I thought I fixed this yesterday with Bug 145140, but that actually only dealt with the overflow scrolling case. If I had followed the lessons of the 'isRubberBandInProgress', I would have noticed that we also need to recognize when a scrolling thread scroll is running the rubber band snap. This patch completes the work I started in Bug 145140.
<rdar://problem/21023785>
Created attachment 253404 [details] Patch
Committed r184596: <http://trac.webkit.org/changeset/184596>