1. Have a window with some lines in it, so there is a scrollback history.
2. Click in it and scroll up to the top and then back down with arrow keys.
3. Now wait a while for some new messages to show up.

Expected behavior: Since I scrolled "all the way down", I should see new
messages displayed as they come in.

Actual behavior: New messages are hidden, because pidgin thinks I am
still scrolled up. Specifically, I seem to be scrolled back by 1 pixel.

Screenshot attached of window in this state. If I then click on the down
arrow, the basically invisible gap I was scrolled up goes away, and the
arrow dims, and I am finally scrolled down.