third_party/web_platform_tests/intersection-observer/resources/nested-cross-origin-child-iframe.sub.html

<!DOCTYPE html>
<script src="/common/get-host-info.sub.js"></script>
<script src="/css/cssom-view/support/scroll-behavior.js"></script>
<style>
.spacer {
  height: calc(100vh + 100px);
}
</style>
<div class="spacer"></div>
<iframe id="iframe"></iframe>
<script>
iframe.src =      // secure port
  get_host_info().HTTPS_NOTSAMESITE_ORIGIN + "/intersection-observer/resources/nested-cross-origin-grand-child-iframe.html";

window.addEventListener("message", async event => {
  if (event.data == "scroll") {
    iframe.scrollIntoView({ behavior: "instant" });
    await waitForScrollEnd(document.scrollingElement);
    window.parent.postMessage("scrollEnd", "*");
  }
});
</script>