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.

if (widget.height != null) return widget.height;
    if (!widget.node.parentNode || widget.node.parentNode.nodeType != 1)
      removeChildrenAndAdd(widget.cm.display.measure, elt("div", [widget.node], null, "position: relative"));
    return widget.height = widget.node.offsetHeight;
  }
  function addLineWidget(cm, handle, node, options) {