Since the shadow tiling patch landed, listbox rendering has been incorrect. Selecting an item, paints the selection color on all items above that item. On investigation, it appears as though the shadow is always being applied.
Created attachment 67876 [details] Patch for this issue
Looks good to me.
Xan r+'d this, but he is having connectivity issues.
Committed r67725: <http://trac.webkit.org/changeset/67725>