Unity global menu of several programs disappears randomly

[Joschi Poschi]

On Ubuntu 13.04 (Raring Ringtail) and also before on 12.10 (Quantal Quetzal) the global menu of a lot of programs randomly disappears from the panel. I always had the latest updates installed.

What should happen:
Global menu should be visible in any program whenever I hover the cursor over the panel.

What happens:
Randomly the global menu of several programs (Nautilus, Firefox, Thunderbird, GIMP, gedit etc.) does not show when hovering over the panel.

I'm really don't know what triggers the mentioned behaviour for now it seems totally random for which program and when it will occur. Usually I work with some programs maximized and switch between them by Alt+Tab or the Compiz 'Scale' option. For Firefox and Thunderbird it helps to minimize the window and maximze it directly to get the global menu shown again on hover over. For the other programs minimizing/maximizing doesn't help.