title: React's mouseEvent doesn't have offsetX/offsetY
body: <p>I try get position of click relative to element, but event doesn't have offsetX.</p>
  
  <pre><code>onClick(e) {
    console.log(e.offsetX) // returns undefined
    console.log(e.target.offsetX) // returns undefined
  }
  
  render() {
    return &lt;img src='http://placehold.it/1000x500' onClick={this.onClick} /&gt;
  }
  </code></pre>
  
  <p>How I can get position of click in element?</p>