Adds "Message" control on Scratch forums; allows arbitrary length messages

scratch-messenger.user.js

// ==UserScript==
// @name        Scratch Messenger
// @namespace   ScratchMessenger
// @author      Zro617, MegaApuTurkUltra
// @description Lets you quickly write a message directly to the author of a forum post on Scratch
// @include     https://scratch.mit.edu/discuss/topic/*
// @version     1.0
// @grant       none
// ==/UserScript==

var isloggedin = document.getElementsByTagName("textarea"); 
if(!isloggedin.length) return;

function chunkString(str, length) {
  return str.match(new RegExp('.{1,' + length + '}', 'g'));
}

var posts = document.getElementsByClassName("blockpost roweven firstpost"),
    users = document.getElementsByClassName("black username");
if (posts.length) {
  for (var p=0,msgauthor,btn,id,ul;p<posts.length;p++){
    msgauthor = document.createElement("li");
    msgauthor.setAttribute("class","message-author");
    btn = document.createElement("a");
    btn.setAttribute("id",users[p].innerHTML);
    //btn.setAttribute("id",posts[p].id.match(/\d+/)[0]);
    btn.innerHTML = "Message " + users[p].innerHTML;
    btn.onclick = function() {
      var status = document.createElement("span");
      btn.appendChild(status);
      var csrf=document.cookie.match(/scratchcsrftoken=([0-9a-zA-Z]+)/);
      if(!csrf)return;
      csrf=csrf[1];
      var user = this.id;
      var comment = prompt("Write your comment to "+this.id+".");
      if(comment && comment.length){
       comment = chunkString(comment, 500);
       var xhr = new XMLHttpRequest();
       console.log("Posting part 0");
       status.textContent = " (posting...)";
       xhr.responseType = 'document';
       xhr.open("POST","https://scratch.mit.edu/site-api/comments/user/" + user + "/add/",true);
       xhr.setRequestHeader("X-CSRFToken", csrf);
       xhr.setRequestHeader("X-Requested-With","XMLHttpRequest");
       xhr.onload=function(){
           if(this.status != 200){
               alert("Sorry, your comment couldn't be posted");
               status.parentNode.removeChild(status);
               return;
           }
           var id = this.responseXML.getElementsByClassName("comment")[0].getAttribute("data-comment-id");
           console.log("Got id: " + id);
           var i = 1;
           function next(){
               if(i >= comment.length){
                   alert("Message posted!");
                   status.parentNode.removeChild(status);
                   return;
               }
               console.log("Posting part " + i);
               status.textContent = " (posting " + i + "/" + comment.length + ")";
               var req = new XMLHttpRequest();
               req.open("POST","https://scratch.mit.edu/site-api/comments/user/" + user + "/add/",true);
               req.setRequestHeader("X-CSRFToken", csrf);
               req.setRequestHeader("X-Requested-With","XMLHttpRequest");
               req.onload = function(){
                   if(this.status != 200){
                       alert("Sorry, your comment couldn't be posted");
                       status.parentNode.removeChild(status);
                       return;
                   }
                   if(this.responseText.indexOf("isFlood") > -1){
                       // uh oh, spam filter
                       // retry soon
                       status.textContent = " (" + i + "/" + comment.length + " | waiting for spam filter)";
                       setTimeout(next, 10000);
                       return;
                   }
                   i++;
                   setTimeout(next, 2000);
               };
               req.send(JSON.stringify({content:comment[i], parent_id:id, commentee_id:""}));
           }
           next();
       };
       xhr.send(JSON.stringify({content:comment[0], parent_id:"", commentee_id:""}));
      }
    };
    msgauthor.appendChild(btn);
    msgauthor.appendChild(document.createTextNode(" | "));
    ul = posts[p].getElementsByClassName("postfootright")[0].getElementsByTagName("ul")[0];
    ul.insertBefore(msgauthor,ul.firstChild.nextSibling);
  }
}

Hmm, an interesting fork, but I don't like the idea of spamming short messages. I think I'll adapt the multi-message part but lift the 200 character limit to the comment maximum (with some modifications).

@Zro617 Lol I forgot 200 characters wasn't the limit. I should change it to 500