Restore sanity to RenderLayerStackingNode::shouldBeNormalFlowOnly

Source/core/rendering/RenderLayerStackingNode.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>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 290 matching lines @@
     dirtyStackingContextZOrderLists();
 
     if (isStackingContext)
         dirtyZOrderLists();
     else
         clearZOrderLists();
 }
 
 bool RenderLayerStackingNode::shouldBeNormalFlowOnly() const
 {
-    RenderLayerModelObject* renderer = this->renderer();
-
-    const bool couldBeNormalFlow = renderer->hasOverflowClip()
-        || renderer->hasReflection()
-        || renderer->hasMask()
-        || renderer->isCanvas()
-        || renderer->isVideo()
-        || renderer->isEmbeddedObject()
-        || renderer->isRenderIFrame()
-        || (renderer->style()->specifiesColumns() && !layer()->isRootLayer());
-
-    const bool preventsElementFromBeingNormalFlow = renderer->isPositioned()
-        || renderer->hasTransform()
-        || renderer->hasClipPath()
-        || renderer->hasFilter()
-        || renderer->hasBlendMode()
-        || layer()->isTransparent();
-
-    return couldBeNormalFlow && !preventsElementFromBeingNormalFlow;
+    return !isStackingContext() && !renderer()->isPositioned();

[Julien - ping for review, 2014/06/03 00:53:20]

Based on the comments in the file, it really looks like "normal flow" just means
overflow-only layer (which is close to being a non-self-painting-layer). We
should amend these comments else it's going to be confusing :-)

Now CSS 2.1 defines normal flow as not absolutely / fixed positioned and not
floating. I think we should use this definition here so:
- it's missing a isFloating() check.
- isPositioned() is basically position != static so it should be
isOutOfFlowPositioned() instead.
- Is the stackingContext() call needed? If so, I would like a comment.

 }
 
 void RenderLayerStackingNode::updateIsNormalFlowOnly()
 {
     bool isNormalFlowOnly = shouldBeNormalFlowOnly();
     if (isNormalFlowOnly == this->isNormalFlowOnly())
         return;
 
     m_isNormalFlowOnly = isNormalFlowOnly;
     if (RenderLayer* p = layer()->parent())
@@ skipping 10 matching lines @@
     }
     return 0;
 }
 
 RenderLayerModelObject* RenderLayerStackingNode::renderer() const
 {
     return m_layer->renderer();
 }
 
 } // namespace WebCore