mozilla-release/testing/web-platform/tests/intersection-observer/isIntersecting-threshold.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>
#scroller { width: 100px; height: 100px; overflow: scroll; }
#scroller > div { height: 800px; }
#target { margin-top: 25px; height: 50px; background-color: blue; }
</style>
<div id="scroller">
  <div>
    <div id="target"></div>
  </div>
</div>

<script>
let entries = [];

window.onload = function() {
  runTestCycle(step2, "At initial scroll position");

  scroller.scrollTop = 0;
  let observer = new IntersectionObserver(
    es => entries = entries.concat(es),
    { threshold: 1 }
  );
  observer.observe(target);
};

function step2() {
  runTestCycle(step3, "Scrolled to half way through target element");

  assert_equals(entries.length, 1);
  assert_equals(entries[0].intersectionRatio, 1);
  assert_equals(entries[0].isIntersecting, true);
  scroller.scrollTop = 50;
}

function step3() {
  runTestCycle(step4, "Scrolled to target element completely off screen");

  assert_equals(entries.length, 2);
  assert_true(entries[1].intersectionRatio >= 0.5 &&
              entries[1].intersectionRatio < 1);
  assert_equals(entries[1].isIntersecting, true);
  scroller.scrollTop = 100;
}

function step4() {
  assert_equals(entries.length, 2);
}
</script>