LayoutTests/compositing/shared-backing/make-sharing-layer-composited.html

<!DOCTYPE HTML>
<html class="reftest-wait">
<head>
<style>
div {
  position: relative;
  width: 200px;
  height: 200px;
}

.composited {
  transform: translateZ(0px);
}
</style>
</head>
<body>

<div class="composited" style="position: absolute"></div>

<!-- div composited due to overlap, with two backing sharing descendants -->
<div>
  <div id="sharing">
    <div></div>
  </div>
</div>

<div id="mutation"></div>

<p>This test should not crash.</p>
<script>
if (window.testRunner)
  testRunner.dumpAsText();

async function test() {
    await new Promise(requestAnimationFrame);
    await new Promise(requestAnimationFrame);
 
    // Make the first sharing layer composited (synchronously) 
    document.getElementById("sharing").classList.add("composited");
    
    // Wait for a rendering update so that the second sharing layer gets updated to no longer share
    await new Promise(requestAnimationFrame);
  
    // Trigger an unrelated style change to update layer positions and verify that it was left in
    // a valid state after the previous rendering update.
    document.getElementById("mutation").style.backgroundColor = "blue";
    document.documentElement.classList.remove("reftest-wait");
}
window.addEventListener('load', test, false);
</script>
</body>
</html>