chromium/third_party/blink/web_tests/fast/forms/select/menulist-dynamic-style-change.html

<!DOCTYPE html>
<title>crbug.com/1103773</title>
<style>
select {
  padding: 1rem;
}

.update {
  padding-top: 1.5rem;
  padding-bottom: 0.5rem;
}
</style>
<body>
<select><option>abcd</select>
<script>
const select = document.querySelector('select');
select.offsetTop;
if (window.testRunner)
  testRunner.waitUntilDone();
requestAnimationFrame(() => {
  requestAnimationFrame(() => {
    select.classList.add('update');
    testRunner.notifyDone();
  });
});
</script>
</body>