A nag popup where the dismiss button just changes itself instead of dismissing the window? Oof, that's a new one.
The Kill Sticky bookmarklet is your friend: https://alisdair.mcdiarmid.org/kill-sticky-headers/
There's a newer version, but it breaks a number of sites for me, so I don't use it: https://github.com/t-mart/kill-sticky
The internet is a hostile place without Kill Sticky, uBlock Origin, and uBlock Matrix.
javascript:( function(){ let i, elements = document.querySelectorAll('body *');
for (i = 0; i < elements.length; i++) {
if(getComputedStyle(elements[i]).position === 'fixed' || getComputedStyle(elements[i]).position === 'sticky') { elements[i].parentNode.removeChild(elements[i]); } } document.body.style.overflow = "auto"; document.body.style.position = "static"; })()