By default the focus chain uses the same order as the list returned
by clutter_actor_get_children(), which corresponds to the z-order.
This doesn't work well in the window picker, where clones follow
the stacking of windows to ensure a correct overview transition,
but previews are laid out purely based on space efficiency. As
a result, the order in which window previews are navigated when
tabbing around is essentially random. Fix this by providing a
focus chain implementation that is based on the visual layout
of the previews rather than the stacking.
https://bugzilla.gnome.org/show_bug.cgi?id=786546
8143f734f8