Comment 2 for bug 484527

Because taking a screenshot forces a redraw and corrects the problem, here are pics of what's actually on the monitor. The image sequence is:

174 - correct display
175 - scroll down one mousewheel tick: notice the doubled new region at the bottom. Is the off-screen rendered image segment being copied in twice by accident?
176 - scroll up one tick: same problem at the top, and we still have one bad region on the bottom
177 - scroll down one tick: problem compounds
178 - scroll up one tick: this image SHOULD be identical to 174, and clicking on the titlebar makes it so.