Set the default viewing mode to Theater Mode.
// ==UserScript==
// @name YouTube - Always Theater Mode
// @namespace r-a-y/youtube/theater
// @description Set the default viewing mode to Theater Mode.
// @include https://www.youtube.com/*
// @version 1.4.4
// @grant none
// @run-at document-start
// @license GPL v3
// ==/UserScript==
// Toggle Theater Mode after YouTube finishes loading a video.
window.addEventListener("yt-navigate-finish", function(event) {
var newPlayer = document.querySelector('button.ytp-size-button')
setTimeout(()=> {
if ( newPlayer && null === document.getElementsByTagName('ytd-watch-flexy')[0].getAttribute('full-bleed-player') ) {
newPlayer.click()
}
}, 600)
})