Compute margin block start for 1st block in LayoutNG root constraint space

third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.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_block_layout_algorithm.h"
 
 #include "core/layout/ng/ng_constraint_space.h"
 #include "core/layout/ng/ng_fragment_builder.h"
 #include "core/layout/ng/ng_fragment.h"
 #include "core/layout/ng/ng_length_utils.h"
@@ skipping 142 matching lines @@
   // 1) there is no padding or border between parent (current child) and its
   //    first/last in-flow child
   // 2) parent's logical height is auto.
   if (current_child_->Style()->logicalHeight().isAuto() &&
       !border_and_padding.block_end) {
     curr_margin_strut.AppendMarginBlockEnd(margins.block_end);
   } else {
     curr_margin_strut.SetMarginBlockEnd(margins.block_end);
   }
 
+  // Compute the margin block start for
+  // 1) adjoining blocks
+  // 2) 1st block in LayoutNG root element's constraint space.

[mstensho (USE GERRIT), 2016/09/28 18:32:33]

Oops, some "LayoutNG" here too...

+  LayoutUnit margin_block_start;
+  if (is_fragment_margin_strut_block_start_updated_ || space.IsRoot()) {
+    margin_block_start = ComputeCollapsedMarginBlockStart(
+        prev_child_margin_strut_, curr_margin_strut);
+  }
+
   // Update the parent fragment's margin strut
   UpdateMarginStrut(curr_margin_strut);
 
-  // Compute the margin block start for adjoining blocks.
-  LayoutUnit margin_block_start;
-  if (is_fragment_margin_strut_block_start_updated_) {
-    margin_block_start = ComputeCollapsedMarginBlockStart(
-        prev_child_margin_strut_, curr_margin_strut);
-  }
   prev_child_margin_strut_ = curr_margin_strut;
   return margin_block_start;
 }
 
 void NGBlockLayoutAlgorithm::UpdateMarginStrut(const NGMarginStrut& from) {
   if (!is_fragment_margin_strut_block_start_updated_) {
     builder_->SetMarginStrutBlockStart(from);
     is_fragment_margin_strut_block_start_updated_ = true;
   }
   builder_->SetMarginStrutBlockEnd(from);
 }
 
 }  // namespace blink