Handle nested position:sticky elements

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

+<script>
+if (window.testRunner)
+  testRunner.waitUntilDone();
+
+function doTest() {
+  finishTestInXFrames(2);
+}
+
+function finishTestInXFrames(frames) {
+  if (frames > 0) {
+    requestAnimationFrame(finishTestInXFrames.bind(null, frames - 1));
+    return;
+  }
+  finishTest();
+}
+
+function finishTest() {
+  for (const scroller of document.querySelectorAll('.scroller')) {
+    scroller.scrollTop = 400;
+  }
+  if (window.testRunner)
+    testRunner.notifyDone();
+}
+
+window.addEventListener('load', doTest, false);
+</script>
+
+<style>
+body {
+  margin: 0;
+}
+
+.scroller {
+  will-change: transform;
+
+  overflow: hidden; /* hide scrollbars */
+  width: 200px;
+  height: 500px;
+  outline: 2px solid black;
+}
+
+.container {
+  width: 200px;
+  height: 1000px;
+}
+
+.sticky {
+  will-change: transform;
+
+  position: sticky;
+  width: 200px;
+  top: 0;
+}
+
+.first {
+  height: 500px;
+  background: red;
+}
+
+.second {
+  height: 400px;
+  background: green;
+}
+
+.third {
+  height: 300px;
+  background: blue;
+  top: 100px;
+}
+
+.fourth {
+  height: 200px;
+  background: pink;
+  top: 100px;
+}
+
+.fifth {
+  height: 100px;
+  background: yellow;
+}
+</style>
+
+<div class="scroller">
+  <div class="container">
+    <div class="first sticky">
+      <div class="second sticky">
+        <div class="third sticky">
+          <div class="fourth sticky">
+            <div class="fifth sticky"></div>
+          </div>
+        </div>
+      </div>
+    </div>
+  </div>
+</div>