Example is fast/multicol/pagination/LeftToRight-tb.html There should never be a vertical scrollbar shown, since this is filled rather than balanced. It's like the column code didn't update properly when the horizontal scrollbar had to appear.
<rdar://problem/16656284>
Created attachment 229825 [details] Patch
Created attachment 229826 [details] Patch
Landed in r167817.
OOps I meant r167617.