buffer-before-onload.html

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This test has a WPT meta file that expects 1 subtest issues.
This WPT test may be referenced by the following Test IDs:
- /element-timing/buffer-before-onload.html - WPT Dashboard Interop Dashboard

<!DOCTYPE html>

<html>

<meta charset=utf-8 />

<title>Element Timing: buffer elements before onload</title>

<script src=/resources/testharness.js></script>

<script src=/resources/testharnessreport.js></script>

<script src="resources/element-timing-helpers.js"></script>

<body>

<img src=resources/slow-image.py?name=square20.png&sleep=500>

<script>

/*

  In this test, a slow image is added to the frame to delay onload. The entry

  is available from the observer with the buffered flag set to true.

*/

  async_test(function(t) {

    assert_implements(window.PerformanceElementTiming, "PerformanceElementTiming is not implemented");

    beforeRender = performance.now();

    const img = document.createElement('img');

    img.src = 'resources/square20.jpg';

    img.setAttribute('elementtiming', 'my_image');

    img.setAttribute('id', 'my_id');

    document.body.appendChild(img);

    // this PerformanceObserver should be notified about the previously

    // buffered element entry

    new PerformanceObserver(t.step_func((entryList, observer) => {

      assert_equals(entryList.getEntries().length, 1);

      entryList.getEntries().forEach(entry => {

        assert_equals(entry.entryType, "element");

        const pathname = window.location.origin + '/element-timing/resources/square20.jpg';

        checkElement(entry, pathname, 'my_image', 'my_id', beforeRender, img);

        checkNaturalSize(entry, 20, 20);

        observer.disconnect();

        t.done();

});

    })).observe({

      type: "element",

      buffered: true

});

  }, "Element Timing: image loads before onload available from buffered flag.");

</script>

</body>

</html>