Previously it was possible to inadvertently activate a menu item by
clicking on a submenu item that was drawn on top of the parent menu.

The root cause was that hide() initiates an asynchronous mechanism
through exitModalState() that eventually destroys the MenuWindow, but
the MouseSourceState timer callbacks and event handlers sometimes still
had a chance to do a state update. Since the submenus have just been
destroyed the update could mistakenly conclude to activate one of the
items of the now lone parent.