fix: make titlebar opaque while fullscreen (#39759)

shell/browser/native_window_mac.mm

@@ -1692,6 +1692,9 @@ void NativeWindowMac::NotifyWindowEnterFullScreen() {
   // Restore the window title under fullscreen mode.
   if (buttons_proxy_)
     [window_ setTitleVisibility:NSWindowTitleVisible];
+
+  if (transparent() || !has_frame())
+    [window_ setTitlebarAppearsTransparent:NO];
 }
 
 void NativeWindowMac::NotifyWindowLeaveFullScreen() {
@@ -1701,6 +1704,9 @@ void NativeWindowMac::NotifyWindowLeaveFullScreen() {
     [buttons_proxy_ redraw];
     [buttons_proxy_ setVisible:YES];
   }
+
+  if (transparent() || !has_frame())
+    [window_ setTitlebarAppearsTransparent:YES];
 }
 
 void NativeWindowMac::NotifyWindowWillEnterFullScreen() {