Update scroll parent correctly for squashing layers.

Source/core/rendering/compositing/CompositedLayerMapping.cpp

 /*
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  *
  * 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
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@@ skipping 852 matching lines @@
 
     updateHasGpuRasterizationHint(renderer()->style());
     updateContentsRect();
     updateBackgroundColor();
     updateDrawsContent();
     updateContentsOpaque();
     updateAfterWidgetResize();
     updateRenderingContext();
     updateShouldFlattenTransform();
     updateChildrenTransform();
+    updateScrollParent(m_owningLayer.scrollParent());

[abarth-chromium, 2014/03/12 18:45:40]

This function walks up the RenderLayer tree.  Instead, we should remember this
information as we walk down the RenderLayer tree.

[Ian Vollick, 2014/03/12 20:03:32]

Agreed, though it's a little tricky since a layer's scroll parent depends on its
containing block. We'll need to pass down a collection of scroll parents for the
variously positioned descendants (like we do for RenderGeometryMap). This is a
biggish refactor and I'm hoping I can address it in a separate patch. I've
created a bug (http://crbug.com/351851) and added FIXMES if you're ok with that.

     registerScrollingLayers();
 
     updateCompositingReasons();
 
     return updateType;
 }
 
 void CompositedLayerMapping::registerScrollingLayers()
 {
     // Register fixed position layers and their containers with the scrolling coordinator.
@@ skipping 506 matching lines @@
 
     // Only the topmost layer has a scroll parent. All other layers have a null scroll parent.
     if (layer != topmostLayer)
         scrollParent = 0;
 
     scrollingCoordinator->updateScrollParentForGraphicsLayer(layer, scrollParent);
 }
 
 void CompositedLayerMapping::updateScrollParent(RenderLayer* scrollParent)
 {
+    if (!scrollParent && m_squashedLayers.size())
+        scrollParent = m_squashedLayers[0].renderLayer->scrollParent();
 
     if (ScrollingCoordinator* scrollingCoordinator = scrollingCoordinatorFromLayer(m_owningLayer)) {
-        GraphicsLayer* topmostLayer = localRootForOwningLayer();
+        GraphicsLayer* topmostLayer = childForSuperlayers();
+        updateScrollParentForGraphicsLayer(m_squashingContainmentLayer.get(), topmostLayer, scrollParent, scrollingCoordinator);
         updateScrollParentForGraphicsLayer(m_ancestorClippingLayer.get(), topmostLayer, scrollParent, scrollingCoordinator);
         updateScrollParentForGraphicsLayer(m_graphicsLayer.get(), topmostLayer, scrollParent, scrollingCoordinator);
     }
 }
 
 void CompositedLayerMapping::updateClipParent(RenderLayer* clipParent)
 {
     if (ScrollingCoordinator* scrollingCoordinator = scrollingCoordinatorFromLayer(m_owningLayer))
         scrollingCoordinator->updateClipParentForGraphicsLayer(m_graphicsLayer.get(), clipParent);
 }
@@ skipping 713 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore