css/css-scroll-anchoring/subtree-exclusion.html

<!DOCTYPE html>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>

body { height: 4000px }
#A, #B { width: 100px; background-color: #afa; }
#B { height: 100px; }
#inner { width: 100px; height: 100px; background-color: pink; }
#A { overflow-anchor: none; }

</style>
<div id="changer1"></div>
<div id="A">
  <div id="inner"></div>
  <div id="changer2"></div>
</div>
<div id="B"></div>
<script>

// Tests that an element with overflow-anchor: none is excluded, along with its
// DOM descendants, from the anchor selection algorithm.

test(() => {
  var changer1 = document.querySelector("#changer1");
  var changer2 = document.querySelector("#changer2");

  document.scrollingElement.scrollTop = 50;
  changer1.style.height = "100px";
  changer2.style.height = "50px";

  // We should anchor to #B, not #A or #inner, despite them being visible.
  assert_equals(document.scrollingElement.scrollTop, 200);

  document.querySelector("#A").style.overflowAnchor = "auto";
  document.scrollingElement.scrollTop = 150;

  changer1.style.height = "200px";
  changer2.style.height = "100px";

  // We should now anchor to #inner, which is moved only by #changer1.
  assert_equals(document.scrollingElement.scrollTop, 250);
}, "Subtree exclusion with overflow-anchor.");

</script>