The functional change here is that our code to stop
autoscrolling on certain events, particularly mousemove,
now only runs during system initiated autoscroll events.
If the user had been replying to a message, then the feature
to stop autoscroll was too aggressive.
This patch also starts to put more scrolling-related code
into viewport.js, which will hopefully prevent some code
duplication and give us a single place to control things like
stopping animated scrolls.
(imported from commit e7d5946b0ac7fcfda2eff1d0e2b58a78b44ecc1a)
9cedca4dc5