<!DOCTYPE HTML>
<head>
<title>Largest Contentful Paint: initially out-of-viewport image gets an LCP entry once they are visible.</title>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
  .flex-container {
     display: flex;
     flex-direction: row;
     width: 1000px;
     overflow-x: hidden;
     scroll-behavior: auto;
  }
</style>
</head>
<body>
<div>
<div class='flex-container' id="container">
  <img src='/images/lcp-100x50.png?pipe=trickle(d1)' width="1000" height="1000"/>
  <img src='/images/lcp-1x1.png?1' width="1000" height="1000"/>
  <img src='/images/lcp-1x1.png?2' width="1000" height="1000"/>
  <img src='/images/lcp-1x1.png?3' width="1000" height="1000"/>
</div>
</div>
<script>
// Spin the carousel
setup({"hide_test_state": true});
const images = document.querySelectorAll('img');

let selected = 0;
const container = document.getElementById("container");
const transition = () => {
  container.scrollLeft = selected * 1000;
  selected = (selected + 1) % images.length;
}

container.scrollLeft=1000;
setInterval(transition, 1000);

promise_test(async t => {

  return new Promise(resolve => {
    assert_implements(window.LargestContentfulPaint,
                      "LargestContentfulPaint is not implemented");
    const observer = new PerformanceObserver(entryList => {
      entryList.getEntries().forEach(entry => {
        // May receive a text entry. Ignore that entry.
        if (!entry.url) {
          return;
        }
        assert_true(entry.url.includes("lcp-100x50.png"), "Re-visible image has an entry");
        resolve();
      });
    });
    observer.observe({type: 'largest-contentful-paint', buffered: true});
    t.step_timeout(() => {
      assert_unreached("The image should have become visible by now, which should have triggered an LCP entry.");
      t.done();
    }, 2000);
  });
}, 'Image visibility: out-of-viewport images are observable by LargestContentfulPaint once they become visible.');
</script>
</body>
