The timeout GtkAdjustmentWatcher::updateAdjustmentsFromScrollbarsLater has a low enough priority that sometimes scrollbars become out of sync with the page contents. I've seen this cause drawing artifacts as well. We just need to boost the priority slightly.
Created attachment 111144 [details] Patch
Comment on attachment 111144 [details] Patch LGTM
Comment on attachment 111144 [details] Patch I hate these priority problems =( I think it makes sense as it is, but just out of curiosity, did you try having it set to something like G_PRIORITY_HIGH_IDLE itself, which would guarantee it's ran before any GTK+ layout is performed?
Comment on attachment 111144 [details] Patch Landing this now, since I think Alex wants it for the release. To answer kov's question, I'm pretty sure I did test G_PRIORITY_HIGH_IDLE without much luck.
Comment on attachment 111144 [details] Patch Clearing flags on attachment: 111144 Committed r98352: <http://trac.webkit.org/changeset/98352>
All reviewed patches have been landed. Closing bug.