A simple way to reproduce: run minibrowser, enable the gesture, go to webkit.com, swipe back. Go to docs or release announcements, swipe back, it works immediately. The event it's waiting for is RenderTreeSizeThreshold. Looks like ViewGestureGeometryCollector::SetRenderTreeSizeNotificationThreshold() is sent too early, and arrives to the wrong process in case of PSON. And it looks like the same thing should happen on Mac. Adding a requestRenderTreeSizeNotificationIfNeeded() call in ViewGestureController::connectToProcess() fixes it here, but that's probably not the right solution.
Seems to be a duplicate of https://bugs.webkit.org/show_bug.cgi?id=205012 and fixed there. *** This bug has been marked as a duplicate of bug 205012 ***