I think this regressed in <http://trac.webkit.org/projects/webkit/changeset/21544>. I am going to post a test case and a patch.
Created attachment 15600 [details] Test case
Created attachment 15601 [details] Compute item background colors in the client and ensure opaqueness Includes a manual test.
Comment on attachment 15601 [details] Compute item background colors in the client and ensure opaqueness r=me
Committed revision 24481.
<rdar://problem/5380585>