mozilla-release/testing/web-platform/tests/intersection-observer/edge-inclusive-intersection.html

<!DOCTYPE html>
<meta name="viewport" content="width=device-width,initial-scale=1">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="./resources/intersection-observer-test-utils.js"></script>

<style>
pre, #log {
  position: absolute;
  top: 0;
  left: 200px;
}
#root {
  width: 200px;
  height: 200px;
  overflow: visible;
}
#target {
  background-color: green;
}
</style>

<div id="root">
  <div id="target" style="width: 100px; height: 100px; transform: translateY(250px)"></div>
</div>

<script>
var entries = [];

runTestCycle(function() {
  var root = document.getElementById('root');
  assert_true(!!root, "root element exists.");
  var target = document.getElementById('target');
  assert_true(!!target, "target element exists.");
  var observer = new IntersectionObserver(function(changes) {
    entries = entries.concat(changes);
  }, { root: root });
  observer.observe(target);
  entries = entries.concat(observer.takeRecords());
  assert_equals(entries.length, 0, "No initial notifications.");
  runTestCycle(step0, "First rAF.");
}, "IntersectionObserver should detect and report edge-adjacent and zero-area intersections.");

function step0() {
  runTestCycle(step1, "Set transform=translateY(200px) on target.");
  checkLastEntry(entries, 0, [8, 108, 258, 358, 0, 0, 0, 0, 8, 208, 8, 208, false]);
  target.style.transform = "translateY(200px)";
}

function step1() {
  runTestCycle(step2, "Set transform=translateY(201px) on target.");
  checkLastEntry(entries, 1, [8, 108, 208, 308, 8, 108, 208, 208, 8, 208, 8, 208, true]);
  target.style.transform = "translateY(201px)";
}

function step2() {
  runTestCycle(step3, "Set transform=translateY(185px) on target.");
  checkLastEntry(entries, 2);
  target.style.height = "0px";
  target.style.width = "300px";
  target.style.transform = "translateY(185px)";
}

function step3() {
  checkLastEntry(entries, 3, [8, 308, 193, 193, 8, 208, 193, 193, 8, 208, 8, 208, true]);
}
</script>