On the quoted page there is a PRE that is so long it goes past the edge of the viewport. However, the horizontal scroll bar does not allow you to see the text, since they do not go far enough! This is very probably because the PRE's parent has been given an explicit width and so even the idea that a child might be wider doesn't occur to whatever is trying to find the maximum width.

The problem was that the root-frame and scroll-frame code wasn't paying attention to the mCombinedArea member of the reflow metrics struct