Makes the YouTube shortcuts work even if the player is not focused. Install Greasemonkey/Tampermonkey first, then click [Raw] to install.

youtube-global-shortcuts.user.js

// ==UserScript==
// @name         YouTube global shortcuts
// @namespace    https://gist.github.com/stefansundin/
// @homepage     https://gist.github.com/stefansundin/65e3d6db697636d8e7f1
// @downloadURL  https://gist.github.com/stefansundin/65e3d6db697636d8e7f1/raw/youtube-global-shortcuts.user.js
// @version      1.1
// @author       Stefan Sundin
// @description  Makes the YouTube shortcuts work even if the player is not focused.
// @icon         https://www.youtube.com/favicon.ico
// @match        https://www.youtube.com/*
// ==/UserScript==

// Space = Pause/play
// F = Toggle fullscreen
// M = Toggle mute
// P = Previous video (if in playlist)
// N = Next video (if in playlist)
// left and right keys focuses the button, but the first keydown won't trigger seeking unfortunately
// P and N already works without this userscript, but you must hold shift.
// Use Ctrl+Space to scroll down, Shift+Space to scroll up

window.addEventListener('keydown', function(e) {
  var button = document.getElementsByClassName('ytp-play-button')[0] || document.getElementById('movie_player');
  if (!button) return;
  console.log(e);

  // Skip if in comment reply form, or search field
  if (['TEXTAREA','INPUT'].indexOf(e.srcElement.tagName) !== -1 || ['yt-commentbox-text','yt-simplebox-text'].indexOf(e.srcElement.className) !== -1) return;

  var c = String.fromCharCode(e.keyCode).toLowerCase();
  if (c == ' ' || e.keyCode == 37 || e.keyCode == 39) { // 37 = left key, 39 = right key
    if (c == ' ' && (e.shiftKey || e.ctrlKey)) return;
    if (e.srcElement != button) {
      button.focus();
      if (c == ' ') button.click();
    }
  }
  else if (c == 'n') {
    document.getElementsByClassName('ytp-next-button')[0].click();
  }
  else if (c == 'p') {
    document.getElementsByClassName('ytp-prev-button')[0].click();
  }
});

Disclaimer: I have not used this for years and I doubt it still works. I think YouTube does have some global shortcuts these days, like the space key.

USE AT YOUR OWN RISK!

Yeah, thank you for the warning. What I wanted was a little simpler so I didn't use your code but learned from it. 🙏🙏