[LayoutNG] Add comments about "certain zero-height line boxes"

third_party/WebKit/Source/core/layout/ng/inline/ng_line_breaker.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/inline/ng_line_breaker.h"
 
 #include "core/layout/ng/inline/ng_inline_break_token.h"
 #include "core/layout/ng/inline/ng_inline_layout_algorithm.h"
 #include "core/layout/ng/inline/ng_inline_node.h"
 #include "core/layout/ng/inline/ng_text_fragment.h"
@@ skipping 439 matching lines @@
     item_result->borders_paddings_block_end =
         borders.block_end + paddings.block_end;
     if (item.HasStartEdge()) {
       item_result->margins = ComputeMargins(*constraint_space_, style,
                                             constraint_space_->WritingMode(),
                                             constraint_space_->Direction());
       item_result->inline_size = item_result->margins.inline_start +
                                  borders.inline_start + paddings.inline_start;
       position_ += item_result->inline_size;
 
+      // While the spec defines "non-zero margins, padding, or borders" prevents
+      // line boxes to be zero-height, tests indicate that only inline direction
+      // of them do so. See should_create_line_box_.
+      // Force to create a box, because such inline boxes affect line heights.
       item_result->needs_box_when_empty =
           item_result->inline_size || item_result->margins.inline_start;
       if (item_result->needs_box_when_empty && !should_create_line_box_)
         SetShouldCreateLineBox();
     }
   }
   SetCurrentStyle(style);
   MoveToNextOf(item);
 }
 
@@ skipping 182 matching lines @@
 }
 
 RefPtr<NGInlineBreakToken> NGLineBreaker::CreateBreakToken() const {
   const Vector<NGInlineItem>& items = node_.Items();
   if (item_index_ >= items.size())
     return nullptr;
   return NGInlineBreakToken::Create(node_, item_index_, offset_);
 }
 
 }  // namespace blink