Using ToT, go to any page that gets a scrollbar (even google.com works these days). You'll see there's an area to the left of the scrollbar, with width equal to the scrollbar's size, that is completely white. Probably the page is getting an allocation too small because of some bug in this area.
*** Bug 53582 has been marked as a duplicate of this bug. ***
This seems to be fixed now. My guess is that it was fixed by: http://trac.webkit.org/changeset/77459