What problem does this solve or what need does it fill?

There's no way, as far as I can tell, to have an element positioned relative to the viewport.

What solution would you like?

Add a Position::Fixed variant that would act, for all intents and purposes, the same as Position::Absolute, except relative to the viewport rather than most recent positioned ancestor.

Viewport being a rectangle with x and y being zero and size being available space passed into compute_root_layout().

What alternative(s) have you considered?

I have considered having an override in my own element nodes, but that seems unnecessary, and contrary to the purpose of using something like taffy.

Additional context

MDN docs: https://developer.mozilla.org/en-US/docs/Web/CSS/position#fixed

Related: #828