Fullscreen Video Speed Controller

Control playback speed, and seek for fullscreen videos with extended shortcuts
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// ==UserScript==
// @name         Fullscreen Video Speed Controller
// @version      2.3.0
// @description  Control playback speed, and seek for fullscreen videos with extended shortcuts
// @author       Wanten
// @copyright    2025 Wanten
// @license      MIT
// @supportURL   https://gist.github.com/WantenMN/c0afabc32a911d4dd10e06cff6bcb211
// @homepageURL  https://gist.github.com/WantenMN/c0afabc32a911d4dd10e06cff6bcb211
// @namespace    https://greasyfork.org/en/scripts/536243
// @run-at       document-end
// @match        https://*/*
// @grant        none
// ==/UserScript==

(function () {
  "use strict";

  // Helper function to get the fullscreen video element
  function getFullscreenVideo() {
    if (
      document.fullscreenElement &&
      document.fullscreenElement.tagName === "VIDEO"
    ) {
      return document.fullscreenElement;
    }

    const videos = document.getElementsByTagName("video");
    for (const video of videos) {
      if (
        video.offsetWidth === window.innerWidth &&
        video.offsetHeight === window.innerHeight
      ) {
        return video;
      }
    }
    return null;
  }

  // Function to set the video speed
  function setVideoSpeed(speed) {
    const video = getFullscreenVideo();
    if (video && video.playbackRate !== speed) {
      video.playbackRate = speed;

      if (video.playbackRate === 1) {
        // Fix audio/video synchronization issues
        video.currentTime = video.currentTime;
      }
    }
  }

  // Function to seek the video forward or backward
  function seekVideo(offset) {
    const video = getFullscreenVideo();
    if (video) {
      let newTime = video.currentTime + offset;

      // Clamp time within [0, duration]
      if (newTime < 0) newTime = 0;
      if (newTime > video.duration) newTime = video.duration;

      video.currentTime = newTime;
    }
  }

  // Event listeners for key presses
  document.addEventListener(
    "keydown",
    function (event) {
      if (!document.fullscreenElement) return;

      const keys = ["j", "k", "l", "h", ";", "o", "p"];
      if (!keys.includes(event.key)) return;

      event.stopImmediatePropagation();
      event.preventDefault();

      switch (event.key) {
        case "j":
          setVideoSpeed(1);
          break;
        case "k":
          setVideoSpeed(2);
          break;
        case "l":
          setVideoSpeed(3);
          break;
        case "h":
          seekVideo(-5); // Rewind 5 seconds
          break;
        case ";":
          seekVideo(5); // Fast-forward 5 seconds
          break;
        case "o":
          seekVideo(-90); // Jump backward 90 seconds, clamp at 0
          break;
        case "p":
          seekVideo(90); // Jump forward 90 seconds, clamp at video end
          break;
      }
    },
    true,
  );
})();