This can be easily reproduced with MiniBrowser. 1- Start MB with WEBKIT_FORCE_COMPOSITING_MODE=1 2- Wait until webkitgtk.org loads 3- Load google.com (or any other page that make a cross site navigation) This is because when AC mode is forced the provisional page creates a drawing area that is always in AC mode. When swapping processes, the current drawing area is destroyed (which exists AC mode) and the provisional one is set. From that point the web process is in AC mode but the UI process isn't. We need to update the compositing mode when swapping processes.
Created attachment 374206 [details] Patch
Committed r247506: <https://trac.webkit.org/changeset/247506>