Distinguish between row (fragmentainer group) height and column (fragmentainer) height.

third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.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.
 
 #ifndef MultiColumnFragmentainerGroup_h
 #define MultiColumnFragmentainerGroup_h
 
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 #include "platform/wtf/Allocator.h"
 
@@ skipping 26 matching lines @@
 
   const LayoutMultiColumnSet& ColumnSet() const { return column_set_; }
 
   bool IsFirstGroup() const;
   bool IsLastGroup() const;
 
   // Position within the LayoutMultiColumnSet.
   LayoutUnit LogicalTop() const { return logical_top_; }
   void SetLogicalTop(LayoutUnit logical_top) { logical_top_ = logical_top; }
 
-  LayoutUnit LogicalHeight() const { return column_height_; }
+  // Return the amount of block space that this fragmentainer group takes up in
+  // its containing LayoutMultiColumnSet.
+  LayoutUnit GroupLogicalHeight() const { return logical_height_; }
+
+  // Return the block size of a column (or fragmentainer) in this fragmentainer
+  // group. The spec says that this value must always be >= 1px, to ensure

[eae, 2017/05/12 16:31:21]

These comments are great! Thank you.

+  // progress.
+  LayoutUnit ColumnLogicalHeight() const {
+    // If the height hasn't been calculated yet, though, we allow returning 0
+    // (and the caller is then expected to refrain from attempting to fragment).
+    if (!logical_height_)
+      return logical_height_;
+
+    return std::max(LayoutUnit(1), logical_height_);
+  }
 
   LayoutSize OffsetFromColumnSet() const;
 
   // Return the block offset from the enclosing fragmentation context, if
   // nested. In the coordinate space of the enclosing fragmentation context.
   LayoutUnit BlockOffsetInEnclosingFragmentationContext() const;
 
   // The top of our flow thread portion
   LayoutUnit LogicalTopInFlowThread() const {
     return logical_top_in_flow_thread_;
@@ skipping 91 matching lines @@
 
  private:
   LayoutUnit HeightAdjustedForRowOffset(LayoutUnit height) const;
   LayoutUnit CalculateMaxColumnHeight() const;
   void SetAndConstrainColumnHeight(LayoutUnit);
 
   LayoutUnit RebalanceColumnHeightIfNeeded() const;
 
   LayoutRect ColumnRectAt(unsigned column_index) const;
   LayoutUnit LogicalTopInFlowThreadAt(unsigned column_index) const {
-    return logical_top_in_flow_thread_ + column_index * column_height_;
+    return logical_top_in_flow_thread_ + column_index * ColumnLogicalHeight();
   }
 
   // Return the column that the specified visual point belongs to. Only the
   // coordinate on the column progression axis is relevant. Every point belongs
   // to a column, even if said point is not inside any of the columns.
   unsigned ColumnIndexAtVisualPoint(const LayoutPoint& visual_point) const;
 
   const LayoutMultiColumnSet& column_set_;
 
   LayoutUnit logical_top_;
   LayoutUnit logical_top_in_flow_thread_;
   LayoutUnit logical_bottom_in_flow_thread_;
 
-  LayoutUnit column_height_;
+  // Logical height of the group. This will also be the height of each column
+  // in this group, with the difference that, while the logical height can be
+  // 0, the height of a column must be >= 1px.
+  LayoutUnit logical_height_;
 
-  LayoutUnit max_column_height_;  // Maximum column height allowed.
+  // Maximum logical height allowed.
+  LayoutUnit max_logical_height_;
 };
 
 // List of all fragmentainer groups within a column set. There will always be at
 // least one group. Deleting the one group is not allowed (or possible). There
 // will be more than one group if the owning column set lives in multiple outer
 // fragmentainers (e.g. multicol inside paged media).
 class CORE_EXPORT MultiColumnFragmentainerGroupList {
   DISALLOW_NEW();
 
  public:
@@ skipping 34 matching lines @@
 
  private:
   LayoutMultiColumnSet& column_set_;
 
   Vector<MultiColumnFragmentainerGroup, 1> groups_;
 };
 
 }  // namespace blink
 
 #endif  // MultiColumnFragmentainerGroup_h