Composite sticky-positioned elements when they have composited descendants

third_party/WebKit/LayoutTests/compositing/overflow/mixed-composited-nested-sticky.html

+<style>
+body {
+  margin: 0;
+}
+
+.container {
+  width: 200px;
+  height: 2000px;
+}
+
+.outerSticky {
+  position: sticky;
+  top: 50px;
+  background-color: green;
+  width: 200px;
+  height: 200px;
+}
+
+.middleSticky {
+  will-change: transform;
+
+  position: sticky;
+  top: 100px;
+  background-color: yellow;
+  width: 200px;
+  height: 100px;
+}
+
+.innerSticky {
+  will-change: transform;
+
+  position: sticky;
+  top: 125px;
+  background-color: red;
+  width: 200px;
+  height: 50px;
+}
+</style>
+
+<script>
+if (window.testRunner)
+  testRunner.waitUntilDone();
+
+function finishTest() {
+  window.scrollTo(0, 200);
+  if (window.testRunner)
+    testRunner.notifyDone();
+}
+
+window.addEventListener('load', function() {
+  requestAnimationFrame(function() {
+    requestAnimationFrame(finishTest);
+  });
+});
+</script>
+
+<div class="container">
+  <div class="outerSticky">
+    <div class="middleSticky">
+      <div class="innerSticky"></div>
+    </div>
+  </div>
+</div>