LayoutTests/fast/repaint/repaint-float-only-and-moves.html

<style>
.container {
  width: 200px;
  height: 200px;
  overflow: hidden;
}

.child {
  float: left;
  width: 100px;
  height: 100px;
  background-color: green;
}

#expand_this {
  height: 50px;
  background-color: blue;
}
</style>
<div class=container>
  <div id=expand_this class=child></div>
  <div class=child></div>
</div>
<pre id=repaints></pre>
<script src="../../resources/ui-helper.js"></script>
<script>
if (window.testRunner) {
  testRunner.dumpAsText();
  testRunner.waitUntilDone();
}
async function runTest() {
  await UIHelper.renderingUpdate();

  if (window.internals)
    internals.startTrackingRepaints();

  expand_this.style.width = "200px";

  await UIHelper.renderingUpdate();

  if (window.internals) {
    repaints.innerHTML = window.internals.repaintRectsAsText();
    internals.stopTrackingRepaints();
  }

  if (window.testRunner)
    testRunner.notifyDone();
}

window.addEventListener('load', runTest);
</script>