... and its implementation in the X11 backend.

GDK does lots of work trying to reduce the region in expose events
so that when the server sends multiple expose events, touching the
same area we can make sure to only redraw stuff once. However:
(1) this is only relevant of there's tons of delay and multiple
    expose events get sent
(2) we coalesce multiple events into a single expose event anyway
(3) we do this on the frame clock

But most importantly:
(4) Since the invention of compositing, servers caches all contents
    anyway