This has two benefits:
(1) Users can scroll their last message to near the top of the screen,
which decreases how often one needs to scroll down to track current
messages.
(2) It makes scrolling bugs near the bottom of the screen a lot easier
to find, because scrolling down too far isn't blocked by being unable
to scroll messages off the screen.
We should probably later convert this to some formula that varies
dynamically based on the height of the last message.
(imported from commit 41954fecd9efb43820ed1ccb5210283c17752f51)
e5c96fb3ad