Reland of Improve how the column balancer handles top margins on floats.

third_party/WebKit/Source/core/layout/ColumnBalancer.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/LayoutMultiColumnSet.h"
 
 namespace blink {
 
 // A column balancer traverses a portion of the subtree of a flow thread that
 // belongs to one or more fragmentainer groups within one column set, in order
@@ skipping 55 matching lines @@
            logicalBottomInFlowThread <= m_logicalBottomInFlowThread;
   }
 
   // Examine and collect column balancing data from a layout box that has been
   // found to intersect with the flow thread portion we're examining. Does not
   // recurse into children. flowThreadOffset() will return the offset from |box|
   // to the flow thread. Two hooks are provided here. The first one is called
   // right after entering and before traversing the subtree of the box, and the
   // second one right after having traversed the subtree.
   virtual void examineBoxAfterEntering(const LayoutBox&,
+                                       LayoutUnit childLogicalHeight,
                                        EBreak previousBreakAfterValue) = 0;
-  virtual void examineBoxBeforeLeaving(const LayoutBox&) = 0;
+  virtual void examineBoxBeforeLeaving(const LayoutBox&,
+                                       LayoutUnit childLogicalHeight) = 0;
 
   // Examine and collect column balancing data from a line that has been found
   // to intersect with the flow thread portion. Does not recurse into layout
   // objects on that line.
   virtual void examineLine(const RootInlineBox&) = 0;
 
   // Examine and collect column balancing data for everything in the flow thread
   // portion. Will trigger calls to examineBoxAfterEntering(),
   // examineBoxBeforeLeaving() and examineLine() for interesting boxes and
   // lines.
@@ skipping 34 matching lines @@
   // column logical height required to avoid fragmentation where it shouldn't
   // occur (inside unbreakable content, between orphans and widows, etc.). This
   // will be used as a hint to the column balancer to help set a good initial
   // column height.
   LayoutUnit tallestUnbreakableLogicalHeight() const {
     return m_tallestUnbreakableLogicalHeight;
   }
 
  private:
   void examineBoxAfterEntering(const LayoutBox&,
+                               LayoutUnit childLogicalHeight,
                                EBreak previousBreakAfterValue);
-  void examineBoxBeforeLeaving(const LayoutBox&);
+  void examineBoxBeforeLeaving(const LayoutBox&, LayoutUnit childLogicalHeight);
   void examineLine(const RootInlineBox&);
 
   // Record that there's a pagination strut that ends at the specified
   // |offsetInFlowThread|, which is an offset exactly at the top of some column.
   void recordStrutBeforeOffset(LayoutUnit offsetInFlowThread, LayoutUnit strut);
 
   // Return the accumulated space used by struts at all column boundaries
   // preceding the specified flowthread offset.
   LayoutUnit spaceUsedByStrutsAt(LayoutUnit offsetInFlowThread) const;
 
@@ skipping 72 matching lines @@
  public:
   MinimumSpaceShortageFinder(const LayoutMultiColumnSet&,
                              LayoutUnit logicalTopInFlowThread,
                              LayoutUnit logicalBottomInFlowThread);
 
   LayoutUnit minimumSpaceShortage() const { return m_minimumSpaceShortage; }
   unsigned forcedBreaksCount() const { return m_forcedBreaksCount; }
 
  private:
   void examineBoxAfterEntering(const LayoutBox&,
+                               LayoutUnit childLogicalHeight,
                                EBreak previousBreakAfterValue);
-  void examineBoxBeforeLeaving(const LayoutBox&);
+  void examineBoxBeforeLeaving(const LayoutBox&, LayoutUnit childLogicalHeight);
   void examineLine(const RootInlineBox&);
 
   void recordSpaceShortage(LayoutUnit shortage) {
     // Only positive values are interesting (and allowed) here. Zero space
     // shortage may be reported when we're at the top of a column and the
     // element has zero height.
     if (shortage > 0)
       m_minimumSpaceShortage = std::min(m_minimumSpaceShortage, shortage);
   }
 
   // The smallest amout of space shortage that caused a column break.
   LayoutUnit m_minimumSpaceShortage;
 
   // Set when breaking before a block, and we're looking for the first
   // unbreakable descendant, in order to report correct space shortage for that
   // one.
   LayoutUnit m_pendingStrut;
 
   unsigned m_forcedBreaksCount;
 };
 
 }  // namespace blink