chromium/third_party/blink/web_tests/paint/invalidation/flexbox/repaint-opacity-change.html

<!DOCTYPE html>
<script src="../resources/text-based-repaint.js"></script>
<script>
function repaintTest() {
  target.style.opacity = '1';
}
onload = runRepaintAndPixelTest;
</script>
<style>
.item {
  margin: 10px;
  padding: 10px;
  opacity: 0.5;
  background: green;
}
.flex {
  display: flex;
  flex-direction: column;
}
</style>
<div class="flex">
    <div class="item" id="target"></div>
    <div class="item"></div>
</div>