iframe-background-attachment-fixed.html

<!DOCTYPE html>
<iframe id="iframe" src="resources/background-attachment-fixed.html"></iframe>

<div style="height: 2000px;">
</div>

<script>
function scrollingReasons(reasons) {
  return reasons ? "scrolls on main thread, reasons: " + reasons : "scrolls on impl";
}

if (window.internals) {
  internals.settings.setPreferCompositingToLCDTextEnabled(true);
}

if (window.testRunner) {
  testRunner.dumpAsText(true);
  onload = () => {
    var reasons = internals.mainThreadScrollingReasons(document);
    text = "Main frame " + scrollingReasons(reasons);
    text += "\nThe iFrame " + scrollingReasons(iframe.contentDocument.getElementById("mainThreadScrollingReasons").innerText);
    testRunner.setCustomTextOutput(text);
  };
}
</script>

chromium/third_party/blink/web_tests/compositing/layer-creation/iframe-background-attachment-fixed.html