For Qt, we would like an option to force the compositing path for filters, and the right place to do so seems to be RenderLayerCompositor and the triggers in ChromeClient.
Created attachment 121349 [details]
Created attachment 121492 [details]
Comment on attachment 121492 [details]
Clearing flags on attachment: 121492
Committed r104347: <http://trac.webkit.org/changeset/104347>
All reviewed patches have been landed. Closing bug.
This patch broke the cases when some of the filters do not render in composited mode. That's because RenderLayerBacking::paintIntoLayer has no logic for applying filters yet. I've added https://bugs.webkit.org/show_bug.cgi?id=75842 to fix that.