drag-image-descendant-iframe-composited.html

<!DOCTYPE html>
<style>
div {
  width: 100px;
  height: 100px;
}
iframe {
  width: 100px;
  height: 100px;
  pointer-events: none;
  border: 0;
  background-color: red;
}
</style>
<div draggable="true">
  <iframe srcdoc="
    <!DOCTYPE html>
    <body style='margin: 0; overflow: hidden; background-color: green; transform: translateZ(0)'></body>">
  </iframe>
</div>
<script>
onload = function() {
  testRunner.dumpDragImage();

  let bounds = document.querySelector('div').getBoundingClientRect();
  let startX = bounds.left + bounds.width / 2;
  let startY = bounds.top + bounds.height / 2;

  eventSender.dragMode = true;
  eventSender.mouseMoveTo(startX, startY);
  eventSender.mouseDown();
  eventSender.mouseMoveTo(startX + bounds.width, startY + bounds.height);
  eventSender.mouseUp();
};
</script>

chromium/third_party/blink/web_tests/images/drag-image-descendant-iframe-composited.html