twitter-native-video-player.user.js

// ==UserScript==
// @name        twitter native video player
// @match       https://twitter.com/*
// @grant       none
// @version     1.0
// @author      yukulele
// @description replace not user-friendly twitter player with native browser player. This allow to: seek with left/right keyboard keys change volume with up/down keyboard keys, fullscreen with double-click, change playback rate (via context menu), share playback rate between videos.
// @inject-into auto
// ==/UserScript==

function nativePlayer(video) {
  video.playbackRate = window.localStorage.playbackRate || 1
  video.addEventListener('ratechange', () => window.localStorage.playbackRate = video.playbackRate)
  video.addEventListener('mouseenter', () => video.controls = true)
  video.addEventListener('mouseleave', () => video.controls = false)
  video.style.zIndex = 1
}

const mutationObserver = new MutationObserver(mutationsList => {
  for (var mutation of mutationsList) {
    if (mutation.type !== 'childList') return
    for (const node of mutation.addedNodes) {
      if (!(node instanceof HTMLElement)) continue
      for (const video of node.querySelectorAll('video')) nativePlayer(video)
    }
  }
})

mutationObserver.observe(document.documentElement, { 
  childList: true, 
  subtree: true,
 })