In Bug 141973 (<https://trac.webkit.org/changeset/180987>) I introduced a regression for mechanical wheel mice users. After my change, WebKit always creates a snap scroll animation state context, which triggers new scroll snap point logic when processing wheel events. If you work with a mechanical wheel mouse, the scroll snap point handler does not understand the gesture (since mechanical mice only produce "stateless" wheel events), and complains via an assertion.
<rdar://problem/20088167>
Note: additional work for mechanical wheel mice is being done under Bug 142501.
Created attachment 248301 [details] Patch
Created attachment 248302 [details] Patch
Committed r181300: <http://trac.webkit.org/changeset/181300>