multicol-rule-actual-columns-change.html

<!DOCTYPE html>
<script src="../../../resources/run-after-layout-and-paint.js"></script>
<script>
runAfterLayoutAndPaint(function() {
  content.style.height = '1000px';
}, true);
</script>
<div style="column-fill: auto; columns: 4; column-rule: 10px solid blue; width: 400px; height: 400px">
  <div id="content" style="height: 500px; background: gray"></div>
</div>

chromium/third_party/blink/web_tests/paint/invalidation/multicol/multicol-rule-actual-columns-change.html