https://bugs.webkit.org/show_bug.cgi?id=86158 made sure to run the overlay scrollbars painting pass when in tiled drawing mode. However, it was a little aggressive in allowing that pass to go forward. Patch forthcoming. <rdar://problem/11437428>
Created attachment 141538 [details] Patch
Thanks Dan! Committed change: http://trac.webkit.org/changeset/116831