Ignore zero-height fragments during margin collapsing

third_party/WebKit/Source/core/layout/ng/ng_units.cc

 // 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.
 
 #include "core/layout/ng/ng_units.h"
 
 #include "core/layout/ng/ng_writing_mode.h"
 
 namespace blink {
 
@@ skipping 43 matching lines @@
             block_offset,
             container_size.height - inline_offset - inner_size.height);
       else
         return NGPhysicalOffset(block_offset, inline_offset);
     default:
       ASSERT_NOT_REACHED();
       return NGPhysicalOffset();
   }
 }
 
+bool NGBoxStrut::IsEmpty() const {
+  return *this == NGBoxStrut();
+}
+
+bool NGBoxStrut::operator==(const NGBoxStrut& other) const {
+  return std::tie(other.inline_start, other.inline_end, other.block_start,
+                  other.block_end) ==
+         std::tie(inline_start, inline_end, block_start, block_end);
+}
+
 void NGMarginStrut::AppendMarginBlockStart(const LayoutUnit& value) {
   if (value < 0) {
     negative_margin_block_start =
         -std::max(value.abs(), negative_margin_block_start.abs());
   } else {
     margin_block_start = std::max(value, margin_block_start);
   }
 }
 
 void NGMarginStrut::AppendMarginBlockEnd(const LayoutUnit& value) {
@@ skipping 30 matching lines @@
 
 bool NGMarginStrut::operator==(const NGMarginStrut& other) const {
   return std::tie(other.margin_block_start, other.margin_block_end,
                   other.negative_margin_block_start,
                   other.negative_margin_block_end) ==
          std::tie(margin_block_start, margin_block_end,
                   negative_margin_block_start, negative_margin_block_end);
 }
 
 }  // namespace blink