Since r258829, the drawing area proxy of a provisional page ignores all messages until the load is committed. This is causing 2 problems for coordinated graphics drawing area. When not in accelerated compositing mode, Update message is sent before the commit is loaded, and the web process keeps waiting for the DidUpdate response message forever. When accelerated compositing mode is forced, the EnterAcceleratedCompositing message is also sent before the load is committed and ignored, so the UI process doesn't know it's in accelerated mode.
Created attachment 394918 [details] Patch
Committed r259283: <https://trac.webkit.org/changeset/259283>
<rdar://problem/61097420>