The compositor tries to avoid rendering the same stuff twice, using a mechanism that was only applicable back when we had a separate OpenGL window.
Created attachment 132420 [details] Patch
Comment on attachment 132420 [details] Patch I trust you.
Comment on attachment 132420 [details] Patch Let's try to get it in.
Comment on attachment 132420 [details] Patch Clearing flags on attachment: 132420 Committed r111180: <http://trac.webkit.org/changeset/111180>
All reviewed patches have been landed. Closing bug.