Always give RenderParts a RenderLayer.

Source/core/rendering/RenderBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 1055 matching lines @@
     return max<LayoutUnit>(0, height);
 }
 
 // Hit Testing
 bool RenderBox::nodeAtPoint(const HitTestRequest& request, HitTestResult& result, const HitTestLocation& locationInContainer, const LayoutPoint& accumulatedOffset, HitTestAction action)
 {
     LayoutPoint adjustedLocation = accumulatedOffset + location();
 
     // Check kids first.
     for (RenderObject* child = slowLastChild(); child; child = child->previousSibling()) {
-        if (!child->hasLayer() && child->nodeAtPoint(request, result, locationInContainer, adjustedLocation, action)) {
+        if ((!child->hasLayer() || !toRenderLayerModelObject(child)->layer()->isSelfPaintingLayer()) && child->nodeAtPoint(request, result, locationInContainer, adjustedLocation, action)) {
             updateHitTestResult(result, locationInContainer.point() - toLayoutSize(adjustedLocation));
             return true;
         }
     }
 
     // Check our bounds next. For this purpose always assume that we can only be hit in the
     // foreground phase (which is true for replaced elements like images).
     LayoutRect boundsRect = borderBoxRect();
     boundsRect.moveBy(adjustedLocation);
     if (visibleToHitTestRequest(request) && action == HitTestForeground && locationInContainer.intersects(boundsRect)) {
@@ skipping 3608 matching lines @@
         return 0;
 
     if (!layoutState && !flowThreadContainingBlock())
         return 0;
 
     RenderBlock* containerBlock = containingBlock();
     return containerBlock->offsetFromLogicalTopOfFirstPage() + logicalTop();
 }
 
 } // namespace WebCore