TamperMonkey UserScript to stop video playback in background

stop-video.js

// ==UserScript==
// @name         Pause video in background
// @namespace    http://tampermonkey.net/
// @version      1.0
// @description  Pause video when tab loses focus, resume when active
// @match        *://*/*
// @grant        none
// ==/UserScript==

(function () {
  'use strict';

  let lastPlayed = new WeakSet();

  function pauseVideos() {
    document.querySelectorAll('video').forEach(video => {
      if (!video.paused && !video.ended) {
        lastPlayed.add(video);
        video.pause();
      }
    });
  }

  function resumeVideos() {
    document.querySelectorAll('video').forEach(video => {
      if (lastPlayed.has(video)) {
        video.play().catch(() => {});
        lastPlayed.delete(video);
      }
    });
  }

  document.addEventListener('visibilitychange', () => {
    if (document.hidden) pauseVideos();
    else resumeVideos();
  });

  window.addEventListener('blur', pauseVideos);
})();