Source code

Revision control

Copy as Markdown

Other Tools

Test Info: Warnings

<meta charset=utf-8>
<title>Layout Instability: PerformanceObserver sees entries with buffered flag</title>
#myDiv { position: relative; width: 300px; height: 100px; background: blue; }
<div id='myDiv'></div>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="resources/util.js"></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 =;
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 =;
// 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));
})).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.');