[LayoutNG] Add NGPhysicalLineBoxFragment::IsEmptyLineBox()

This patch adds IsEmptyLineBox() function that returns
whethe the line box is "empty", or "certain zero-height
line box" as defined in CSS2[1] or not, and change where
we used to determine it by checking if the number of children
is 0.

[1] https://drafts.csswg.org/css2/visuren.html#phantom-line-box

Bug: 636993, 903578
Change-Id: I2b9961e1ecb743063238b39a3a5be53f15ffce8c
Reviewed-on: https://chromium-review.googlesource.com/c/1332972Reviewed-by: Aleks Totic <atotic@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608419}

third_party/blink/renderer/core/layout/ng/inline/ng_inline_layout_algorithm.cc

@@ -356,8 +356,10 @@ void NGInlineLayoutAlgorithm::CreateLine(NGLineInfo* line_info,
 
   // Even if we have something in-flow, it may just be empty items that
   // shouldn't trigger creation of a line. Exit now if that's the case.
-  if (line_info->IsEmptyLine())
+  if (line_info->IsEmptyLine()) {
+    container_builder_.SetIsEmptyLineBox();
     return;
+  }
 
   DCHECK(!line_box_metrics.IsEmpty());
 

third_party/blink/renderer/core/layout/ng/inline/ng_line_box_fragment_builder.cc

@@ -17,10 +17,15 @@ namespace blink {
 void NGLineBoxFragmentBuilder::Reset() {
   children_.resize(0);
   offsets_.resize(0);
+  line_box_type_ = NGPhysicalLineBoxFragment::kNormalLineBox;
   metrics_ = NGLineHeightMetrics();
   size_.inline_size = LayoutUnit();
 }
 
+void NGLineBoxFragmentBuilder::SetIsEmptyLineBox() {
+  line_box_type_ = NGPhysicalLineBoxFragment::kEmptyLineBox;
+}
+
 NGLineBoxFragmentBuilder::Child*
 NGLineBoxFragmentBuilder::ChildList::FirstInFlowChild() {
   for (auto& child : *this) {
@@ -78,6 +83,11 @@ void NGLineBoxFragmentBuilder::AddChildren(ChildList& children) {
 }
 
 scoped_refptr<NGLayoutResult> NGLineBoxFragmentBuilder::ToLineBoxFragment() {
+#if DCHECK_IS_ON()
+  if (line_box_type_ == NGPhysicalLineBoxFragment::kEmptyLineBox)
+    DCHECK_EQ(children_.size(), 0u);
+#endif
+
   writing_mode_ = ToLineWritingMode(writing_mode_);
 
   if (!break_token_)

third_party/blink/renderer/core/layout/ng/inline/ng_line_box_fragment_builder.h

@@ -9,6 +9,7 @@
 #include "third_party/blink/renderer/core/layout/ng/inline/ng_inline_break_token.h"
 #include "third_party/blink/renderer/core/layout/ng/inline/ng_inline_node.h"
 #include "third_party/blink/renderer/core/layout/ng/inline/ng_line_height_metrics.h"
+#include "third_party/blink/renderer/core/layout/ng/inline/ng_physical_line_box_fragment.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_container_fragment_builder.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_layout_result.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_positioned_float.h"
@@ -32,6 +33,7 @@ class CORE_EXPORT NGLineBoxFragmentBuilder final
                            TextDirection)
       : NGContainerFragmentBuilder(style, writing_mode, TextDirection::kLtr),
         node_(node),
+        line_box_type_(NGPhysicalLineBoxFragment::kNormalLineBox),
         base_direction_(TextDirection::kLtr) {}
 
   void Reset();
@@ -40,6 +42,9 @@ class CORE_EXPORT NGLineBoxFragmentBuilder final
     return metrics_.LineHeight().ClampNegativeToZero();
   }
 
