The geometry used by within_viewport() is a little off to
begin with, but we don't want to check it at all in this
situation, because if the last messages falls out of the
viewport, we still want to scroll. The relevant thing
to check is that available_space_for_scroll exceeds zero.
(imported from commit a0a6f0d23db2eab8d9f22fc9ad523031cf7f7ec2)