Previously we had various things watching for notify::mapped so
we could be more lazy about updating non-visible actors, but it
was fairly ad-hoc.
The deferred work system unifies these callbacks, and also adds
a timeout so that we don't delay changes arbitrarily; this way we
avoid a storm of work if you stay out of the overview for a while,
then go in.
https://bugzilla.gnome.org/show_bug.cgi?id=603522
d624db18c5