Good point, that would not be very convenient with existing structures. But that page was so old (my first efforts at fiddling around with web design) that it deserved a complete rewrite. The fixed elements there are just a “frame” for housing the other content and not markup that would need nesting, so I removed that. But if anything, it does illustrate a similar issue.
I think the webkit related browsers have a long history of problematic fixed positioning, there’s quite a few more bugs around. Firefox seems to have taken the approach from the beginning to render fixed layers separately, repainting them most smoothly. And IE’s somewhere in between, I think.