<!DOCTYPE HTML>
<script>
var x;
function doTest() {
  f.contentDocument.body.onclick = function (event) {
    x = event.offsetX;
  }
  f.contentDocument.body.dispatchEvent(f.contentWindow.ev);
}
</script>
<iframe id="f" style="display:none" onload="doTest()"
        src="data:text/html,<script>var ev = new MouseEvent('click', {clientX:0, clientY:0})</script>"></iframe>
