Make Gist editor automatically resize based on content using a TamperMonkey script

gist-editor-resize-userscript.user.js

// ==UserScript==
// @name       gist-editor-resize-userscript
// @namespace  https://gist.github.com/
// @version    0.2 
// @description  Automatically resize the gist editor for easier editing.
// @match      http*://gist.github.com/*
// @copyright  2013+ Joel Day - blog.dayjo.org (updated by @zudsniper - gh.zod.tf)
// ==/UserScript==

var textareas = document.querySelectorAll('textarea.file_contents');
var span = document.createElement('div');
span.style.visibility = "hidden";
document.body.appendChild(span);

$('textarea.file-contents').keyup(function(){
	span.style.width = this.offsetWidth + 'px';
	span.style.fontSize = this.style.fontSize;
	span.innerText = this.value;
	
	this.style.height = span.offsetHeight + 'px';
});