Avoid creating ancestor clipping layers for scroll children

Source/core/layout/compositing/CompositedDeprecatedPaintLayerMapping.cpp

 /*
  * 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 339 matching lines @@
     }
 }
 
 void CompositedDeprecatedPaintLayerMapping::updateCompositingReasons()
 {
     // All other layers owned by this mapping will have the same compositing reason
     // for their lifetime, so they are initialized only when created.
     m_graphicsLayer->setCompositingReasons(m_owningLayer.compositingReasons());
 }
 
-bool CompositedDeprecatedPaintLayerMapping::owningLayerClippedByLayerNotAboveCompositedAncestor()
+bool CompositedDeprecatedPaintLayerMapping::owningLayerClippedByLayerNotAboveCompositedAncestor(DeprecatedPaintLayer* scrollParent)
 {
     if (!m_owningLayer.parent())
         return false;
 
     const DeprecatedPaintLayer* compositingAncestor = m_owningLayer.enclosingLayerWithCompositedDeprecatedPaintLayerMapping(ExcludeSelf);
     if (!compositingAncestor)
         return false;
 
     const LayoutObject* clippingContainer = m_owningLayer.clippingContainer();
     if (!clippingContainer)
         return false;
 
+    if (clippingContainer->enclosingLayer() == scrollParent)
+        return false;
+
     if (compositingAncestor->layoutObject()->isDescendantOf(clippingContainer))
         return false;
 
     // We ignore overflow clip here; we want composited overflow content to
     // behave as if it lives in an unclipped universe so it can prepaint, etc.
     // This means that we need to check if we are actually clipped before
     // setting up m_ancestorClippingLayer otherwise
     // updateAncestorClippingLayerGeometry will fail as the clip rect will be
     // infinite.
     // FIXME: this should use cached clip rects, but this sometimes give
@@ skipping 36 matching lines @@
     if (m_owningLayer.needsCompositedScrolling())
         needsDescendantsClippingLayer = false;
 
     DeprecatedPaintLayer* scrollParent = compositor->preferCompositingToLCDTextEnabled() ? m_owningLayer.scrollParent() : 0;
 
     // This is required because compositing layers are parented according to the z-order hierarchy, yet
     // clipping goes down the layoutObject hierarchy. Thus, a DeprecatedPaintLayer can be clipped by a
     // DeprecatedPaintLayer that is an ancestor in the layoutObject hierarchy, but a sibling in the z-order
     // hierarchy. Further, that sibling need not be composited at all. In such scenarios, an ancestor
     // clipping layer is necessary to apply the composited clip for this layer.
-    bool needsAncestorClip = owningLayerClippedByLayerNotAboveCompositedAncestor();
-
-    if (scrollParent) {
-        // If our containing block is our ancestor scrolling layer, then we'll already be clipped
-        // to it via our scroll parent and we don't need an ancestor clipping layer.
-        if (m_owningLayer.layoutObject()->containingBlock()->enclosingLayer() == m_owningLayer.ancestorScrollingLayer())
-            needsAncestorClip = false;
-    }
+    bool needsAncestorClip = owningLayerClippedByLayerNotAboveCompositedAncestor(scrollParent);
 
     if (updateClippingLayers(needsAncestorClip, needsDescendantsClippingLayer))
         layerConfigChanged = true;
 
     bool scrollingConfigChanged = false;
     if (updateScrollingLayers(m_owningLayer.needsCompositedScrolling())) {
         layerConfigChanged = true;
         scrollingConfigChanged = true;
     }
 
     if (updateOverflowControlsLayers(requiresHorizontalScrollbarLayer(), requiresVerticalScrollbarLayer(), requiresScrollCornerLayer(), needsAncestorClip))
         layerConfigChanged = true;
 
     bool hasPerspective = false;
     // FIXME: Can |style| be really null that late in the DocumentCycle?
     if (const ComputedStyle* style = layoutObject->style())
         hasPerspective = style->hasPerspective();
     bool needsChildTransformLayer = hasPerspective && layoutObject->isBox();
     if (updateChildTransformLayer(needsChildTransformLayer))
         layerConfigChanged = true;
 
     if (updateSquashingLayers(!m_squashedLayers.isEmpty()))
         layerConfigChanged = true;
 
     updateScrollParent(scrollParent);
-    updateClipParent();
+    updateClipParent(scrollParent);
 
     if (layerConfigChanged)
         updateInternalHierarchy();
 
     if (scrollingConfigChanged) {
         if (layoutObject->view())
             compositor->scrollingLayerDidChange(&m_owningLayer);
     }
 
     // A mask layer is not part of the hierarchy proper, it's an auxiliary layer
@@ skipping 1148 matching lines @@
     if (!layer)
         return;
 
     // Only the topmost layer has a scroll parent. All other layers have a null scroll parent.
     if (layer != topmostLayer)
         clipParent = 0;
 
     scrollingCoordinator->updateClipParentForGraphicsLayer(layer, clipParent);
 }
 
-void CompositedDeprecatedPaintLayerMapping::updateClipParent()
+void CompositedDeprecatedPaintLayerMapping::updateClipParent(DeprecatedPaintLayer* scrollParent)
 {
-    if (owningLayerClippedByLayerNotAboveCompositedAncestor())
+    if (owningLayerClippedByLayerNotAboveCompositedAncestor(scrollParent))
         return;
 
     DeprecatedPaintLayer* clipParent = m_owningLayer.clipParent();
     if (clipParent)
         clipParent = clipParent->enclosingLayerWithCompositedDeprecatedPaintLayerMapping(IncludeSelf);
 
     if (ScrollingCoordinator* scrollingCoordinator = scrollingCoordinatorFromLayer(m_owningLayer)) {
         GraphicsLayer* topmostLayer = childForSuperlayers();
         updateClipParentForGraphicsLayer(m_squashingContainmentLayer.get(), topmostLayer, clipParent, scrollingCoordinator);
         updateClipParentForGraphicsLayer(m_ancestorClippingLayer.get(), topmostLayer, clipParent, scrollingCoordinator);
@@ skipping 698 matching lines @@
     } else if (graphicsLayer == m_scrollingBlockSelectionLayer.get()) {
         name = "Scrolling Block Selection Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink