chromium/third_party/blink/web_tests/fast/loader/iframe-window-open-stealing-focus.html

<input id="input" type="text" autofocus="true" onkeydown="window.open('about:blank', 'i')" onfocus="setTimeout(test, 0)">
<iframe name="i"></iframe>
<script>
if (window.testRunner) {
  testRunner.dumpAsText();
  testRunner.waitUntilDone();
}

function test() {
  if (!window.eventSender) {
    document.body.appendChild(document.createTextNode(
        "To run manually, type into the input field and see if text appears"));
    return;
  }
  eventSender.keyDown("a");
  document.body.appendChild(document.createTextNode(
  input.value == "a" ? "PASS" : "FAIL"));
  if (window.testRunner)
    testRunner.notifyDone();
}
</script>