code: function(el) {
    var mirrorEl = document.createElement('div');
    mirrorEl.className = 'preview-code';
    mirrorEl.style.position = 'absolute';
    mirrorEl.style.left = '-9999px';
    bodyEl.appendChild(mirrorEl);
    var maxHeight = parseInt(
      window.getComputedStyle(el).getPropertyValue('max-height'),
      10);
    var codeDidChange = function(ev) {
      mirrorEl.textContent = this.value + '\n';
      var height = mirrorEl.offsetHeight + 2; // Account for borders.
      if (height >= maxHeight) {
        this.style.overflow = 'auto';
      } else {
        this.style.overflow = 'hidden';
      }
      this.style.height = (mirrorEl.offsetHeight + 2) + 'px';
    };
    el.addEventListener('keypress', codeDidChange);
    el.addEventListener('keyup', codeDidChange);
    codeDidChange.call(el);
  }
docs: Add an element with the same styles and content as the textarea to calculate the height of the textarea content.