react-google-maps OverlayView zIndex

Trying to get zIndex working on an OverlayView. [This fiddle doesn't work -- missing react-google-maps]

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.24/browser.js"></script>
<script src="https://fb.me/react-with-addons-0.14.6.js"></script>
<script src="https://fb.me/react-dom-0.14.6.js"></script>
<script src="https://facebook.github.io/react/js/jsfiddle-integration-babel.js"></script>
<script src="https://maps.googleapis.com/maps/api/js"></script>
<!-- this doesn't actually work: -->
<script type="text/babel" src="https://rawgit.com/tomchentw/react-google-maps/master/lib/index.js"></script>

<div id="container">
  <!-- This element's contents will be replaced with your component. -->
  The fiddle can't run, because it can't import react-google-maps. (But check the JS source).
</div>

CSS

.overlay {
  background: white;
  border: 1px solid #ccc;
  padding: 5px;
  max-width: 40em;
}

JavaScript 1.7

import {
  GoogleMap,
  OverlayView,
  Marker
} from "react-google-maps";

class OverlayViewExample extends React.Component {
  render() {
    const pos = {
      lat: -34.397,
      lng: 150.644
    };
    const mapPane = OverlayView.OVERLAY_MOUSE_TARGET;
    const getOffset = this.getPixelPositionOffset.bind(this);

    return ( <
      GoogleMap defaultZoom = {
        8
      }
      defaultCenter = {
        pos
      } >
      <
      OverlayView position = {
        pos
      }
      mapPaneName = {
        mapPane
      }
      getPixelPositionOffset = {
        getOffset
      } >
      <
      div style = {
        {
          zIndex: 2
        }
      }
      className = "overlay"
      ref = "childDiv" > This should be in front < /div> < /
      OverlayView > <
      OverlayView position = {
        pos
      }
      mapPaneName = {
        mapPane
      }
      getPixelPositionOffset = {
        getOffset
      } >
      <
      div style = {
        {
          zIndex: 1
        }
      }
      className = "overlay" > Another overlay should be in front of me < /div> < /
      OverlayView > <
      /GoogleMap>
    );
  }

  getPixelPositionOffset(width, height) {
    // uncomment this for a hack workaround:
    // if (this.refs.childDiv) {
    //  this.refs.childDiv.parentNode.style.zIndex = 2;
    // }
    return {
      x: -(width / 2),
      y: -(height / 2)
    };
  }
}


ReactDOM.render( <
  OverlayViewExample / > ,
  document.getElementById('container')
);