layout-instability/idlharness.html

<!doctype html>
<title>Layout Instability IDL tests</title>
<meta name="timeout" content="long">
<link rel="help" href="https://wicg.github.io/layout-instability/">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="/resources/WebIDLParser.js"></script>
<script src="/resources/idlharness.js"></script>
<script src="resources/util.js"></script>
<script>
'use strict';

idl_test(
  ['layout-instability'],
  ['performance-timeline', 'geometry', 'dom', 'hr-time'],
  async (idl_array, t) => {
    idl_array.add_objects({
      LayoutShift: ['layoutShift'],
      LayoutShiftAttribution: ['layoutShiftAttribution'],
    });

    // If LayoutShift isn't supported, avoid the timeout below and just let the
    // objects declared above be null. The tests will still fail, but we will
    // consistently generate the same set of subtests on all platforms.
    if (!PerformanceObserver ||
        !PerformanceObserver.supportedEntryTypes ||
        !PerformanceObserver.supportedEntryTypes.includes('layout-shift')) {
      return;
    }

    // Make sure that the image has initially been laid out, so that the movement
    // later is counted as a layout shift.
    await waitForAnimationFrames(2);

    self.layoutShift = await new Promise((resolve, reject) => {
      const observer = new PerformanceObserver(entryList => {
        resolve(entryList.getEntries()[0]);
      });
      observer.observe({type: 'layout-shift', buffered: true});
      t.step_timeout(() => reject('Timed out waiting for LayoutShift entry'), 3000);

      // Move the image, to cause layout shift.
      image.style.marginTop = '100px';
    });
    self.layoutShiftAttribution = layoutShift.sources[0];
  }
);
</script>

<img id="image" src="/images/green-100x50.png">