anchor-scroll-scrollable-anchor.html

<!DOCTYPE html>
<meta name="fuzzy" content="1;0-50">
<title>Basic of anchor positioned scrolling: scroll of a scrollable anchor should not affect anchor positioning</title>
<link rel="help" href="https://drafts.csswg.org/css-anchor-1/">
<link rel="match" href="anchor-scroll-scrollable-anchor-ref.html">
<style>
#scroll-container {
  width: 400px;
  height: 400px;
  overflow: scroll;
  will-change: scroll-position;
}

#scroll-contents {
  width: 1000px;
  height: 1000px;
  position: relative;
}

.placefiller {
  height: 500px;
}

#anchor {
  anchor-name: --anchor;
  height: 100px;
  width: 100px;
  overflow: scroll;
  will-change: scroll-position;
}

#anchored {
  background: green;
  position: absolute;
  width: 100px;
  height: 100px;
  left: anchor(left);
  bottom: anchor(top);
  position-anchor: --anchor;
}
</style>

<div id="scroll-container">
  <div id="scroll-contents">
    <div class="placefiller"></div>
    <div id="anchor">
      <div class="placefiller"></div>
    </div>
  </div>
</div>
<div id="anchored"></div>

<script>
document.getElementById('scroll-container').scrollTop = 300;
document.getElementById('anchor').scrollTop = 300;
</script>

chromium/third_party/blink/web_tests/external/wpt/css/css-anchor-position/anchor-scroll-scrollable-anchor.html