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

body { height: 2000px; overflow-anchor: none; }
#scroller { overflow: scroll; width: 500px; height: 300px; }
.anchor {
  position:relative; height: 100px; width: 150px;
  background-color: #afa; border: 1px solid gray;
}
#forceScrolling { height: 500px; background-color: #fcc; }

</style>
<div id="outerChanger"></div>
<div id="outerAnchor" class="anchor"></div>
<div id="scroller">
  <div id="innerChanger"></div>
  <div id="innerAnchor" class="anchor"></div>
  <div id="forceScrolling"></div>
</div>
<script>

// Tests that scroll anchoring can be disabled per-scroller with the
// overflow-anchor property.

var divScroller = document.querySelector("#scroller");
var docScroller = document.scrollingElement;
var innerChanger = document.querySelector("#innerChanger");
var outerChanger = document.querySelector("#outerChanger");

function setup() {
  divScroller.scrollTop = 100;
  docScroller.scrollTop = 100;
  innerChanger.style.height = "200px";
  outerChanger.style.height = "150px";
}

function reset() {
  document.body.style.overflowAnchor = "";
  divScroller.style.overflowAnchor = "";
  divScroller.scrollTop = 0;
  docScroller.scrollTop = 0;
  innerChanger.style.height = "";
  outerChanger.style.height = "";
}

test(() => {
  setup();

  assert_equals(divScroller.scrollTop, 300,
      "Scroll anchoring should apply to #scroller.");

  assert_equals(docScroller.scrollTop, 100,
      "Scroll anchoring should not apply to the document scroller.");

  reset();
}, "Disabled on document, enabled on div.");

test(() => {
  document.body.style.overflowAnchor = "auto";
  divScroller.style.overflowAnchor = "none";
  setup();

  assert_equals(divScroller.scrollTop, 100,
      "Scroll anchoring should not apply to #scroller.");

  assert_equals(docScroller.scrollTop, 250,
      "Scroll anchoring should apply to the document scroller.");

  reset();
}, "Enabled on document, disabled on div.");

</script>
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