Comment 31 for bug 10905

I've read the code in unity/services/panel-service.c
about PanelService: close a menu and re-send the keyevent when handling a
    combination Or when we try to open HUD/Dash. Also, close the active
    menu if a new application is opened and focused. (LP: #10905,
    #1234457, #1197071),

but it still does not resolve the problem about PrintScreen key to snapshot context menu.

The root reason is at the XGrabKeyboard in popup_grab_on_window() of gtk/gtkmenu.c.

Guys, don't bother yourself any more, as you know linux means freedom, we could get rid of XGrabKeyboard by ourselves:
http://stackoverflow.com/a/20071469/264181