Right now we use codemirror's JavaScript mode on the website but there's a JSX mode now so we should start using that instead.

var display = cm.display, pos = cursorCoords(cm, cm.doc.sel.head, "div");
    display.cursor.style.left = pos.left + "px";
    display.cursor.style.top = pos.top + "px";
    display.cursor.style.height = Math.max(0, pos.bottom - pos.top) * cm.options.cursorHeight + "px";
    display.cursor.style.display = "";
    if (pos.other) {
      display.otherCursor.style.display = "";
      display.otherCursor.style.left = pos.other.left + "px";
      display.otherCursor.style.top = pos.other.top + "px";
      display.otherCursor.style.height = (pos.other.bottom - pos.other.top) * .85 + "px";
    } else { display.otherCursor.style.display = "none"; }
  }
  
  function updateSelectionRange(cm) {
    var display = cm.display, doc = cm.doc, sel = cm.doc.sel;
    var fragment = document.createDocumentFragment();
    var clientWidth = display.lineSpace.offsetWidth, pl = paddingLeft(cm.display);
    function add(left, top, width, bottom) {
      if (top < 0) top = 0;
      fragment.appendChild(elt("div", null, "CodeMirror-selected", "position: absolute; left: " + left +
                               "px; top: " + top + "px; width: " + (width == null ? clientWidth - left : width) +
                               "px; height: " + (bottom - top) + "px"));
    }
    function drawForLine(line, fromArg, toArg) {