chromium/content/test/data/frame_tree/busy_frame.html

<!DOCTYPE html>
<html>
<script>
function countToInfinity() {
  var i = 0;
  while(true) {
    i = i + 1;
  }
}
function lookBusy() {requestAnimationFrame(() => {setTimeout("countToInfinity()")})};
</script>
<body>
This page counts to infinity after submitting a compositor frame.
A cross-origin iframe is added to ensure that a hit test query is sent to this frame.
<iframe src="/cross-site/bar.com/title1.html"></iframe>
</body>
</html>