I think this regressed when we switched to always start the wayland compositor. In AcceleratedDrawingAreaProxy::waitForAndDispatchDidUpdateBackingStoreState() we check if the nested compositor is running, which is always true now (if EGL and the required extensions are available). We should check if we are in AC mode instead.
Created attachment 320532 [details]
Committed r222027: <http://trac.webkit.org/changeset/222027>