NSWindow,
+    NSWindowButton as Objc2NSWindowButton, NSWorkspace,
 };
 use objc2_foundation::{NSPoint as Objc2NSPoint, NSRect as Objc2NSRect, NSURL};
 use parking_lot::Mutex;
@@ -101,14 +102,6 @@ const NSFloatingWindowLevel: NSInteger = 3;
 #[allow(non_upper_case_globals)]
 const NSPopUpWindowLevel: NSInteger = 101;
 #[allow(non_upper_case_globals)]
-const NSTrackingMouseEnteredAndExited: NSUInteger = 0x01;
-#[allow(non_upper_case_globals)]
-const NSTrackingMouseMoved: NSUInteger = 0x02;
-#[allow(non_upper_case_globals)]
-const NSTrackingActiveAlways: NSUInteger = 0x80;
-#[allow(non_upper_case_globals)]
-const NSTrackingInVisibleRect: NSUInteger = 0x200;
-#[allow(non_upper_case_globals)]
 const NSWindowAnimationBehaviorUtilityWindow: NSInteger = 4;
 #[allow(non_upper_case_globals)]
 const NSViewLayerContentsRedrawDuringViewResize: NSInteger = 2;
@@ -1198,15 +1191,29 @@ impl MacWindow {
             let main_window: id = msg_send![app, mainWindow];
             let mut sheet_parent = None;
 
+            // SAFETY: This is the live GPUIView (an NSView subclass) added to the
+            // content view above, and window creation runs on the main thread.
+            let tracking_view = &*native_view.cast::<Objc2NSView>();
+
             match kind {
                 WindowKind::Normal | WindowKind::Floating => {
                     if kind == WindowKind::Floating {
                         // Let the window float keep above normal windows.
                         native_window.setLevel_(NSFloatingWindowLevel);
+                        native_window.setAcceptsMouseMovedEvents_(YES);
                     } else {
                         native_window.setLevel_(NSNormalWindowLevel);
+                        native_window.setAcceptsMouseMovedEvents_(NO);
+                        add_mouse_tracking_area(
+                            tracking_view,
+                            NSTrackingAreaOptions::MouseEnteredAndExited
+                                | NSTrackingAreaOptions::MouseMoved
+                                // Track while this application is active, even if another
+                                // window in the application has focus.
+                                | NSTrackingAreaOptions::ActiveInActiveApp
+                                | NSTrackingAreaOptions::InVisibleRect,
+                        );
                     }
-                    native_window.setAcceptsMouseMovedEvents_(YES);
 
                     if let Some(tabbing_identifier) = tabbing_identifier {
                         let tabbing_id = ns_string(tabbing_identifier.as_str());
@@ -1218,19 +1225,15 @@ impl MacWindow {
                 // `AnchoredPopup` is rejected in `MacPlatform::open_window`, grouped here only
                 // for exhaustiveness.
                 WindowKind::PopUp | WindowKind::AnchoredPopup(_) => {
-                    // Use a tracking area to allow receiving MouseMoved events even when
-                    // the window or application aren't active, which is often the case
-                    // e.g. for notification windows.
-                    let tracking_area: id = msg_send![class!(NSTrackingArea), alloc];
-                    let _: () = msg_send![
-                        tracking_area,
-                        initWithRect: NSRect::new(NSPoint::new(0., 0.), NSSize::new(0., 0.))
-                        options: NSTrackingMouseEnteredAndExited | NSTrackingMouseMoved | NSTrackingActiveAlways | NSTrackingInVisibleRect
-                        owner: native_view
-                        userInfo: nil
-                    ];
-                    let _: () =
-                        msg_send![native_view, addTrackingArea: tracking_area.autorelease()];
+                    add_mouse_tracking_area(
+                        tracking_view,
+                        NSTrackingAreaOptions::MouseEnteredAndExited
+                            | NSTrackingAreaOptions::MouseMoved
+                            // Track even when another application is active, e.g. so
+                            // notification windows can respond to hover.
+                            | NSTrackingAreaOptions::ActiveAlways
+                            | NSTrackingAreaOptions::InVisibleRect,
+                    );
 
                     native_window.setLevel_(NSPopUpWindowLevel);
                     let _: () = msg_send![
@@ -2480,6 +2483,21 @@ extern "C" fn dealloc_view(this: &Object, _: Sel) {
     }
 }
 
+fn add_mouse_tracking_area(native_view: &Objc2NSView, options: NSTrackingAreaOptions) {
+    // SAFETY: NSView provides the tracking-event callbacks, and the owner is
+    // the same view that retains the tracking area. No user info is supplied.
+    let tracking_area = unsafe {
+        NSTrackingArea::initWithRect_options_owner_userInfo(
+            NSTrackingArea::alloc(),
+            Objc2NSRect::ZERO,
+            options,
+            Some(native_view),
+            None,
+        )
+    };
+    native_view.addTrackingArea(&tracking_area);
+}
+
 extern "C" fn reset_cursor_rects(this: &Object, _: Sel) {
     // SAFETY: AppKit invokes cursor-rect updates on the main thread for GPUIView instances,
     // whose WINDOW_STATE_IVAR is initialized when the view is created. The cursor registered