Handle nested position:sticky elements

third_party/WebKit/Source/core/frame/FrameView.cpp

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index e8d7bb641938d8b15026853b0377334102b61dda..70e0065dc65003c3747b70dca1235efd4656d24e 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -1944,11 +1944,17 @@ void FrameView::updateLayersAndCompositingAfterScrollIfNeeded() {
   if (!hasViewportConstrainedObjects())
     return;
 
-  // Update sticky position objects which are stuck to the viewport.
+  // Update sticky position objects which are stuck to the viewport. In order to
+  // correctly compute the sticky position offsets, the layers must be visited
+  // top-down, so start at the 'root' sticky elements and recurse downwards.
   for (const auto& viewportConstrainedObject : *m_viewportConstrainedObjects) {
     LayoutObject* layoutObject = viewportConstrainedObject;
     PaintLayer* layer = toLayoutBoxModelObject(layoutObject)->layer();
-    if (layoutObject->style()->position() == StickyPosition) {
+    StickyConstraintsMap constraintsMap =
+        layer->getScrollableArea()->stickyConstraintsMap();
+    if (layoutObject->style()->position() == StickyPosition &&
+        constraintsMap.contains(layer) &&
+        !constraintsMap.get(layer).hasAncestorStickyElement()) {
       // TODO(skobes): Resolve circular dependency between scroll offset and
       // compositing state, and remove this disabler. https://crbug.com/420741
       DisableCompositingQueryAsserts disabler;