fix: make titlebar opaque while fullscreen (#39779)

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Samuel Attard <[email protected]>

shell/browser/native_window_mac.mm

@@ -1721,6 +1721,9 @@ 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() {
@@ -1730,6 +1733,9 @@ void NativeWindowMac::NotifyWindowLeaveFullScreen() {
     [buttons_proxy_ redraw];
     [buttons_proxy_ setVisible:YES];
   }
+
+  if (transparent() || !has_frame())
+    [window_ setTitlebarAppearsTransparent:YES];
 }
 
 void NativeWindowMac::NotifyWindowWillEnterFullScreen() {