third_party/web_platform_tests/intersection-observer/v2/inline-occlusion.html

<!DOCTYPE html>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="../resources/intersection-observer-test-utils.js"></script>

<style>
body, html {
  margin: 0;
}
pre, #log {
  position: absolute;
  top: 0;
  left: 200px;
}
.testdiv {
  font-size: 24px;
}
</style>

<div class="testdiv">This is the <span id="target">target</span>.</div>
<div class="testdiv" id="occluder">This is the occluder.</div>

<script>
var delay = 100;
var entries = [];
var target;
var occluder;

runTestCycle(function() {
  target = document.getElementById("target");
  occluder = document.getElementById("occluder");
  assert_true(!!target, "target exists");
  assert_true(!!occluder, "occluder exists");
  var observer = new IntersectionObserver(function(changes) {
    entries = entries.concat(changes)
  }, {trackVisibility: true, delay: delay});
  observer.observe(target);
  entries = entries.concat(observer.takeRecords());
  assert_equals(entries.length, 0, "No initial notifications.");
  runTestCycle(step0, "First rAF.", delay);
}, "IntersectionObserverV2 in a single document using the implicit root, with an occluding element.", delay);

function step0() {
  occluder.style.marginTop = "-10px";
  runTestCycle(step1, "occluder.style.marginTop = '-10px'", delay);
  assert_equals(entries.length, 1);
  assert_true(entries[0].isVisible);
}

function step1() {
  // Occluding elements with opacity=0 should not affect target visibility.
  occluder.style.opacity = "0";
  runTestCycle(step2, "occluder.style.opacity = 0", delay);
  assert_equals(entries.length, 2);
  assert_false(entries[1].isVisible);
}

function step2() {
  assert_equals(entries.length, 3);
  assert_true(entries[2].isVisible);
}
</script>