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 43718b3c8601adccf367e8306bbb5b33f57556d1..dbca09d33dc7908a0025d8c90bfc882d77eec534 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -1928,16 +1928,23 @@ 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) {
-      // TODO(skobes): Resolve circular dependency between scroll offset and
-      // compositing state, and remove this disabler. https://crbug.com/420741
-      DisableCompositingQueryAsserts disabler;
-      layer->updateLayerPositionsAfterOverflowScroll();
-      layoutObject->setMayNeedPaintInvalidationSubtree();
+    StickyConstraintsMap constraintsMap =
+        layer->getScrollableArea()->stickyConstraintsMap();
+    if (layoutObject->style()->position() == StickyPosition &&
+        constraintsMap.contains(layer)) {
+      if (!constraintsMap.get(layer).hasAncestorStickyElement()) {

[flackr, 2017/01/18 14:42:56]

nit: Add condition to outer if rather than nesting if conditions.

[smcgruer, 2017/01/18 19:10:50]

Done.

+        // TODO(skobes): Resolve circular dependency between scroll offset and
+        // compositing state, and remove this disabler. https://crbug.com/420741
+        DisableCompositingQueryAsserts disabler;
+        layer->updateLayerPositionsAfterOverflowScroll();
+        layoutObject->setMayNeedPaintInvalidationSubtree();
+      }
     }
   }