File: anchor-updates-after-explicit-scroll.html

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

#scroller {
  height: 200px;
  width: 200px;
  overflow: scroll;
}
#a1, #space1, #a2, #space2 {
  height: 200px;
}
#a1, #a2 {
  background-color: #8f8;
}

</style>
<div id="scroller">
  <div id="space0"></div>
  <div id="a1"></div>
  <div id="space1"></div>
  <div id="a2"></div>
  <div id="space2"></div>
</div>
<script>

// Tests that the anchor node is recomputed after an explicit change to the
// scroll position.

test(() => {
  var scroller = document.querySelector("#scroller");
  scroller.scrollTop = 500;

  // We should now be anchored to #a2.
  document.querySelector("#space1").style.height = "300px";
  assert_equals(scroller.scrollTop, 600);

  scroller.scrollTop = 100;

  // We should now be anchored to #a1. Make sure there is no adjustment from
  // moving #a2.
  document.querySelector("#space1").style.height = "400px";
  assert_equals(scroller.scrollTop, 100);

  // Moving #a1 should produce an adjustment.
  document.querySelector("#space0").style.height = "100px";
  assert_equals(scroller.scrollTop, 200);
}, "Anchor node recomputed after an explicit scroll occurs.");

</script>