Hopefully this gives you a better idea of what I'm doing:
That should give you rudimentary illustration of what I'm trying to do. The scrollbar is offset outside the body (which has the light grey background and encompasses the header and content element), although the scrollbar is still *technically* inside the "content" element, which has a transparent background. It only appears to be outside the content element because of the transparent background.
The problem comes if the scrollbar's width is increased or decreased. It would alter the position of the content.