[New Multicolumn] Support dynamic insertion and removal of multicol content.

Source/core/rendering/RenderMultiColumnFlowThread.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
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 // make sure that the columns are painted and hit-tested correctly.
 //
 // If there is any column content inside the multicol container, we create a
 // RenderMultiColumnSet. We only need to create multiple sets if there are spanners
 // (column-span:all) in the multicol container. When a spanner is inserted, content preceding it
 // gets its own set, and content succeeding it will get another set. The spanner itself will also
 // get its own placeholder between the sets (RenderMultiColumnSpannerPlaceholder), so that it gets
 // positioned and sized correctly. The column-span:all element is inside the flow thread, but its
 // containing block is the multicol container.
 //
+// Some invariants for the render tree structure for multicol:
+// - A multicol container is always a RenderBlockFlow
+// - Every multicol container has one and only one RenderMultiColumnFlowThread
+// - All multicol DOM children and pseudo-elements associated with the multicol container are
+//   reparented into the flow thread
+// - The RenderMultiColumnFlowThread is the first child of the multicol container
+// - A multicol container may only have RenderMultiColumnFlowThread, RenderMultiColumnSet and
+//   RenderMultiColumnSpannerPlaceholder children
+// - A RenderMultiColumnSet may not be adjacent to another RenderMultiColumnSet; there are no
+//   use-cases for it, and there are also implementation limitations behind this requirement.
+// - The flow thread is not in the containing block chain for children that are not to be laid out
+//   in columns. This means column spanners and absolutely positioned children whose containing
+//   block is outside column content
+// - Each spanner (column-span:all) establishes a RenderMultiColumnSpannerPlaceholder
+//
 // The width of the flow thread is the same as the column width. The width of a column set is the
 // same as the content box width of the multicol container; in other words exactly enough to hold
 // the number of columns to be used, stacked horizontally, plus column gaps between them.
 //
 // Since it's the first child of the multicol container, the flow thread is laid out first, albeit
 // in a slightly special way, since it's not to take up any space in its ancestors. Afterwards, the
 // column sets are laid out. Column sets get their height from the columns that they hold. In single
 // column-row constrained height non-balancing cases without spanners this will simply be the same
 // as the content height of the multicol container itself. In most other cases we'll have to
 // calculate optimal column heights ourselves, though. This process is referred to as column
@@ skipping 86 matching lines @@
     void columnRuleStyleDidChange();
 
 protected:
     RenderMultiColumnFlowThread();
     void setProgressionIsInline(bool isInline) { m_progressionIsInline = isInline; }
 
     virtual void layout() override;
 
 private:
     void calculateColumnCountAndWidth(LayoutUnit& width, unsigned& count) const;
-    void createAndInsertMultiColumnSet();
-    void createAndInsertSpannerPlaceholder(RenderBox* spanner);
+    void createAndInsertMultiColumnSet(RenderBox* insertBefore = 0);
+    void createAndInsertSpannerPlaceholder(RenderBox* spanner, RenderBox* insertBefore = 0);
     virtual bool descendantIsValidColumnSpanner(RenderObject* descendant) const;
 
     virtual const char* renderName() const override;
     virtual void addRegionToThread(RenderMultiColumnSet*) override;
     virtual void willBeRemovedFromTree() override;
     virtual LayoutUnit skipColumnSpanner(RenderBox*, LayoutUnit logicalTopInFlowThread) override;
     virtual void flowThreadDescendantWasInserted(RenderObject*) override;
     virtual void flowThreadDescendantWillBeRemoved(RenderObject*) override;
     virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;
     virtual void updateLogicalWidth() override;
@@ skipping 11 matching lines @@
     LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
     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;
 };
 
 } // namespace blink
 
 #endif // RenderMultiColumnFlowThread_h