On Linux (exact platform see above) the signals QMenu::aboutToShow() and QMenu::aboutToHide() are not emitted on a menu which is set on a tray icon with QSystemTrayIcon::setContextMenu().

This can be easily reproduced with the attached little example source code:
The QLCDNumber in the main window is incremented on aboutToShow() and decremented on aboutToHide().
This works fine with the main menu.
It doesnt work with the tray icon menu on linux (but it does on Windows7).