Initial support for nested multicol layout.

Source/core/layout/LayoutMultiColumnFlowThread.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 150 matching lines @@
     unsigned columnCount() const { return m_columnCount; }
     LayoutUnit columnHeightAvailable() const { return m_columnHeightAvailable; }
     void setColumnHeightAvailable(LayoutUnit available) { m_columnHeightAvailable = available; }
     bool progressionIsInline() const { return m_progressionIsInline; }
 
     LayoutSize columnOffset(const LayoutPoint&) const final;
 
     // Do we need to set a new width and lay out?
     virtual bool needsNewWidth() const;
 
+    bool isPageLogicalHeightKnown() const final;
+
+    LayoutSize flowThreadTranslationAtOffset(LayoutUnit) const;
+
     LayoutPoint visualPointToFlowThreadPoint(const LayoutPoint& visualPoint) const override;
 
     LayoutMultiColumnSet* columnSetAtBlockOffset(LayoutUnit) const final;
 
     void layoutColumns(bool relayoutChildren, SubtreeLayoutScope&);
 
+    bool isInInitialLayoutPass() const { return !m_inBalancingPass; }
+
     bool recalculateColumnHeights();
 
     void columnRuleStyleDidChange();
 
     // Remove the spanner placeholder and return true if the specified object is no longer a valid spanner.
     bool removeSpannerPlaceholderIfNoLongerValid(LayoutBox* spannerObjectInFlowThread);
 
+    LayoutMultiColumnFlowThread* enclosingFlowThread() const;
+    LayoutUnit blockOffsetInEnclosingFlowThread() const { ASSERT(enclosingFlowThread()); return m_blockOffsetInEnclosingFlowThread; }
+
+    // Return true if we have a fragmentainer group that can hold a column at the specified flow thread block offset.
+    bool hasFragmentainerGroupForColumnAt(LayoutUnit offsetInFlowThread) const;
+
+    // If we've run out of columns in the last fragmentainer group (column row), we have to insert
+    // another fragmentainer group in order to hold more columns. This means that we're moving to
+    // the next outer column (in the enclosing fragmentation context).
+    void appendNewFragmentainerGroupIfNeeded(LayoutUnit offsetInFlowThread);
+
     const char* name() const override { return "LayoutMultiColumnFlowThread"; }
 
 protected:
     LayoutMultiColumnFlowThread();
     void setProgressionIsInline(bool isInline) { m_progressionIsInline = isInline; }
 
     void layout() override;
 
 private:
     void calculateColumnCountAndWidth(LayoutUnit& width, unsigned& count) const;
     void createAndInsertMultiColumnSet(LayoutBox* insertBefore = nullptr);
     void createAndInsertSpannerPlaceholder(LayoutBox* spannerObjectInFlowThread, LayoutObject* insertedBeforeInFlowThread);
     void destroySpannerPlaceholder(LayoutMultiColumnSpannerPlaceholder*);
     virtual bool descendantIsValidColumnSpanner(LayoutObject* descendant) const;
 
     void addColumnSetToThread(LayoutMultiColumnSet*) override;
     void willBeRemovedFromTree() override;
     void skipColumnSpanner(LayoutBox*, LayoutUnit logicalTopInFlowThread) override;
     void flowThreadDescendantWasInserted(LayoutObject*) final;
     void flowThreadDescendantWillBeRemoved(LayoutObject*) final;
     void flowThreadDescendantStyleWillChange(LayoutObject*, StyleDifference, const ComputedStyle& newStyle) override;
     void flowThreadDescendantStyleDidChange(LayoutObject*, StyleDifference, const ComputedStyle& oldStyle) override;
     void computePreferredLogicalWidths() override;
     void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;
     void updateLogicalWidth() override;
     void setPageBreak(LayoutUnit offset, LayoutUnit spaceShortage) override;
     void updateMinimumPageHeight(LayoutUnit offset, LayoutUnit minHeight) override;
     bool addForcedColumnBreak(LayoutUnit, LayoutObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = nullptr) override;
-    bool isPageLogicalHeightKnown() const override;
 
     // The last set we worked on. It's not to be used as the "current set". The concept of a
     // "current set" is difficult, since layout may jump back and forth in the tree, due to wrong
     // top location estimates (due to e.g. margin collapsing), and possibly for other reasons.
     LayoutMultiColumnSet* m_lastSetWorkedOn;
 
     unsigned m_columnCount; // The used value of column-count
     LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
+
+    // Cached block offset from this flow thread to the enclosing flow thread, if any. In the
+    // coordinate space of the enclosing flow thread.
+    LayoutUnit m_blockOffsetInEnclosingFlowThread;
+
     bool m_inBalancingPass; // Set when relayouting for column balancing.
     bool m_needsColumnHeightsRecalculation; // Set when we need to recalculate the column set heights after layout.
     bool m_progressionIsInline; // Always true for regular multicol. False for paged-y overflow.
     bool m_isBeingEvacuated;
 };
 
 // Cannot use DEFINE_LAYOUT_OBJECT_TYPE_CASTS here, because isMultiColumnFlowThread() is defined in
 // LayoutFlowThread, not in LayoutObject.
 DEFINE_TYPE_CASTS(LayoutMultiColumnFlowThread, LayoutFlowThread, object, object->isLayoutMultiColumnFlowThread(), object.isLayoutMultiColumnFlowThread());
 
 } // namespace blink
 
 #endif // LayoutMultiColumnFlowThread_h