The Mozilla theme drawing code does not expose the "trough-under-steppers" property of the theme. This is necessary to correctly draw scrollbar.
Created attachment 65042 [details] Patch for this issue
Comment on attachment 65042 [details] Patch for this issue OK!
Comment on attachment 65042 [details] Patch for this issue Clearing flags on attachment: 65042 Committed r65850: <http://trac.webkit.org/changeset/65850>
All reviewed patches have been landed. Closing bug.