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chromium/third_party/blink/web_tests/fast/forms/suggestion-picker/month-suggestion-picker-appearance.html 

<!DOCTYPE html>
<meta name=fuzzy content="maxDifference=0-3; totalPixels=0-1000">
<script src="../../../resources/testdriver.js"></script>
<script src="../../../resources/testdriver-vendor.js"></script>
<script src="../resources/picker-common.js"></script>

<body style="background-color: #eeffff;">
<input type=month value="10000-12" list="suggestions" style="width: 200px;">
<datalist id=suggestions>
  <option>2012-00</option> <!--invalid-->
  <option>2012-01-01</option> <!--invalid-->
  <option>foo</option> <!--invalid-->
  <option label="This Month">2012-01</option>
  <option>2012-02</option>
  <option>2012-03</option>
  <option>2012-04</option>
  <option>2012-05</option>
  <option>2012-06</option>
  <option>2012-07</option>
  <option>2012-08</option>
  <option>2012-09</option>
  <option>2012-10</option>
  <option>2012-11</option>
  <option>2012-12</option>
  <option>2013-01</option>
  <option>2013-02</option>
  <option>2013-03</option>
  <option>2013-04</option>
  <option>2013-05</option>
  <option>2013-06</option>
  <option>2013-07</option>
</datalist>

<script>
  window.onload = openPickerAppearanceOnly(document.querySelector('input'),() => {
    internals.pagePopupWindow.focus();
    eventSender.keyDown('ArrowDown');
  });
</script>