Previously we incorrectly would sometimes trigger autoscroll when
messages arrived via get_old_messages (i.e. we loaded some historical
messages from the server). This resulted in some confusing and
erroneous scrolling behavior.
I kinda hate the fact that we need to pass these values all the way
through the rendering process, but fundamentally there's no other way
for _render to find out, and pushing the autoscrolling higher up in
the stack is hard since we don't return anything like the block of new
messages.
(imported from commit c5ed27f1b21691a21c68ed7e09cff8e3e4a75e76)
f10d3d5517