Handle nested position:sticky elements correctly (compositor)

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

+<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;

[flackr, 2017/03/09 20:25:20]

I don't think you have a test of mixed composited and non composited sticky
elements yet do you?

[smcgruer, 2017/03/14 22:04:06]

Tried, but everything appears to be getting composited anyway 'due to
association with an element with a "position: fixed" or "position: sticky"
style'

Uploaded that test, ptal.

+
+  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>
+
+<script>
+if (window.testRunner)
+  testRunner.waitUntilDone();
+
+function finishTest() {
+  document.querySelector('.scroller').scrollTop = 400;
+  if (window.testRunner)
+    testRunner.notifyDone();
+}
+
+window.addEventListener('load', function() {
+  requestAnimationFrame(function() {
+    requestAnimationFrame(finishTest);
+  });
+});
+</script>
+
+<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>