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chromium/third_party/blink/web_tests/paint/invalidation/compositing/composited-iframe-resizer.html 

<!DOCTYPE html>
<iframe id="iframe" style="width: 200px; height: 200px; resize: both; will-change: transform"></iframe>
<script src="../../../resources/run-after-layout-and-paint.js"></script>
<script>
runAfterLayoutAndPaint(function() {
  iframe.style.width = '400px';
}, true);
</script>