Since WebPageProxy can be created and methods called when no underlying WebPage exists yet, a call to WebPageProxy::setUseFixedLayout(...) will only be delivered to WebProcess by the IPC mechanism after the WebPage is created. But it is WebPage constructor that creates a DrawingArea, that calls enterAcceleratedCompositingMode(), that creates a CoordinatedLayerTreeHost, that calls WebKit::WebPage::settings()->setScrollingCoordinatorEnabled(true), if fixed layout is true. The call to WebPage::setUseFixedLayout() comes too late, and the scrolling coordinated is never enabled.
Created attachment 203622 [details] Patch
Comment on attachment 203622 [details] Patch LGTM, please ask for owner review
Comment on attachment 203622 [details] Patch Clearing flags on attachment: 203622 Committed r151132: <http://trac.webkit.org/changeset/151132>
All reviewed patches have been landed. Closing bug.