LayoutView's paint layer should clip to the full viewport size.

third_party/WebKit/Source/core/paint/PaintLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 2409 matching lines @@
     if (layerBounds.intersects(damageRect))
       return true;
   }
 
   // Otherwise we need to compute the bounding box of this single layer and see
   // if it intersects the damage rect.
   return physicalBoundingBox(offsetFromRoot).intersects(damageRect);
 }
 
 LayoutRect PaintLayer::logicalBoundingBox() const {
-  return layoutObject()->visualOverflowRect();
+  LayoutRect rect = layoutObject()->visualOverflowRect();
+
+  if (isRootLayer()) {
+    rect.unite(
+        LayoutRect(rect.location(), layoutObject()->view()->viewRect().size()));
+  }
+
+  return rect;
 }
 
 static inline LayoutRect flippedLogicalBoundingBox(
     LayoutRect boundingBox,
     LayoutObject* layoutObjects) {
   LayoutRect result = boundingBox;
   if (layoutObjects->isBox())
     toLayoutBox(layoutObjects)->flipForWritingMode(result);
   else
     layoutObjects->containingBlock()->flipForWritingMode(result);
@@ skipping 93 matching lines @@
 
   if (!ancestorLayer)
     ancestorLayer = this;
 
   // FIXME: This could be improved to do a check like
   // hasVisibleNonCompositingDescendantLayers() (bug 92580).
   if (this != ancestorLayer && !hasVisibleContent() && !hasVisibleDescendant())
     return LayoutRect();
 
   // Without composited scrolling, the root layer is the size of the document.
-  if (isRootLayer() && !needsCompositedScrolling())
-    return LayoutRect(m_layoutObject->view()->documentRect());
+  if (isRootLayer() && !needsCompositedScrolling()) {
+    IntRect documentRect = layoutObject()->view()->documentRect();
+
+    if (FrameView* frameView = layoutObject()->document().view()) {
+      documentRect.unite(IntRect(IntPoint(), frameView->visibleContentSize()));
+    }
+
+    return LayoutRect(documentRect);
+  }
 
   // The layer created for the LayoutFlowThread is just a helper for painting
   // and hit-testing, and should not contribute to the bounding box. The
   // LayoutMultiColumnSets will contribute the correct size for the layout
   // content of the multicol container.
   if (layoutObject()->isLayoutFlowThread())
     return LayoutRect();
 
   // If there is a clip applied by an ancestor to this PaintLayer but below or
   // equal to |ancestorLayer|, use that clip as the bounds rather than the
@@ skipping 694 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->enclosingLayer());
 }
 #endif