In a Swing applet running within Mac OS/Safari, when the applet window is maximised, the JMenuItem appear behind the Safari browser.
This is readily reproduced with a test case and can be seen in production.
This is a visual defect, the JMenuItems can still respond.
To recover, simply click on JMenu and the JMenuItem will be redrawn.
This is seen in
1. OS X 10.8/Safari 6.1
2. OS X 10.9/Safari 7.0