WIP - compositor worker mega patch.

third_party/WebKit/ManualTests/compositor-worker/sticky/js/dom-util.js

+function getViewportRect(target) {
+  // Q: What's the correct way to get the scroller bounds (if the scroller is the document)?
+  if (target == document.scrollingElement) {
+    return {
+      left: 0,
+      top: 0,
+      width: window.innerWidth,
+      height: window.innerHeight,
+      right: window.innerWidth,
+      bottom: window.innerHeight
+    };
+  }
+  return target.getBoundingClientRect();
+}
+
+function getContainingBlockElement(node) {
+  if (node.style.position == 'absolute') {
+    do {
+      node = node.parentNode;
+    } while (['absolute', 'fixed', 'relative', 'sticky'].indexOf(getComputedStyle(node).position) == -1);
+    return node || document;
+  }
+    do {
+      node = node.parentNode;
+    } while (['block', 'inline-block', 'list-item', 'run-in', 'table', 'table-cell'].indexOf(getComputedStyle(node).display) == -1);
+    return node || document;
+}
+
+function isScrollable(element) {
+  // Q: Why is scrollHeight > clientHeight on body when document is scrollable node?
+  return element.scrollHeight > element.clientHeight || element == document.scrollingElement;
+}
+
+function getContainingScrollingElement(element) {
+  do {
+    element = getContainingBlockElement(element);
+  } while (element && !isScrollable(element));
+  return element;
+}