I've found Kill Sticky [1] to be an incredibly useful bookmarket lately. It finds any DOM element with "position: fixed" and deletes it. Wish I didn't need to use it so often, but here we are.

Sometimes for annoying popups covering content, sometimes for the sidebar of brightly colored floating social widgets, sometimes just the enormous fixed banner that continues to waste screen real estate after you've scrolled down.

I use it a lot on my laptop and haven't tried JS bookmarklets in mobile browsers, but I'd hope that they work by now?

[1] https://alisdair.mcdiarmid.org/kill-sticky-headers/