common_comments_box.user.js

// ==UserScript==
// @name Common Comments Box (Stack Overflow)
// @namespace about:blank
// @include http*://stackoverflow.com/questions/*
// @version 1
// @grant none
// ==/UserScript==

var question_comments = [
  "Are you getting an error? If so, what is the error and stack trace? Is your code running, but producing unexpected output? If so, what output are you getting, and what output did you expect to get?",
  "Something went wrong with your code formatting. Consult [Markdown help - Code and Preformatted Text](http://stackoverflow.com/editing-help#code) and [edit] your post."
];

var answer_comments = [
  "Glad to have been of help! Feel free to [accept my answer](http://meta.stackexchange.com/questions/5234/how-does-accepting-an-answe‌​r-work) if you feel it was useful to you. :-)"
];

function getElementsByExactClassName(element, name){
  var candidates = element.getElementsByClassName(name);
  var ret = [];
  for(var i = 0; i < candidates.length; i+=1){
    if (candidates[i].className.trim() == name){
      ret.push(candidates[i]);
    }
  }
  return ret;
}

function get_comment_links(){
  return getElementsByExactClassName(document, "js-add-link comments-link");
}

function create_common_popup(form, comments){
    var box = document.createElement("div");
    box.className = "popup";
    box.style.width = "800px";
    box.style.display = "block";
    box.style.left = "calc(50% - 416px)";
    box.style.position = "fixed";
    box.style.top = "159.5px";

    var checkboxes = [];

    for(var i = 0; i < comments.length; i+=1){
        var input = document.createElement("input");
        input.type = "checkbox";
        checkboxes.push(input);
        box.appendChild(input);
        
        var text = document.createElement("span");
        text.innerHTML = comments[i];
        box.appendChild(text);
        
        box.appendChild(document.createElement("br"));
    }

    button = document.createElement("input");
    button.type = "button";
    button.value = "Add";

    button.onclick = function(){
      var text_to_add = "";
      for(var i = 0; i < checkboxes.length; i+=1){
          if (checkboxes[i].checked){
              if (text_to_add.length > 0){
                  text_to_add += " ";
              }
              text_to_add += comments[i];
          }
      }
      var textarea = form.getElementsByTagName("TEXTAREA")[0];
      textarea.value = text_to_add;
      box.parentNode.removeChild(box);
      textarea.focus();
    }

    box.appendChild(button);
    document.body.appendChild(box);
}

function create_common_link(form, comments){
    var button_title = "common";

    //don't create the link if it already exists. This can happen if the user clicked "add comment" more than once.
    var existing_buttons = form.getElementsByTagName("A");
    for (var i = 0; i < existing_buttons.length; i+=1){
        if (existing_buttons[i].innerHTML == button_title){
            return;
        }
    }
    
    var add_comment_button = form.getElementsByTagName("INPUT")[0];

    var link = document.createElement("a");
    link.innerHTML = button_title;
    link.className = "comment-help-link";
    add_comment_button.parentNode.insertBefore(link, add_comment_button.nextSibling);

    link.onclick = function(){
        create_common_popup(form, comments);
    }
}

function register_click_behavior(link, comments){
  link.onclick = function(){
    var containing_td = link.parentNode.parentNode;   
    var form = containing_td.getElementsByClassName("comment-form")[0];
    //wait for the page's native onclick trigger to finish, or else the form elements won't be present
    setTimeout(
        function(){
          create_common_link(form, comments);
        },
        100
    );
  }
}

var links = get_comment_links();
for (var i = 0; i < links.length; i +=1){
  register_click_behavior(links[i], i == 0 ? question_comments : answer_comments);
}