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chromium/third_party/blink/web_tests/external/wpt/css/css-scroll-snap/snap-after-relayout/adding-only-snap-area.html 

<!DOCTYPE html>
<title>
  Adding a new snap area when there are none should make the scroller snap to it.
</title>
<link rel="help" href="https://drafts.csswg.org/css-scroll-snap/#re-snap" />
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<style>
div {
  position: absolute;
  margin: 0;
}

#scroller {
  height: 500px;
  width: 500px;
  overflow: hidden;
  scroll-snap-type: both mandatory;
}

#target {
  width: 300px;
  height: 300px;
  top: 100px;
  left: 100px;
  background-color: green;
  scroll-snap-align: start;
}

.area {
  width: 2000px;
  height: 2000px;
}
</style>

<div id="scroller">
  <div class="area"></div>
  <div id="target"></div>
</div>

<script>
const target = document.getElementById("target");
const scroller = document.getElementById("scroller");

test(() => {
  scroller.removeChild(target);
  scroller.scrollTo(0,0);
  assert_equals(scroller.scrollTop, 0);
  assert_equals(scroller.scrollLeft, 0);

  scroller.appendChild(target);
  assert_equals(scroller.scrollTop, 100);
  assert_equals(scroller.scrollLeft, 100);
}, "Adding a new snap area when there are none should make the scroller snap to it.");
</script>