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

Index: third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc b/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
index 3c77a6bce448f472f72ccb3eea153facd476dbd9..477e89b9d2f9fd80a0b92929f9448ae5a0cf9e02 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
+++ b/third_party/WebKit/Source/core/layout/ng/ng_block_layout_algorithm.cc
@@ -66,8 +66,8 @@ bool NGBlockLayoutAlgorithm::Layout(const NGConstraintSpace* constraint_space,
         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,
@@ -108,13 +108,32 @@ bool NGBlockLayoutAlgorithm::Layout(const NGConstraintSpace* constraint_space,
 
 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);