Adapt and reland old position sticky implementation

third_party/WebKit/Source/core/paint/DeprecatedPaintLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 479 matching lines @@
         // While we're updating the compositing inputs, these values may differ.
         // We should never be asking for this value when that is the case.
         ASSERT(m_needsDescendantDependentCompositingInputsUpdate == m_needsAncestorDependentCompositingInputsUpdate);
         return m_needsDescendantDependentCompositingInputsUpdate;
     }
 
     void updateAncestorDependentCompositingInputs(const AncestorDependentCompositingInputs&);
     void updateDescendantDependentCompositingInputs(const DescendantDependentCompositingInputs&);
     void didUpdateCompositingInputs();
 
-    const AncestorDependentCompositingInputs& ancestorDependentCompositingInputs() const { ASSERT(!m_needsAncestorDependentCompositingInputsUpdate); return m_ancestorDependentCompositingInputs; }
+    const AncestorDependentCompositingInputs& ancestorDependentCompositingInputs() const
+    {
+        // TODO(flackr): When we process scroll updates after compositing updates we won't be querying ancestor
+        // dependent compositing inputs while they're still needed: http://crbug.com/420741.
+        ASSERT(isAllowedToQueryCompositingState() || !m_needsAncestorDependentCompositingInputsUpdate);
+        return m_ancestorDependentCompositingInputs;
+    }
     const DescendantDependentCompositingInputs& descendantDependentCompositingInputs() const { ASSERT(!m_needsDescendantDependentCompositingInputsUpdate); return m_descendantDependentCompositingInputs; }
 
     IntRect clippedAbsoluteBoundingBox() const { return ancestorDependentCompositingInputs().clippedAbsoluteBoundingBox; }
     const DeprecatedPaintLayer* opacityAncestor() const { return ancestorDependentCompositingInputs().opacityAncestor; }
     const DeprecatedPaintLayer* transformAncestor() const { return ancestorDependentCompositingInputs().transformAncestor; }
     const DeprecatedPaintLayer* filterAncestor() const { return ancestorDependentCompositingInputs().filterAncestor; }
     const LayoutObject* clippingContainer() const { return ancestorDependentCompositingInputs().clippingContainer; }
     const DeprecatedPaintLayer* ancestorScrollingLayer() const { return ancestorDependentCompositingInputs().ancestorScrollingLayer; }
     const DeprecatedPaintLayer* nearestFixedPositionLayer() const { return ancestorDependentCompositingInputs().nearestFixedPositionLayer; }
     DeprecatedPaintLayer* scrollParent() const { return const_cast<DeprecatedPaintLayer*>(ancestorDependentCompositingInputs().scrollParent); }
@@ skipping 252 matching lines @@
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const blink::DeprecatedPaintLayer*);
 void showLayerTree(const blink::LayoutObject*);
 #endif
 
 #endif // Layer_h