css/css-anchor-position/anchor-center-visibility-change.html

<!DOCTYPE html>
<html class=reftest-wait>
<title>Tests 'anchor-center' value when target visibility changes</title>
<link rel="help" href="https://drafts.csswg.org/css-anchor-position-1/#valdef-justify-self-anchor-center">
<link rel="author" href="mailto:plampe@igalia.com">
<link rel="match" href="anchor-center-visibility-change-ref.html">
<script src="/common/reftest-wait.js"></script>
<script src="/common/rendering-utils.js"></script>
<style>
.container {
  width: 100px;
  height: 100px;
  border: solid 3px;
  position: relative;
  margin: 50px;
}

.anchor {
  anchor-name: --anchor;
  position: relative;
  width: 50px;
  height: 50px;
  left: 40px;
  top: 5px;
  background: lime;
}

.target {
  position-anchor: --anchor;
  position: fixed;
  background: cyan;
  justify-self: anchor-center;
  display: none;
}
</style>

<div class="container">
  <div class="anchor"></div>
  <div id="target" class="target">
    <div style="width:30px;height:20px;"></div>
  </div>
</div>

<script>
  // #target should be invisible initially.
  waitForAtLeastOneFrame().then(() => {
    // Change #target to be visible.
    document.getElementById('target').style.display = 'flow';
    waitForAtLeastOneFrame().then(() => {
      // #target should be visible and anchor-centered now.
      takeScreenshot();
    });
  });
</script>