Revert of Revert of Put the squashing layer underneath the ancestor clipping layer if present.

Source/core/rendering/compositing/CompositedLayerMapping.cpp

Index: Source/core/rendering/compositing/CompositedLayerMapping.cpp
diff --git a/Source/core/rendering/compositing/CompositedLayerMapping.cpp b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
index f03f62614a7c6c6d19eca779878a3618ab8fa14f..7a67e4caed9214ffd420db7074fde67742d0eb04 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
@@ -878,7 +878,11 @@
         }
     }
 
-    updateSquashingLayerGeometry(delta);
+    {
+        IntPoint squashingDelta(delta);
+        squashingDelta.moveBy(-graphicsLayerParentLocation);
+        updateSquashingLayerGeometry(squashingDelta);
+    }
 
     if (m_owningLayer.scrollableArea() && m_owningLayer.scrollableArea()->scrollsOverflow())
         m_owningLayer.scrollableArea()->positionOverflowControls();
@@ -970,16 +974,16 @@
     // The squashing containment layer, if it exists, becomes a no-op parent.
     if (m_squashingLayer) {
         ASSERT(compositor()->layerSquashingEnabled());
-        ASSERT(m_squashingContainmentLayer);
-
-        m_squashingContainmentLayer->removeAllChildren();
-
-        if (m_ancestorClippingLayer)
-            m_squashingContainmentLayer->addChild(m_ancestorClippingLayer.get());
-        else
+        ASSERT((m_ancestorClippingLayer && !m_squashingContainmentLayer) || (!m_ancestorClippingLayer && m_squashingContainmentLayer));
+
+        if (m_squashingContainmentLayer) {
+            m_squashingContainmentLayer->removeAllChildren();
             m_squashingContainmentLayer->addChild(m_graphicsLayer.get());
-
-        m_squashingContainmentLayer->addChild(m_squashingLayer.get());
+            m_squashingContainmentLayer->addChild(m_squashingLayer.get());
+        } else {
+            // The ancestor clipping layer is already set up and has m_graphicsLayer under it.
+            m_ancestorClippingLayer->addChild(m_squashingLayer.get());
+        }
     }
 }
 
@@ -1459,23 +1463,23 @@
             m_squashingLayer->setDrawsContent(true);
 
             // FIXME: containment layer needs a new CompositingReason, CompositingReasonOverlap is not appropriate.
-            m_squashingContainmentLayer = createGraphicsLayer(CompositingReasonLayerForSquashingContainer);
+            if (!m_ancestorClippingLayer)
+                m_squashingContainmentLayer = createGraphicsLayer(CompositingReasonLayerForSquashingContainer);
             layersChanged = true;
         }
 
-        ASSERT(m_squashingLayer && m_squashingContainmentLayer);
+        ASSERT(m_squashingLayer);
     } else {
         if (m_squashingLayer) {
             m_squashingLayer->removeFromParent();
             m_squashingLayer = nullptr;
-            // FIXME: do we need to invalidate something here?
-
-            ASSERT(m_squashingContainmentLayer);
+            layersChanged = true;
+        }
+        if (m_squashingContainmentLayer) {
             m_squashingContainmentLayer->removeFromParent();
             m_squashingContainmentLayer = nullptr;
             layersChanged = true;
         }
-
         ASSERT(!m_squashingLayer && !m_squashingContainmentLayer);
     }