Initial support for nested multicol layout.

Source/core/layout/LayoutMultiColumnSet.h

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 46 matching lines @@
 // is inside another fragmentation context (e.g. paged media, or an another multicol container), we
 // may need to group the columns, so that we get one MultiColumnFragmentainerGroup for each outer
 // fragmentainer (page / column) that the inner multicol container lives in. Each fragmentainer
 // group has its own column height, but the column height is uniform within a group.
 class CORE_EXPORT LayoutMultiColumnSet : public LayoutBlockFlow {
 public:
     static LayoutMultiColumnSet* createAnonymous(LayoutFlowThread&, const ComputedStyle& parentStyle);
 
     const MultiColumnFragmentainerGroup& firstFragmentainerGroup() const { return m_fragmentainerGroups.first(); }
     const MultiColumnFragmentainerGroup& lastFragmentainerGroup() const { return m_fragmentainerGroups.last(); }
-    MultiColumnFragmentainerGroup& fragmentainerGroupAtFlowThreadOffset(LayoutUnit);
-    const MultiColumnFragmentainerGroup& fragmentainerGroupAtFlowThreadOffset(LayoutUnit) const;
+    MultiColumnFragmentainerGroup& fragmentainerGroupAtFlowThreadOffset(LayoutUnit flowThreadOffset)
+    {
+        return m_fragmentainerGroups[fragmentainerGroupIndexAtFlowThreadOffset(flowThreadOffset)];
+    }
+    const MultiColumnFragmentainerGroup& fragmentainerGroupAtFlowThreadOffset(LayoutUnit flowThreadOffset) const
+    {
+        return m_fragmentainerGroups[fragmentainerGroupIndexAtFlowThreadOffset(flowThreadOffset)];
+    }
     const MultiColumnFragmentainerGroup& fragmentainerGroupAtVisualPoint(const LayoutPoint&) const;
 
     bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectLayoutMultiColumnSet || LayoutBlockFlow::isOfType(type); }
     bool canHaveChildren() const final { return false; }
 
     // Return the width and height of a single column or page in the set.
     LayoutUnit pageLogicalWidth() const { return flowThread()->logicalWidth(); }
     LayoutUnit pageLogicalHeightForOffset(LayoutUnit) const;
     LayoutUnit pageRemainingLogicalHeightForOffset(LayoutUnit, PageBoundaryRule) const;
     bool isPageLogicalHeightKnown() const;
 
     LayoutFlowThread* flowThread() const { return m_flowThread; }
 
     LayoutBlockFlow* multiColumnBlockFlow() const { return toLayoutBlockFlow(parent()); }
     LayoutMultiColumnFlowThread* multiColumnFlowThread() const { return toLayoutMultiColumnFlowThread(flowThread()); }
 
     LayoutMultiColumnSet* nextSiblingMultiColumnSet() const;
     LayoutMultiColumnSet* previousSiblingMultiColumnSet() const;
 
+    MultiColumnFragmentainerGroup& appendNewFragmentainerGroup();
+
     LayoutUnit logicalTopInFlowThread() const;
     LayoutUnit logicalBottomInFlowThread() const;
     LayoutUnit logicalHeightInFlowThread() const { return logicalBottomInFlowThread() - logicalTopInFlowThread(); }
     LayoutRect flowThreadPortionRect() const;
     LayoutRect flowThreadPortionOverflowRect() const;
     LayoutRect overflowRectForFlowThreadPortion(const LayoutRect& flowThreadPortionRect, bool isFirstPortion, bool isLastPortion) const;
 
     // The used CSS value of column-count, i.e. how many columns there are room for without overflowing.
     unsigned usedColumnCount() const { return multiColumnFlowThread()->columnCount(); }
 
@@ skipping 53 matching lines @@
 
     // The "CSS actual" value of column-count. This includes overflowing columns, if any.
     unsigned actualColumnCount() const;
 
     const char* name() const override { return "LayoutMultiColumnSet"; }
 
 protected:
     LayoutMultiColumnSet(LayoutFlowThread*);
 
 private:
+    unsigned fragmentainerGroupIndexAtFlowThreadOffset(LayoutUnit) const;
+
     void insertedIntoTree() final;
     void willBeRemovedFromTree() final;
 
     bool isSelfCollapsingBlock() const override { return false; }
 
     void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;
     PositionWithAffinity positionForPoint(const LayoutPoint&) override;
 
     void paintObject(const PaintInfo&, const LayoutPoint& paintOffset) override;
 
     void addOverflowFromChildren() override;
 
     MultiColumnFragmentainerGroupList m_fragmentainerGroups;
     LayoutFlowThread* m_flowThread;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutMultiColumnSet, isLayoutMultiColumnSet());
 
 } // namespace blink
 
 #endif // LayoutMultiColumnSet_h