Overlay scrollbars must respect ancestor clip

Source/core/rendering/compositing/CompositedLayerMapping.h

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 72 matching lines @@
 // but this is likely to evolve soon.
 class CompositedLayerMapping FINAL : public GraphicsLayerClient {
     WTF_MAKE_NONCOPYABLE(CompositedLayerMapping); WTF_MAKE_FAST_ALLOCATED;
 public:
     explicit CompositedLayerMapping(RenderLayer&);
     virtual ~CompositedLayerMapping();
 
     RenderLayer& owningLayer() const { return m_owningLayer; }
 
     bool updateGraphicsLayerConfiguration();
-    void updateGraphicsLayerGeometry(const RenderLayer* compositingContainer, Vector<RenderLayer*>& layersNeedingPaintInvalidation);
+    void updateGraphicsLayerGeometry(const RenderLayer* compositingContainer, const RenderLayer* compositingStackingContext, Vector<RenderLayer*>& layersNeedingPaintInvalidation);
 
     // Update whether layer needs blending.
     void updateContentsOpaque();
 
     GraphicsLayer* mainGraphicsLayer() const { return m_graphicsLayer.get(); }
 
     // Layer to clip children
     bool hasClippingLayer() const { return m_childContainmentLayer; }
     GraphicsLayer* clippingLayer() const { return m_childContainmentLayer.get(); }
 
@@ skipping 115 matching lines @@
     void updateScrollingBlockSelection();
 
 private:
     static const GraphicsLayerPaintInfo* containingSquashedLayer(const RenderObject*,  const Vector<GraphicsLayerPaintInfo>& layers);
 
     // Helper methods to updateGraphicsLayerGeometry:
     void computeGraphicsLayerParentLocation(const RenderLayer* compositingContainer, const IntRect& ancestorCompositingBounds, IntPoint& graphicsLayerParentLocation);
     void updateSquashingLayerGeometry(const LayoutPoint& offsetFromCompositedAncestor, const IntPoint& graphicsLayerParentLocation, const RenderLayer& referenceLayer, Vector<GraphicsLayerPaintInfo>& layers, GraphicsLayer*, LayoutPoint* offsetFromTransformedAncestor, Vector<RenderLayer*>& layersNeedingPaintInvalidation);
     void updateMainGraphicsLayerGeometry(const IntRect& relativeCompositingBounds, const IntRect& localCompositingBounds, IntPoint& graphicsLayerParentLocation);
     void updateAncestorClippingLayerGeometry(const RenderLayer* compositingContainer, const IntPoint& snappedOffsetFromCompositedAncestor, IntPoint& graphicsLayerParentLocation);
+    void updateOverflowControlsHostLayerGeometry(const RenderLayer* compositingStackingContext);
     void updateChildContainmentLayerGeometry(const IntRect& clippingBox, const IntRect& localCompositingBounds);
     void updateChildTransformLayerGeometry();
     void updateMaskLayerGeometry();
     void updateTransformGeometry(const IntPoint& snappedOffsetFromCompositedAncestor, const IntRect& relativeCompositingBounds);
     void updateForegroundLayerGeometry(const FloatSize& relativeCompositingBoundsSize, const IntRect& clippingBox);
     void updateBackgroundLayerGeometry(const FloatSize& relativeCompositingBoundsSize);
     void updateReflectionLayerGeometry(Vector<RenderLayer*>& layersNeedingPaintInvalidation);
     void updateScrollingLayerGeometry(const IntRect& localCompositingBounds);
     void updateChildClippingMaskLayerGeometry();
 
     void createPrimaryGraphicsLayer();
     void destroyGraphicsLayers();
 
     PassOwnPtr<GraphicsLayer> createGraphicsLayer(CompositingReasons);
     bool toggleScrollbarLayerIfNeeded(OwnPtr<GraphicsLayer>&, bool needsLayer, CompositingReasons);
 
