Collapse bottom margins of a last child and its parent if parent's height=auto

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 94 matching lines @@
   *out = nullptr;
   return true;
 }
 
 LayoutUnit NGBlockLayoutAlgorithm::CollapseMargins(
     const NGBoxStrut& margins,
     const NGMarginStrut& children_margin_strut) {
   // Calculate margin strut for the current child.
   NGMarginStrut curr_margin_strut = children_margin_strut;
   curr_margin_strut.AppendMarginBlockStart(margins.block_start);
-  curr_margin_strut.AppendMarginBlockEnd(margins.block_end);
+  if (current_child_->Style()->logicalHeight().isAuto()) {
+    // bottom margin of a last in-flow child is only collapsed if
+    // the parent has 'auto' computed height
+    curr_margin_strut.AppendMarginBlockEnd(margins.block_end);
+  } else {
+    curr_margin_strut.SetMarginBlockEnd(margins.block_end);
+  }
 
   // Set the margin strut for the resultant fragment if this is the first or
   // last child fragment.
   if (current_child_ == first_child_)
     builder_->SetMarginStrutBlockStart(curr_margin_strut);
   if (!current_child_->NextSibling())
     builder_->SetMarginStrutBlockEnd(curr_margin_strut);
 
   // Compute the margin block start for adjoining blocks.
   LayoutUnit margin_block_start;
   if (current_child_ != first_child_)
     margin_block_start = ComputeCollapsedMarginBlockStart(
         prev_child_margin_strut_, curr_margin_strut);
 
   prev_child_margin_strut_ = curr_margin_strut;
   // TODO(layout-ng): support other Margin Collapsing use cases,
   // i.e. support 0 height elements etc.
   return margin_block_start;
 }
 
 }  // namespace blink