Ignore zero-height fragments during margin collapsing

third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.h

 // Copyright 2016 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 NGBlockLayoutAlgorithm_h
 #define NGBlockLayoutAlgorithm_h
 
 #include "core/CoreExport.h"
 #include "core/layout/ng/ng_box.h"
 #include "core/layout/ng/ng_fragment_builder.h"
@@ skipping 34 matching lines @@
     visitor->trace(current_child_);
   }
 
  private:
   // Computes collapsed margins for 2 adjoining blocks and updates the resultant
   // fragment's MarginStrut if needed.
   // See https://www.w3.org/TR/CSS2/box.html#collapsing-margins
   //
   // @param space Constraint space for the block.
   // @param child_margins Margins information for the current child.
-  // @param children_margin_strut MarginStrut for children of the current child.
+  // @param fragment Current child's fragment.
   // @return Margin block start based on collapsed margins result.
   LayoutUnit CollapseMargins(const NGConstraintSpace& space,
                              const NGBoxStrut& child_margins,
-                             const NGMarginStrut& children_margin_strut);
+                             const NGFragment& fragment);
+
+  // Updates block-{start|end} of the currently constructed fragment.
+  //
+  // This method is supposed to be called on every child but it only updates
+  // the block-start once (on the first non-zero height child fragment) and
+  // keeps updating block-end (on every non-zero height child).
+  void UpdateMarginStrut(const NGMarginStrut& from);
 
   RefPtr<const ComputedStyle> style_;
   Member<NGBox> first_child_;
 
   enum State { kStateInit, kStateChildLayout, kStateFinalize };
   State state_;
   Member<NGFragmentBuilder> builder_;
   Member<NGConstraintSpace> constraint_space_for_children_;
   Member<NGBox> current_child_;
   NGBoxStrut border_and_padding_;
   LayoutUnit content_size_;
   LayoutUnit max_inline_size_;
   // MarginStrut for the previous child.
   NGMarginStrut prev_child_margin_strut_;
+  // Whether the block-start was set for the currently built
+  // fragment's margin strut.
+  bool is_fragment_margin_strut_block_start_updated_ : 1;
 };
 
 }  // namespace blink
 
 #endif  // NGBlockLayoutAlgorithm_h