layout-instability/main-frame.html

<!DOCTYPE html>
<html>
<body>
<title>Layout Instability: subframe layout shift score</title>
<link rel="help" href="https://wicg.github.io/layout-instability/" />
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  #i {
    border: 0;
    position: absolute;
    left: 0;
    top: 0;
    background-color: pink;
  }
</style>
<iframe id="i" width="400" height="300" src="sub-frame.html"></iframe>

<script>
const loadPromise = new Promise(resolve => {
  window.addEventListener("load", () => {
    resolve(true);
  });
});

let iframe = document.getElementById('i');
const load_promise = new Promise(resolve => {
  iframe.addEventListener('load', function() {
    resolve(true);
  });
});

checkMainFrameLoad = async () => {
  await loadPromise;
  return true;
};

checkIFrameLoad = async () => {
  // Wait for the iframe finishing loading
  await load_promise;
  return true;
};

promise_test(async t => {
  checkMainFrameLoad();
  // Wait for the iframe finishing loading
  checkIFrameLoad();

  // Wait for the message sent from the iframe after it receives all the layout
  // shifts.
  await new Promise(resolve => {
    window.addEventListener("message", (event) => {
      if (event.data.type == "layout shift score") {
        t.step(() => {
          assert_equals(event.data.score, event.data.expectedScore);
        });
        resolve();
      }
    }, false);
  });
}, "");
</script>
</body>
</html>