Afaict position-fallback-bounds only affects whether the abspos is considered to overflow for the purposes of triggering fallback positions.

I'm having a hard time understanding what use case it might have that wouldn't be covered by a position-container that actually changes the containing block (and would have many more use cases besides, see e.g. #9662 and the Apple proposal which approximated this by targetting the anchor box).

Assuming that a fixedpos would continue to check its overflow against the viewport in addition to its containing block, do we really need both of these features?

CC @una