This website will have modals. You'll probably position: fixed the modal so that it's predictably positioned regardless of scrolling. It's possible the modal itself will scroll.
Notice the modal has a close button. We can position: absolute that in the upper-right corner, but if our modal scrolls, it's a problem.
Should be a job for more position: fixed, right? Not really, sadly, as there is no such thing as a local context for position: fixed.
Well... actually there kinda (weirdly) is. If the modal has any sort of CSS transform on it (and it might already if you center it using the ol' left: 50%; transform: translateX(-50%); trick) then the fixed position close button will come back home:
But... as the transform helped pull the close button back into place, you can see it behaves like position: absolute and not position: fixed. ¯\_(ツ)_/¯
There is a possibility here, though. The idea is that position: sticky is, in a sense, a locally scoped position: fixed. If we position the close button at the top: 0 anyway, it'll just stick there as the modal scrolls down.
I just thought this was an interesting possibility. Ultimately, to be honest, if I had modals I worried about scrolling, I'd probably have like a .modal-header container and a .modal-content container. The header would sit on top always and the container would be the thing that could scroll.