YouTube Playback Rate Resync Hotfix

youtube-speed-desync-fix.user.js

// ==UserScript==
// @name         YouTube Playback Rate Resync Hotfix
// @namespace    https://tampermonkey.net/
// @version      1.2
// @description  Re-sync YouTube audio/video after slowing playback speed on YouTube in Firefox
// @match        https://www.youtube.com/*
// @run-at       document-idle
// @grant        none
// ==/UserScript==

(function () {
  'use strict';

  function attachHotfix(video) {
    if (!video || video.dataset.ytSpeedHotfixAttached === 'true') return;
    video.dataset.ytSpeedHotfixAttached = 'true';

    let delayId = null;
    let lastPlaybackRate = video.playbackRate;

    video.addEventListener('ratechange', () => {
      const oldPlaybackRate = lastPlaybackRate;
      const newPlaybackRate = video.playbackRate;
      const slowedDown = newPlaybackRate < oldPlaybackRate;

      clearTimeout(delayId);

      if (slowedDown) {
        delayId = setTimeout(() => {
          if (!video.paused && !video.seeking) {
            video.currentTime = video.currentTime;
            console.log(
              `YouTube playback resynced after slowing from ${oldPlaybackRate}x to ${newPlaybackRate}x`
            );
          }
        }, 300);
      }

      lastPlaybackRate = newPlaybackRate;
    });
  }

  function findAndAttach() {
    const video = document.querySelector('video');
    if (video) attachHotfix(video);
  }

  findAndAttach();

  const observer = new MutationObserver(findAndAttach);
  observer.observe(document.documentElement, {
    childList: true,
    subtree: true,
  });
})();