Right now, to determine if there are overlay scrollbars, we always consult the native theme. This is not good because that is not relevant for RenderScrollbars (CSS Scrollbars). I have a fix. <rdar://problem/9034318>
Created attachment 83381 [details] Patch
Attachment 83381 [details] did not build on chromium: Build output: http://queues.webkit.org/results/7944405
Created attachment 83384 [details] Patch that hopefully doesn't break things
Thanks Sam! Fixed with revision 79360.