File: observe-layoutjank.html

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<!DOCTYPE HTML>
<meta charset=utf-8>
<title>Layout Jank: observe jank fraction via PerformanceObserver</title>
<body>
<style>
#myDiv { position: relative; width: 300px; height: 100px; }
</style>
<div id='myDiv'></div>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script>
  async_test(function (t) {
    const observer = new PerformanceObserver(
      t.step_func_done(function(entryList) {
        assert_equals(entryList.getEntries().length, 1);
        const entry = entryList.getEntries()[0];
        assert_equals(entry.entryType, "layoutJank");
        assert_equals(entry.name, "");
        assert_equals(entry.startTime, 0.0);
        assert_equals(entry.duration, 0.0);
        // The layout jank fraction should be: 300 * (100 + 60) / viewport size.
        assert_equals(entry.fraction, 300 * (100 + 60) /
          (document.documentElement.clientWidth * document.documentElement.clientHeight));
      })
    );
    observer.observe({entryTypes: ['layoutJank']});
    window.onload = () => {
      // Modify the position of the div.
      document.getElementById('myDiv').style = "top: 60px";
    };
  }, 'LayoutJank is observable via PerformanceObserver.');
</script>

</body>