If we don't have a render tree size (because the UI process has restarted), we should keep the blank white snapshot view up until either the first layer tree commit from the Web process or the watchdog fires. We should *not* remove the snapshot immediately upon ending the gesture, because that will instantly reveal the previous page's tiles.
Created attachment 233096 [details] patch
This is built on top of https://bugs.webkit.org/show_bug.cgi?id=133885
http://trac.webkit.org/changeset/170036
<rdar://problem/17416500>