Sometimes the first swipe gesture in a WKWebView would fail to initiate a back/forward swipe despite the view being pinned to the edge and non-rubber-banding. This happens because of a race condition in EventDispatcher::wheelEvent, where setting the rubber-band state is dispatched to the scrolling thread, whereas querying the rubber-banding state happens immediately right after that on the event dispatcher thread, under tryToHandleWheelEvent().
Created attachment 347941 [details]
Set the rubber-band state from the event dispatcher thread
Fixed in <https://trac.webkit.org/r235236>.