The problem occurs when the height of the scroll bar is reduced past its normal minimum size and manifests as the top and bottom portions of the widget are drawn overlapping. This is specific to the Nimbus theme engine.
I observed this 2+ years ago, but didn't know it hadn't been fixed. Adobe Reader demonstrates this when displaying e.g.: http://www.usenix.org/events/lisa09/tech/full_papers/tripathi.pdf I'll file a bug later today if someone hasn't done so already. -Albert
