The change in <http://trac.webkit.org/changeset/184296> introduced a regression on certain sites that handle scrolling manually. The change in r184296 was designed to make sure that the logic to notify scroll bars that they should re-hide was not bypassed when the wheel event had zero movement, which is typical for "end scroll" state. Unfortunately, on certain sites this had the undesirable side-effect of triggering a programmatic scroll event in JavaScript at the end of the gesture. I'm working on a reduced test case for this behavior, but I have a fix for the web page showing the problem.
<rdar://problem/21524942>
Created attachment 255903 [details] Patch
Committed r186185: <http://trac.webkit.org/changeset/186185>