layout: adjust fixed positioning

In CSS, `position: fixed' either a) moves the box on scroll (with
`@media screen'), or b) repeats the box on every page (with
`@media print').

a) would completely mess up our document model, and even if it didn't,
the renderer couldn't handle a redraw on every single scroll.
b) sounds better, but still doesn't work because it's incompatible with
`@media screen' semantics. e.g. in a) I can read text despite any banner
on the bottom of the screen, because the box moves when I scroll, but
in b), the same box will obscure some text on every single page.

So instead, make the `position: fixed' containing box
width: max(:root.width, 100vw); height: max(:root.height, 100vh)).
This is completely non-standard, but at least both TOC-like fixed boxes
*and* banners are placed somewhere that makes some sense.

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