Currently when the summary boxpointer is ungrabbed automatically
because the keyboard focus was moved outside the message tray
(for instance by selecting the overview search entry or opening
the right-click menu of a dash item), after the popup is hidden
_updateState() will grab focus and show the popup again.
Work around this by unsetting the clicked summary item when losing
focus to an actor outside the message tray.
https://bugzilla.gnome.org/show_bug.cgi?id=685156