I've used a position:fixed form to create the input field for a downward-scrolling, bottom-affixed chat view (think Slack). I don't think they have any real use in web "documents", though.

This isn't even a problem though. Try pressing pgdn on any page with a fixed header. Modern browsers only scroll by 80% or so.

EDIT: why the downvotes? Try it yourself on http://legitmix.com

I've found the best way to avoid downvotes on HN is to pose your assertion as a question (as sincerely as possible) :)

"This isn't even a problem though." -> "Is this really a problem? I thought that... correct me if I'm wrong!"

Your karma will move in the positive direction almost every time.

Yeah, it's generally a good tip for interaction in general, where constantly making assertions comes across as argumentative and annoys people who realise that not everything they say is a fact.

Downvotes are likely because one's brain learns, "After you hit the space bar, look at the top of the text you were reading, except 15% lower. That's where the next line will be." Sites screw with this assumption and become very hard to read - even if the text is all visible after a scroll.

Browsers have scrolled less than 100% since forever, but there are differences. Most noticeably, Firefox scrolls (or used to scroll) more than Chromium, and has problems with content hiding more often.