     RenderLayerModelObject* renderer() const { return m_owningLayer.renderer(); }
     RenderLayerCompositor* compositor() const { return m_owningLayer.compositor(); }
 
     void updateInternalHierarchy();
     void updatePaintingPhases();
     bool updateClippingLayers(bool needsAncestorClip, bool needsDescendantClip);
     bool updateChildTransformLayer(bool needsChildTransformLayer);
-    bool updateOverflowControlsLayers(bool needsHorizontalScrollbarLayer, bool needsVerticalScrollbarLayer, bool needsScrollCornerLayer);
+    bool updateOverflowControlsLayers(bool needsHorizontalScrollbarLayer, bool needsVerticalScrollbarLayer, bool needsScrollCornerLayer, bool needsAncestorClip);
     bool updateForegroundLayer(bool needsForegroundLayer);
     bool updateBackgroundLayer(bool needsBackgroundLayer);
     bool updateMaskLayer(bool needsMaskLayer);
     bool updateClippingMaskLayers(bool needsChildClippingMaskLayer);
     bool requiresHorizontalScrollbarLayer() const { return m_owningLayer.scrollableArea() && m_owningLayer.scrollableArea()->horizontalScrollbar(); }
     bool requiresVerticalScrollbarLayer() const { return m_owningLayer.scrollableArea() && m_owningLayer.scrollableArea()->verticalScrollbar(); }
     bool requiresScrollCornerLayer() const { return m_owningLayer.scrollableArea() && !m_owningLayer.scrollableArea()->scrollCornerAndResizerRect().isEmpty(); }
     bool updateScrollingLayers(bool scrollingLayers);
     void updateScrollParent(RenderLayer*);
     void updateClipParent();
@@ skipping 115 matching lines @@
     //         + root content layer
     //
     // With the hierarchy set up like this, the root content layer is able to scroll without affecting
     // the background layer (or repainting).
     OwnPtr<GraphicsLayer> m_foregroundLayer; // Only used in cases where we need to draw the foreground separately.
     OwnPtr<GraphicsLayer> m_backgroundLayer; // Only used in cases where we need to draw the background separately.
 
     OwnPtr<GraphicsLayer> m_layerForHorizontalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForVerticalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForScrollCorner;
+
+    // This layer exists to simplify the reparenting of overflow control that is occasionally required
+    // to ensure that scrollbars appear above scrolling content.
     OwnPtr<GraphicsLayer> m_overflowControlsHostLayer;
 
+    // The reparented overflow controls sometimes need to be clipped by a non-ancestor. In just the same
+    // way we need an ancestor clipping layer to clip this CLM's internal hierarchy, we add another layer
+    // to clip the overflow controls. It would be possible to make m_overflowControlsHostLayer be
+    // responsible for applying this clip, but that could require repositioning all of the overflow
+    // controls since the this clip may apply an offset. By using a separate layer, the overflow controls
+    // can remain ignorant of the layers above them and still work correctly.

[chrishtr, 2014/07/24 18:44:57]

Starting with a basic question..

Can you give me a concrete example? In particular, I don't yet understand why
the controls would be clipped by something other than the thing which clips
m_graphicsLayer content. Also please diagram
the layers for this situation, similar to the comment on lines 404-408.

+    OwnPtr<GraphicsLayer> m_overflowControlsClippingLayer;
+
     // A squashing CLM has two possible squashing-related structures.
     //
     // If m_ancestorClippingLayer is present:
     //
     // m_ancestorClippingLayer
     //   + m_graphicsLayer
     //   + m_squashingLayer
     //
     // If not:
     //
@@ skipping 14 matching lines @@
     unsigned m_isMainFrameRenderViewLayer : 1;
     unsigned m_requiresOwnBackingStoreForIntrinsicReasons : 1;
     unsigned m_requiresOwnBackingStoreForAncestorReasons : 1;
     unsigned m_backgroundLayerPaintsFixedRootBackground : 1;
     unsigned m_scrollingContentsAreEmpty : 1;
 };
 
 } // namespace blink
 
 #endif // CompositedLayerMapping_h