If these two bars are equally long
You are in standards mode (content-box)…
Two of them!

Super content box demo

This works pretty well :). It uses IE's proprietary CSS extension 'expression()' to manually calculate the correct height and width (as IE doesn't do that automatically). It automatically updates when resizing, too. I could have done it with CSS alone as well (albeit IE-specific hacky CSS), were it not that when IE renders a scrollbar, it includes the border and the margin in the scrolling area as well >.<, and that IE doesn't scroll when no specific height is given.

Click to resize | Start moving | Start sizing

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