Comment # 13 on bug 1123424 from Stefan Hundhammer

BTW that "Clean Up" menu is the only one that is a "tear-off" menu that you can
rip off the menu bar for repeated use, so it has a WM title. All the other
menus don't; they only have the tiny menu content.

It looks to me like the QMainWindow and its QMenuBar and its QStatusBar are the
only widgets that at least give it an honest try to display something if the
widget font is invalid (zero size?). Most other widgets appear to use font
metrics for their size hint and obviously return a zero size.