Overlay scrollbars that have grown are sometimes askew in the track. This started happening after we adopted the setPresentationValue ScrollbarPainter API which allowed us to update the position of the scrollbar knob from our secondary scrolling thread. This bug occurs when the scrollbar grows while it still thinks it is in presentation-value mode. Whenever the scrollbar grows, it should be in non-presentation value mode.
Created attachment 225801 [details]
Thanks, Simon! http://trac.webkit.org/changeset/165069