Do not collapse margins with padding/border b/w parent and first/last child

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 48 matching lines @@
       return false;
     }
     case kStateChildLayout: {
       if (current_child_) {
         NGFragment* fragment;
         if (!current_child_->Layout(constraint_space_for_children_, &fragment))
           return false;
         NGBoxStrut child_margins = computeMargins(
             *constraint_space_for_children_, *current_child_->Style());
 
-        LayoutUnit margin_block_start =
-            CollapseMargins(child_margins, fragment->MarginStrut());
+        LayoutUnit margin_block_start = CollapseMargins(
+            child_margins, fragment->MarginStrut(), *constraint_space);
 
         // TODO(layout-ng): Support auto margins
         builder_->AddChild(fragment,
                            NGLogicalOffset(border_and_padding_.inline_start +
                                                child_margins.inline_start,
                                            content_size_ + margin_block_start));
 
         content_size_ += fragment->BlockSize() + margin_block_start;
         max_inline_size_ =
             std::max(max_inline_size_, fragment->InlineSize() +
@@ skipping 20 matching lines @@
       return true;
     }
   };
   NOTREACHED();
   *out = nullptr;
   return true;
 }
 
 LayoutUnit NGBlockLayoutAlgorithm::CollapseMargins(
     const NGBoxStrut& margins,
-    const NGMarginStrut& children_margin_strut) {
+    const NGMarginStrut& children_margin_strut,
+    const NGConstraintSpace& space) {
   // Calculate margin strut for the current child.
   NGMarginStrut curr_margin_strut = children_margin_strut;
-  curr_margin_strut.AppendMarginBlockStart(margins.block_start);
-  if (current_child_->Style()->logicalHeight().isAuto()) {
-    // bottom margin of a last in-flow child is only collapsed if
-    // the parent has 'auto' computed height
+  LayoutUnit margin_block_start;
+
+  // Calculate borders and padding for the current child.
+  LayoutUnit border_and_padding_before =
+      computeBorderAndPaddingBlockStart(space, *current_child_->Style());
+  LayoutUnit border_and_padding_after =
+      computeBorderAndPaddingBlockEnd(space, *current_child_->Style());
+
+  // Collapse TOP margins if there is no padding or border between

[ikilpatrick, 2016/09/14 18:45:25]

s/TOP/BLOCK-START

[Gleb Lanbin, 2016/09/14 18:58:31]

Done.

+  // parent(current child) and its first in-flow child.

[ikilpatrick, 2016/09/14 18:45:25]

insert space between "parent ("

[Gleb Lanbin, 2016/09/14 18:58:31]

Done.

+  if (border_and_padding_before) {
+    curr_margin_strut.SetMarginBlockStart(margins.block_start);
+  } else {
+    curr_margin_strut.AppendMarginBlockStart(margins.block_start);
+  }
+
+  // Collapse BOTOM margins if

[ikilpatrick, 2016/09/14 18:45:25]

s/BOTTOM/BLOCK-END

[Gleb Lanbin, 2016/09/14 18:58:31]

Done.

+  // 1) there is no padding or border between parent(current child) and its

[ikilpatrick, 2016/09/14 18:45:25]

This this change if box-sizing: border-box is applied? (I'm just curious, it
probably doesn't).

[ikilpatrick, 2016/09/14 18:45:25]

insert space between "parent ("

[Gleb Lanbin, 2016/09/14 18:58:31]

Setting box-sizing doesn't have any affect on margin collapsing logic using the
existing code.

[Gleb Lanbin, 2016/09/14 18:58:31]

Done.

+  //    first/last in-flow child
+  // 2) parent's logical height is auto.
+  if (current_child_->Style()->logicalHeight().isAuto() &&
+      !border_and_padding_after) {
     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