The hide animation causes hover notifications for the actors inside the
boxpointer. PopupBaseMenuItems, in particular, grab the keyboard focus on
hover notifications to enable keyboard navigation on menus. This, in turn,
breaks modal dialogs' keyboard navigation since key focus is taken away from a
just created dialog when the menu is hiding.
Since input events aren't useful while menus are animating we just prevent
them from propagating.
https://bugzilla.gnome.org/show_bug.cgi?id=662493