X11 menus do not open upon a mouse click, if the
menubutton has no text but just an image and if
the menu items are just images.
Affected platforms are MacOSX/X11 _and_
Debian 4.0/X11 with the Gnome desktop (focus follows
mouse must be enabled on the latter!).
The menu can still be opened, when the mouse is clicked
on a neighboring menubutton and then moved (with the
mouse button held down) over the menu that does not
open in the first place.
Disabling focus follows mouse in Gnome also helps.