Something is wrong here, testcase will show. This occurred on the site of Jon Hicks, and he fixed it with what seems like a hack to me. See for yourself in the upcoming testcase.
Created attachment 8168 [details] Testcase
Scrolling the testcase will show the bug.
Created attachment 8171 [details] Even simpler test case. This is an incremental repaint bug. If you resize the window the glitch goes away.
I don't think this is a repaint bug. Forcing a repaint (by switching to a different tab) doesn't clear the problem. Resizing does, but that also does a layout.
Bug 9659 wins since it appears to have a more extensive test case. *** This bug has been marked as a duplicate of 9659 ***
Everyone wins if this bug is fixed :)