Whenever we get a narrowing event, it's possible for new messages
to appear visible, and we need to call process_visible_unread_messages().
This has been a bug, but it's mostly obscured by the fact that we
call process_visible_unread_messages() as part of focus/scrolling
events.
(imported from commit b9447977f8e2272d45865ca67b436cacafd58a03)