title: offseX values are different in different browsers
body: <p>I have a slider. On clicking below slider axis, <code>onClickHorizontal</code> function is called but the value of <code>d3.event.offsetX</code> is different in Chrome(50.0.2661.102) and Firefox(42.0) due to which slider position is different even though I click at the same position of slider in both the browsers</p>
  
  <p><a href="https://github.com/MasterMaps/d3-slider/blob/master/d3.slider.js#L216" rel="nofollow">https://github.com/MasterMaps/d3-slider/blob/master/d3.slider.js#L216</a></p>
  
  <pre><code>function onClickHorizontal() {
      if (toType(value) != "array") {
          var pos = Math.max(0, Math.min(sliderLength, d3.event.offsetX || d3.event.layerX));
          moveHandle(scale.invert ?
                      stepValue(scale.invert(pos / sliderLength)) :
                      nearestTick(pos / sliderLength));
      }
  }
  </code></pre>