Turn off/disable YouTube autoplay feature

disable_youtube_autoplay.js

// To run, install GreaseMonkey or TamperMonkey extension in your browser
// Copy this code into new user script, and enable
// !! If the toggle button is not been toggled automaticly:
// !! That means youtube changed for reasons the id of the toggle button again.
// !! In Chrome/Firefox: right-click the toggle button and choose "Inspect Element (Q)"
// !! You will find something like "<div id="TOGGLE_BUTTON_ID" class="toggle-button...."
// !! Copy whatever TOGGLE_BUTTON_ID is and replace down in "function disableAfterLoad()" the place holder named "TOGGLE_BUTTON_ID".
// !! Save script ctrl+S and reload youtube page.


// ==UserScript==
// @name         Disable Youtube autoplay
// @version      1.3
// @description  This script turns off Youtube's newest autoplay feature after the page loads
// @author       Jeff Bellucci, Jakob Wandel
// @match        *://www.youtube.com/*
// @run-at       document-end
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    let debug = false;

    let bottonIds = [];
    let i = 0;
    let MAX_TRIES = 5;
    bottonIds[i++] = 'toggle';
    bottonIds[i++] = 'toggleButton';
    bottonIds[i++] = 'improved-toggle';
    bottonIds[i++] = 'TOGGLE_BUTTON_ID';

    function debugMsg(msg){
     if(debug){
      console.log(msg);
     }
    }

    //https://stackoverflow.com/questions/2705583/how-to-simulate-a-click-with-javascript
    function eventFire(el, etype){
        if (el.fireEvent) {
            el.fireEvent('on' + etype);
        } else {
            var evObj = document.createEvent('Events');
            evObj.initEvent(etype, true, false);
            el.dispatchEvent(evObj);
        }
    }

    function disableAfterLoad(try_nr) {
        for (let key in bottonIds) {
            let autoplayToggle = document.getElementById(bottonIds[key]);
            if (autoplayToggle) {
                debugMsg("I found a Element called '"+bottonIds[key]+"'. I will now try to uncheck it:");
                if (autoplayToggle.hasAttribute('checked')) {
                    eventFire(autoplayToggle, 'click');
                    debugMsg("I assume, that if '"+bottonIds[key]+"' was the right Element, its now untoggled!");
                    delete bottonIds[key];
                    return;
                }else{
                    debugMsg("The element '"+bottonIds[key]+"' has no attribute checked. So I don't click it and check the other elements but not this one anymore.");
                    delete bottonIds[key];
                }
            } else {
                debugMsg("I could not finde an Element called '"+bottonIds[key]+"'. Maybe the site is not loaded jet? I will try again in 1 second.");
            }
        }
        if(try_nr > 0){
            try_nr--;
            setTimeout(function() {
               disableAfterLoad(try_nr);
            }, 1000);
        }
    }

    disableAfterLoad(MAX_TRIES);

})();

Ah, Yes sorry this scripts stops only YouTube from auto playing the next video. But your problem can be solved within the Browser Settings:
Firefox: https://support.mozilla.org/en-US/kb/block-autoplay (thats per default activated)
Chrome: "Launch Chrome and in the Address Bar type: chrome://flags/#autoplay-policy and hit Enter. That will bring you directly to the flag that you need to change. From the drop-down box, change the setting from Default to “Document user activation is required” and relaunch the browser." (source: https://www.groovypost.com/howto/disable-autoplay-videos-on-sites-in-google-chrome/)

Thanks. But this only works when the tab is in the background. As soon as I put it in foreground it plays.
Well, the userscript works. All is good. :)