Doesn't work. Here's what it shows in Console in devtools:

Spoiler (click me)

Uncaught TypeError: Cannot read property 'appendChild' of null
at userscript:GM-Don't fuck with my scroll!!!:323
at userscript:GM-Don't fuck with my scroll!!!:324
at userscript:GM-Don't fuck with my scroll!!!:328

... which leads to this line:

scriptParent is undefined

scriptParent.appendChild(script);

Browser - Qutebrowser v2.0.0 (Backend - QtWebEngine). LocalStorage is turned off. Do I need it turned on?

Changed document-start in @run-at header to document-end, seems to work. I guess, since the script was being run before the DOM was fully loaded, things like document.body returned null, and that caused scriptParent to be undefined.

So, I officially turn this issue into a request for a bugfix.