As Olivier found, this was caused by the gtk_menu_popup() code not being able to
get a keyboard grab before trying to pop up the menu, causing it to give up.
Solution was to steal a bit of xfwm4's code to wait for a grab (with timeout)
before trying to pop up the window. Fixed in trunk and xfce_4_2. Finally!
As Olivier found, this was caused by the gtk_menu_popup() code not being able to
get a keyboard grab before trying to pop up the menu, causing it to give up.
Solution was to steal a bit of xfwm4's code to wait for a grab (with timeout)
before trying to pop up the window. Fixed in trunk and xfce_4_2. Finally!