SO question

Demobug.user.js

// ==UserScript==
// @name         Demobug
// @version      0.1
// @description  Demo bug
// @author       https://stackoverflow.com/users/2675672/gaurang-tandon
// @match       *://*.stackoverflow.com/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    function createModal(buttonBar){
        var div = document.createElement("div"),
            input = document.createElement("input"),
            btn = document.createElement("button");

        div.id = "box";

        input.type = "text";

        btn.innerHTML = "Button";
        btn.onclick = function(e){
            e.preventDefault();
            console.log("I was called");
            input.focus();
            console.dir(e);
            console.trace();
        };

        div.appendChild(input);
        div.appendChild(btn);

        return div;
    }

    setInterval(function () {
        var cont = document.querySelector(".wmd-container:not(.processed)"), ul, buttonBar, div;

        if (cont && (ul = cont.querySelector(".wmd-button-bar ul"))) {
            cont.classList.add("processed");

            buttonBar = cont.querySelector("div[id^=wmd-button-bar]");

            div = createModal(buttonBar);
            buttonBar.appendChild(div);
        }
    }, 500);

})();