Disable clipping on root scroller's ancestors.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.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 431 matching lines @@
 
     if (updateForegroundLayer(compositor->needsContentsCompositingLayer(&m_owningLayer)))
         layerConfigChanged = true;
 
     bool needsDescendantsClippingLayer = compositor->clipsCompositingDescendants(&m_owningLayer);
 
     // Our scrolling layer will clip.
     if (m_owningLayer.needsCompositedScrolling())
         needsDescendantsClippingLayer = false;
 
+    // We disable clipping on ancestor layers of the root scroller to give it
+    // the same behavior w.r.t top controls as the real root layer. See the
+    // RootScrollerController class for more details.
+    if (m_owningLayer.hasRootScrollerAsDescendant())
+        needsDescendantsClippingLayer = false;

[chrishtr, 2016/09/09 18:21:15]

Could you add some unittest examples to CompositedLayerMappingTest.cpp that
explictly checks the masking parameters?
There are examples there already that should make it easy.

[bokan, 2016/09/10 00:05:54]

Done.

+
     const PaintLayer* scrollParent = this->scrollParent();
 
     // This is required because compositing layers are parented according to the z-order hierarchy, yet
     // clipping goes down the layoutObject hierarchy. Thus, a PaintLayer can be clipped by a
     // PaintLayer 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(scrollParent);
+    bool needsAncestorClip = owningLayerClippedByLayerNotAboveCompositedAncestor(scrollParent)
+        && !m_owningLayer.clippingContainer()->enclosingLayer()->hasRootScrollerAsDescendant();
 
     if (updateClippingLayers(needsAncestorClip, needsDescendantsClippingLayer))
         layerConfigChanged = true;
 
     bool scrollingConfigChanged = false;
     if (updateScrollingLayers(m_owningLayer.needsCompositedScrolling())) {
         layerConfigChanged = true;
         scrollingConfigChanged = true;
     }
 
@@ skipping 1212 matching lines @@
 bool CompositedLayerMapping::updateScrollingLayers(bool needsScrollingLayers)
 {
     ScrollingCoordinator* scrollingCoordinator = scrollingCoordinatorFromLayer(m_owningLayer);
 
     bool layerChanged = false;
     if (needsScrollingLayers) {
         if (!m_scrollingLayer) {
             // Outer layer which corresponds with the scroll view.
             m_scrollingLayer = createGraphicsLayer(CompositingReasonLayerForScrollingContainer);
             m_scrollingLayer->setDrawsContent(false);
-            m_scrollingLayer->setMasksToBounds(true);
 
             // Inner layer which renders the content that scrolls.
             m_scrollingContentsLayer = createGraphicsLayer(CompositingReasonLayerForScrollingContents);
 
             if (Node* owningNode = m_owningLayer.layoutObject()->node())
                 m_scrollingContentsLayer->setElementId(createCompositorElementId(DOMNodeIds::idForNode(owningNode), CompositorSubElementId::Scroll));
 
             m_scrollingLayer->addChild(m_scrollingContentsLayer.get());
 
             layerChanged = true;
             if (scrollingCoordinator) {
                 scrollingCoordinator->scrollableAreaScrollLayerDidChange(m_owningLayer.getScrollableArea());
                 scrollingCoordinator->scrollableAreasDidChange();
             }
         }
+
+        m_scrollingLayer->setMasksToBounds(
+            !m_owningLayer.hasRootScrollerAsDescendant());
     } else if (m_scrollingLayer) {
         m_scrollingLayer = nullptr;
         m_scrollingContentsLayer = nullptr;
         layerChanged = true;
         if (scrollingCoordinator) {
             scrollingCoordinator->scrollableAreaScrollLayerDidChange(m_owningLayer.getScrollableArea());
             scrollingCoordinator->scrollableAreasDidChange();
         }
     }
 
@@ skipping 987 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink