Handle nested position:sticky elements

third_party/WebKit/LayoutTests/compositing/overflow/composited-nested-sticky-inline.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 = 200;
+  }
+  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: 350px;
+  outline: 2px solid black;
+}
+
+.container {
+  width: 200px;
+  height: 700px;
+}
+
+.outerSticky {
+  will-change: transform;
+
+  display: inline;
+  position: sticky;
+  top: 0;
+}
+
+.innerSticky {
+  will-change: transform;
+
+  display: inline;
+  position: sticky;
+  top: 25px;
+}
+</style>
+
+<div class="scroller">
+  <div class="container">
+    <div class="outerSticky">1<div class="innerSticky">2</div></div>
+  </div>
+</div>