See bug 5016 comment 17. This is the first issue left over from that bug.
Created attachment 373707 [details]
Meant to attach this previously, oh well.
Created attachment 373715 [details]
testcase take 2
Previous testcase was fine (although IE behaves differently) this one should show the issue at hand.
Looks like a dup of bug 743599 to me, fwiw. Or vice versa.