layout-instability/buffered-flag.html

<!DOCTYPE HTML>
<meta charset=utf-8>
<title>Layout Instability: PerformanceObserver sees entries with buffered flag</title>
<body>
<style>
#myDiv { position: relative; width: 300px; height: 100px; background: blue; }
</style>
<div id='myDiv'></div>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="resources/util.js"></script>
<script>
promise_test(async t => {
  assert_implements(window.LayoutShift, 'Layout Instability is not supported.');
  // Wait for the initial render to complete.
  await waitForAnimationFrames(2);

  const startTime = performance.now();
  return new Promise(resolve => {
    // First observer creates second in callback to ensure the entry has been dispatched by the time
    // the second observer begins observing.
    new PerformanceObserver(() => {
      const endTime = performance.now();
      // Second observer requires 'buffered: true' to see entries.
      new PerformanceObserver(t.step_func(list => {
        assert_equals(list.getEntries().length, 1);
        const entry = list.getEntries()[0];
        assert_equals(entry.entryType, "layout-shift");
        assert_greater_than_equal(entry.startTime, startTime);
        assert_less_than_equal(entry.startTime, endTime);
        assert_equals(entry.duration, 0.0);
        assert_equals(entry.value, computeExpectedScore(300 * (100 + 60),  60));
        resolve();
      })).observe({'type': 'layout-shift', buffered: true});
    }).observe({type: 'layout-shift'});
    // Modify the position of the div to cause a layout-shift entry.
    document.getElementById('myDiv').style = "top: 60px";
  });
}, 'PerformanceObserver with buffered flag sees previous layout-shift entry.');
</script>
</body>