The issue existed from me in both firefox and chrome. Click on outside columns will result in normal scroll. Click or highlight in the center column will result in the jumpy scroll that does not quite scroll one comment at a time with up/down arrow.

It kinda happens to me on firefox, one press of the down arrow scrolls so "Here's a formula for the lemniscate in polar coordinates" in the first reply is at the top of the screen, not helpful.