add placeholder textarea which sync with ACE Editor to GitHub.

github:place-holder-textarea.user.js

// ==UserScript==
// @name        github:place-holder-textarea
// @namespace   http://efcl.info/
// @description place holder textare
// @include     https://github.com/*/*/edit/*/*
// @include     https://github.com/*/*/new/*/*
// @version     1
// @grant       none
// @run-at  document-end
// ==/UserScript==

var div = document.querySelector("div.ace_editor");
var editor = ace.edit(div);
function onChange(textArea, callback){
  textArea.addEventListener('input', function(event) {
    callback(event.target.value);
  }, false);
  textArea.addEventListener('change', function(event) {
    callback(event.target.value);
  }, false);
  textArea.addEventListener('blur', function(event) {
    callback(event.target.value);
  }, false);
}
function createPlaceHolder(aceEditor){
    var textArea = document.createElement("textarea");
    textArea.setAttribute("style", `
    position: absolute;
    overflow: auto;
    top: 80px;
    right: 0;
    `);
    textArea.value = aceEditor.getValue();
    onChange(textArea, function(value){
        aceEditor.setValue(value);
    });

    return textArea;
}
var placeHolderTextArea = createPlaceHolder(editor);
editor.on('change', function(event){
  placeHolderTextArea.value = editor.getValue();
});
document.body.appendChild(placeHolderTextArea);