We now rely on toplevels receiving and forwarding all the events
the windowing should be able to handle. Event masks are no longer a
way to determine whether an event is deliverable ot a widget.
Events will always be delivered in the three captured/target/bubbled
phases, widgets can now just attach GtkEventControllers and let those
handle the events.
In this case we have a bunch of interactive main children
of the overlay, and then a centered overlay that contains both
non-interactive (labels) and interactive (entry) widgets.
This shows off a problem where the non-interactive parts (the labels)
steals input from the overlay main children (breaks button click and
hover effects).
https://bugzilla.gnome.org/show_bug.cgi?id=750568https://bugs.freedesktop.org/show_bug.cgi?id=90917