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chromium/third_party/blink/web_tests/scrollbars/hidden-scrollbars-invisible.html 

<!DOCTYPE html>
<script>
if (window.internals)
  internals.useMockOverlayScrollbars();
</script>
<div id="div" style="overflow: scroll; width: 200px; height: 200px;">
  <div style="background-color: green; height: 3000px; width: 100px"></div>
</div>
<iframe srcdoc="<div style='background-color: red; height: 3000px; width: 100px'></div>"
        id="iframe",
        width="200",
        height="200"></iframe>
<script>
  window.onload = function() {
    if (window.internals) {
      var div = document.querySelector('#div');
      var iframe = document.querySelector('#iframe');
      internals.setScrollbarVisibilityInScrollableArea(div, true);
      internals.setScrollbarVisibilityInScrollableArea(
          iframe.contentDocument, true);

      internals.setScrollbarVisibilityInScrollableArea(div, false);
      internals.setScrollbarVisibilityInScrollableArea(
          iframe.contentDocument, false);
    }
  }
</script>