chromium/chrome/browser/page_load_metrics/integration_tests/data/simple_div_movement.html

<!DOCTYPE html>
<title>Layout Instability: verify simple-block-movement for CLS</title>
<script src="resources/util.js"></script>
<style>
#shifter { position: relative; width: 300px; height: 200px; background: blue; }
</style>
<div id="shifter">shifter</div>
<script>

if (PerformanceObserver.supportedEntryTypes.indexOf("layout-shift") == -1)
    throw new Error("Layout Instability API not supported");

let runTest = async () => {
    await waitUntilAfterNextLayout();
    document.querySelector("#shifter").style = "top: 160px";
};
runTest();

async function waitForTestFinished() {
  return await waitForNumLayoutShiftEntries(1);
}

</script>