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chromium/third_party/blink/web_tests/fast/multicol/resize-with-column-rule.html 

<!DOCTYPE html>
<script src="../../resources/run-after-layout-and-paint.js"></script>
<p>There should be two black vertical lines below.</p>
<div id="multicol" style="columns:3; column-rule:2px solid; column-gap:2px; column-fill:auto; width:304px; height:100px;">
    <div style="height:300px; background:white;"></div>
</div>
<script>
    runAfterLayoutAndPaint(function() {
        document.getElementById("multicol").style.width = "604px";
    }, true);
</script>