mozilla-release/testing/web-platform/tests/intersection-observer/containing-block.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: 170px;
  height: 200px;
  overflow-y: scroll;
}
#target {
  width: 100px;
  height: 100px;
  background-color: green;
  position: absolute;
}
</style>

<div id="root" style="position: absolute">
  <div id="target" style="left: 50px; top: 250px"></div>
</div>

<script>
var entries = [];
var root, target;

runTestCycle(function() {
  root = document.getElementById("root");
  assert_true(!!root, "root element exists.");
  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.");
  target.style.top = "10px";
  runTestCycle(test1, "In containing block and intersecting.");
}, "IntersectionObserver should only report intersections if root is a containing block ancestor of target.");

function test1() {
  runTestCycle(test2, "In containing block and not intersecting.");
  var rootBounds = contentBounds(root);
  checkLastEntry(entries, 0, [58, 158, 18, 118, 58, 158, 18, 118].concat(rootBounds));
  target.style.top = "250px";
}

function test2() {
  runTestCycle(test3, "Not in containing block and intersecting.");
  var rootBounds = contentBounds(root);
  checkLastEntry(entries, 1, [58, 158, 258, 358, 0, 0, 0, 0].concat(rootBounds));
  root.style.position = "static";
  target.style.top = "10px";
}

function test3() {
  runTestCycle(test4, "Not in containing block and not intersecting.");
  checkLastEntry(entries, 1);
  target.style.top = "250px";
}

function test4() {
  checkLastEntry(entries, 1);
  target.style.top = "0";
}
</script>