Make YouTube use your browser's default scrollbar

restore-youtube-scrollbar.user.js

// ==UserScript==
// @name        Restore YouTube Scrollbar
// @namespace   Violentmonkey Scripts
// @match       https://www.youtube.com/*
// @grant       none
// @version     1.0
// @author      https://github.com/lassekongo83
// @description Make YouTube use your browser's default scrollbar
// ==/UserScript==

function restoreScrollbar() {
  'use strict';
  document.body.removeAttribute('standardized-themed-scrollbar');
  // For Firefox
  document.querySelector('html').removeAttribute('standardized-themed-scrollbar');
  Element.prototype.removeAttributes = function(...attrs) {
    attrs.forEach(attr => this.removeAttribute(attr));
  }
  document.querySelector('ytd-app').removeAttributes('scrollbar-rework', 'standardized-themed-scrollbar');
}
restoreScrollbar();
document.querySelector('ytd-app').addEventListener('yt-visibility-refresh', restoreScrollbar);