For each Stack Overflow question, adds an additional box to the "share" button, containing a link with markdown

share_markdown.user.js

// ==UserScript==
// @name        Share Markdown (Stack Overflow)
// @namespace   about:blank
// @include     /^https?:\/\/\w*.?(stackexchange.com|stackoverflow.com|serverfault.com|superuser.com|askubuntu.com|stackapps.com|mathoverflow.net)\/questions/.*/
// @version     8
// @grant       none
// ==/UserScript==

function make_input(text){
    var input = document.createElement("input");
    input.type = "text";
    input.value = text;
    input.style.display = "block";
    input.style.width = "292px";
    input.className = "gm-share-box";
    return input;
}

//add functionality to the "share permalink" box.
function enhance_box(box){
    //don't add the new element if it already exists
    if(box.querySelector(".gm-share-box")){
        return;
    }
  	var input = box.getElementsByTagName("input")[0];
    var url = input.value;
    var title = document.getElementsByClassName("question-hyperlink")[0].childNodes[0].nodeValue;
    title = title.replace("[", "\\[").replace("]", "\\]");
    var text = "[" + title + "](" + url + ")";
    
    //stick it after the first box
  	input.parentElement.appendChild(make_input(text));
}

function whenExists(func, callback, delay){
    //calls `func` every `delay` milliseconds, until it returns a non-null value.
    //when it does, calls `callback(result)`.
    var result = func()
    if (result != null){
        callback(result);
    }
    else{
        window.setTimeout(whenExists, delay, func, callback, delay);
    }
}

function whenHasSibling(element, selector, callback, delay){
    //like whenExists, but for checking whether an element has a sibling matching a selector
    function getChild(){
        return element.parentElement.querySelector(selector);
    }
    whenExists(getChild, callback, delay);
}

try{
	  for (var menu of document.querySelectorAll(".js-share-link")){
			whenHasSibling(menu, ".s-popover", enhance_box);
      console.log(menu);
    }
}
catch(err){
    console.log(err.message);
}