<!DOCTYPE html>
<title>Layout Instability: shift offscreen with scroll anchoring and natural scroll</title>
<link rel="help" href="https://wicg.github.io/layout-instability/" />
<style>
#scroller {
  overflow: scroll;
  left: 20px;
  top: 20px;
  width: 200px;
  height: 200px;
}
#spacer {
  height: 3000px;
}
#ch {
  position: relative;
  background: yellow;
  left: 10px;
  top: 100px;
  width: 150px;
  height: 150px;
}
#offscreenElement {
  width: 300px;
  height: 300px;
  background: lightblue;
}
#onscreenElement {
  width: 300px;
  height: 300px;
  background: lightgreen;
}
</style>
<div id="scroller">
  <div id="offscreenElement"></div>
  <div id="spacer"></div>
  <div id="onscreenElement"></div>
</div>
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="resources/util.js"></script>
<script>

promise_test(async () => {
  const watcher = new ScoreWatcher;

  // Wait for the initial render to complete.
  await waitForAnimationFrames(2);

  // Scroll to show #onscreenElement.
  scroller.scrollTop = 3250;
  await waitForAnimationFrames(1);

  // Resize #offscreernElement and scroll a bit.
  // Visually, #onscreenElement will move by 20px.
  offscreenElement.style.height = '250px';
  scroller.scrollBy(0, 20);

  await waitForAnimationFrames(3);
  // There should be no reported layout shift, because to the user it looks
  // like a natural scroll by 20px.
  assert_equals(watcher.score, 0);
}, "Offscreen shift with scroll annchoring and natural scroll not counted.");

</script>
