To reproduce, open the attached test case, make the browser window narrow, and scroll slowly from side to side. The overflow (beyond the gray rectangle) will not repaint.
Created attachment 7291 [details] Test case
<rdar://problem/4549738>
It's absoluteBoundingBox() in RenderLayer that is to blame for this. However I'm not sure of the best way to fix it. If the check is too broad, then we'll end up doing too much work. The common case is very fast because of this restrictive check.
Hi Mitz, can you still reproduce this? I can't. Tested with QtTestBrowser and Chromium 12.