Previously, if the pointer was high on the page such that there was a
lot of empty space below and the render window was full (a situation
we could get into if we're following new messages arriving), a newly
recieved message would not be rendered even though it would have been
visible on the screen if we had done a rerender. Adding the
_maybe_rerender() call to the end of append() ensures that we don't
end up in this situation.
(imported from commit 925d3cc62e8221b42f1d5ff1788e99c7d07ccc24)
65d2f65bbb