This was caused by the change to remove repaintObjectsBeforeLayout in bug 13496 (r21120 / http://trac.webkit.org/projects/webkit/changeset/21120). See attached test case.
Created attachment 14461 [details] Test case
Created attachment 14462 [details] Test case using script
Created attachment 14468 [details] Simplify layer repaint logic A P1/Critical bug is a good excuse to get rid of checkForRepaintOnResize.
Comment on attachment 14468 [details] Simplify layer repaint logic r=me
Landed in r21390. http://trac.webkit.org/projects/webkit/changeset/21390
This does fix my original document and the test case.