Neil Roberts authored Aug 29, 2013 and Jasper St. Pierre committed Nov 14, 2013

This adds clutter_event_add/remove_filter which adds a callback
function which will receive all Clutter events just before the event
signal is emitted for them. The event filter will be invoked
regardless of any grabs or captures. This will be used by Mutter which
wants to access the events at a lower level then the event bubbling
mechanism. It needs to see all mouse motion events even if there is a
grab in place.

https://bugzilla.gnome.org/show_bug.cgi?id=707560