[New Multicolumn] Minimal support for nested multicol in RenderLayer.

Source/core/rendering/RenderLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. 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 197 matching lines @@
 
     bool canUseConvertToLayerCoords() const
     {
         // These RenderObjects have an impact on their layers without the renderers knowing about it.
         return !renderer()->hasColumns() && !renderer()->hasTransformRelatedProperty() && !renderer()->isSVGRoot();
     }
 
     void convertToLayerCoords(const RenderLayer* ancestorLayer, LayoutPoint&) const;
     void convertToLayerCoords(const RenderLayer* ancestorLayer, LayoutRect&) const;
 
+    // Does the same as convertToLayerCoords() when not in multicol. For multicol, however,
+    // convertToLayerCoords() calculates the offset in flow-thread coordinates (what the layout
+    // engine uses internally), while this method calculates the visual coordinates; i.e. it figures
+    // out which column the layer starts in and adds in the offset. See
+    // http://www.chromium.org/developers/design-documents/multi-column-layout for more info.
     LayoutPoint visualOffsetFromAncestor(const RenderLayer* ancestorLayer) const;
 
     // The hitTest() method looks for mouse events by walking layers that intersect the point from front to back.
     bool hitTest(const HitTestRequest&, HitTestResult&);
     bool hitTest(const HitTestRequest&, const HitTestLocation&, HitTestResult&);
 
     // Pass offsetFromRoot if known.
     bool intersectsDamageRect(const LayoutRect& layerBounds, const LayoutRect& damageRect, const RenderLayer* rootLayer, const LayoutPoint* offsetFromRoot = 0) const;
 
     // Bounding box relative to some ancestor layer. Pass offsetFromRoot if known.
@@ skipping 441 matching lines @@
     // The layer's width/height
     IntSize m_size;
 
     // Cached normal flow values for absolute positioned elements with static left/top values.
     LayoutUnit m_staticInlinePosition;
     LayoutUnit m_staticBlockPosition;
 
     OwnPtr<TransformationMatrix> m_transform;
 
     // Pointer to the enclosing RenderLayer that caused us to be paginated. It is 0 if we are not paginated.
+    //
+    // See RenderMultiColumnFlowThread and
+    // https://sites.google.com/a/chromium.org/dev/developers/design-documents/multi-column-layout
+    // for more information about the multicol implementation. It's important to understand the
+    // difference between flow thread coordinates and visual coordinates when working with multicol
+    // in RenderLayer, since RenderLayer is one of the few places where we have to worry about the
+    // visual ones. Internally we try to use flow-thread coordinates whenever possible.
     RenderLayer* m_enclosingPaginationLayer;
 
     // These compositing reasons are updated whenever style changes, not while updating compositing layers.
     // They should not be used to infer the compositing state of this layer.
     CompositingReasons m_potentialCompositingReasonsFromStyle;
 
     // Once computed, indicates all that a layer needs to become composited using the CompositingReasons enum bitfield.
     CompositingReasons m_compositingReasons;
 
     DescendantDependentCompositingInputs m_descendantDependentCompositingInputs;
@@ skipping 15 matching lines @@
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const blink::RenderLayer*);
 void showLayerTree(const blink::RenderObject*);
 #endif
 
 #endif // RenderLayer_h