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

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

Index: third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.h
diff --git a/third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.h b/third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.h
index 30b072578619e41450c4f3ee5d27c8fb7f875b8a..31c7ad3ce92a2220d6857dcc46ff7830cfc2d213 100644
--- a/third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.h
+++ b/third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.h
@@ -44,7 +44,21 @@ class MultiColumnFragmentainerGroup {
   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;
 
@@ -156,7 +170,7 @@ class MultiColumnFragmentainerGroup {
 
   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
@@ -170,9 +184,13 @@ class MultiColumnFragmentainerGroup {
   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