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chromium/third_party/blink/web_tests/paint/invalidation/reflection/reflection-resize.html 

<!DOCTYPE html>
<div id="target" style="width: 100px; height: 100px; -webkit-box-reflect: below 20px; background: green"></div>
<script src="../../../resources/run-after-layout-and-paint.js"></script>
<script>
runAfterLayoutAndPaint(function() {
  target.style.height = '150px';
}, true);
</script>