WK1 clients only trigger latching when the wheel event is in a momentum phase. This means that if you begin a continuous wheel scrolling event outside of a scrollable region (e.g., an Iframe), you will not latch to the main frame. Instead, the Iframe will begin consuming wheel events, causing the page scroll to stop.
This was corrected for WK2 "Async Scrolling" case in Bug 127386. However, further testing shows that this is not correct in WK1 logic.
Created attachment 223032 [details]
Committed r163338: <http://trac.webkit.org/changeset/163338>