As the bounce back animation from pinch zoom centers the content using the contents point which was in the center of the viewport, this animation cannot be used when stopping a pan animation, as it won't position the content at the place it was when the pan animation was interrupted.
Created attachment 113319 [details] Patch
Comment on attachment 113319 [details] Patch r=me
Comment on attachment 113319 [details] Patch Clearing flags on attachment: 113319 Committed r99065: <http://trac.webkit.org/changeset/99065>
All reviewed patches have been landed. Closing bug.