Created attachment 261143 [details] reproduction Video attached. It seems like there are some unexpected mouse events firing that interfere with the rearrangement animation.
<rdar://problem/22692570>
Created attachment 261154 [details] Proposed Fix
Comment on attachment 261154 [details] Proposed Fix r=me
Comment on attachment 261154 [details] Proposed Fix Clearing flags on attachment: 261154 Committed r189763: <http://trac.webkit.org/changeset/189763>
All reviewed patches have been landed. Closing bug.