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Youtube - Theater Mode Windowed Fullscreen

Youtube - Theater Mode Windowed Fullscreen. YT 影院模式 窗口化全屏

// ==UserScript==
// @name         Youtube - Theater Mode Windowed Fullscreen
// @namespace    http://tampermonkey.net/
// @version      1.0
// @description  Youtube - Theater Mode Windowed Fullscreen. YT 影院模式 窗口化全屏
// @author       Martin______X
// @match        https://www.youtube.com/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=youtube.com
// @grant        none
// @license      MIT
// ==/UserScript==

const theaterInterval = setInterval(() => {
    try {
        /**/
        let ytd_watch_flexy = document.getElementsByTagName("ytd-watch-flexy")[0];
        /**/
        let full_bleed_container = document.getElementById("full-bleed-container");
        /**/
        let masthead_container = document.getElementById("masthead-container");
        /**/
        let default_layout = ytd_watch_flexy.hasAttribute("default-layout");
        /**/
        let masthead_container_height = window.getComputedStyle(masthead_container).getPropertyValue("height");
        /**/
        let height = window.innerHeight - parseFloat(masthead_container_height);
        /**/
        full_bleed_container.style.setProperty("min-height", height + "px");
        /**/
        if(default_layout){
            /**/
            full_bleed_container.style.setProperty("position", "absolute");
        }else{
            /**/
            full_bleed_container.style.setProperty("position", "relative");
        }
    } catch (error) {
        console.error(error);
    }
}, 1);