There's a sample site of a guy who created a Sticky Header in Notion + Potion. It's pretty ugly but it works.
Here is his code:
"So you can actually just add your own html and css for the header or foooter. Doesn't have to be in Notion. And you can just put it once in the snippet injeciton. I didn't think of that."
"So just to check (I haven't got Potion yet, I wanted to see if it could do this first) there's a 'snippet injection' section where I paste the code in?"
"yep, thats right."
-Noah, who was on Support chat with me