+  // Mark this line box is an "empty" line box. See NGLineBoxType.
+  void SetIsEmptyLineBox();
+
   const NGLineHeightMetrics& Metrics() const { return metrics_; }
   void SetMetrics(const NGLineHeightMetrics& metrics) { metrics_ = metrics; }
 
@@ -204,6 +209,7 @@ class CORE_EXPORT NGLineBoxFragmentBuilder final
   NGLineHeightMetrics metrics_;
   Vector<NGPositionedFloat> positioned_floats_;
 
+  NGPhysicalLineBoxFragment::NGLineBoxType line_box_type_;
   TextDirection base_direction_;
 
   friend class NGLayoutResult;

third_party/blink/renderer/core/layout/ng/inline/ng_physical_line_box_fragment.cc

@@ -45,7 +45,7 @@ NGPhysicalLineBoxFragment::NGPhysicalLineBoxFragment(
                                   builder->GetWritingMode(),
                                   children_,
                                   kFragmentLineBox,
-                                  0),
+                                  builder->line_box_type_),
       metrics_(builder->metrics_) {
   style_ = std::move(builder->style_);
   base_direction_ = static_cast<unsigned>(builder->base_direction_);

third_party/blink/renderer/core/layout/ng/inline/ng_physical_line_box_fragment.h

@@ -17,6 +17,16 @@ class NGLineBoxFragmentBuilder;
 class CORE_EXPORT NGPhysicalLineBoxFragment final
     : public NGPhysicalContainerFragment {
  public:
+  enum NGLineBoxType {
+    kNormalLineBox,
+    // This is an "empty" line box, or "certain zero-height line boxes":
+    // https://drafts.csswg.org/css2/visuren.html#phantom-line-box
+    // that are ignored for margin collapsing and for other purposes.
+    // https://drafts.csswg.org/css2/box.html#collapsing-margins
+    // Also see |NGInlineItem::IsEmptyItem|.
+    kEmptyLineBox
+  };
+
   static scoped_refptr<const NGPhysicalLineBoxFragment> Create(
       NGLineBoxFragmentBuilder* builder);
 
@@ -25,6 +35,11 @@ class CORE_EXPORT NGPhysicalLineBoxFragment final
       child.fragment->Release();
   }
 
+  NGLineBoxType LineBoxType() const {
+    return static_cast<NGLineBoxType>(sub_type_);
+  }
+  bool IsEmptyLineBox() const { return LineBoxType() == kEmptyLineBox; }
+
   ChildLinkList Children() const final {
     return ChildLinkList(num_children_, &children_[0]);
   }

third_party/blink/renderer/core/layout/ng/list/ng_unpositioned_list_marker.cc

@@ -70,7 +70,7 @@ bool NGUnpositionedListMarker::AddToBox(
     // If this child is an empty line-box, the list marker should be aligned
     // with the next non-empty line box produced. (This can occur with floats
     // producing empty line-boxes).
-    if (line_box.Children().IsEmpty() && !line_box.BreakToken()->IsFinished())
+    if (line_box.IsEmptyLineBox() && !line_box.BreakToken()->IsFinished())
       return false;
 
     content_metrics = line_box.Metrics();

third_party/blink/renderer/core/layout/ng/ng_block_layout_algorithm.cc

@@ -2102,6 +2102,7 @@ LayoutUnit NGBlockLayoutAlgorithm::ComputeLineBoxBaselineOffset(
     LayoutUnit line_box_block_offset) const {
   NGLineHeightMetrics metrics =
       line_box.BaselineMetrics(request.BaselineType());
+  DCHECK(!metrics.IsEmpty());
 
   // NGLineHeightMetrics is line-relative, which matches to the flow-relative
   // unless this box is in flipped-lines writing-mode.
@@ -2133,9 +2134,9 @@ bool NGBlockLayoutAlgorithm::AddBaseline(const NGBaselineRequest& request,
     const NGPhysicalLineBoxFragment* line_box =
         ToNGPhysicalLineBoxFragment(child);
 
-    // Skip over a line-box which is empty. These don't any baselines which
+    // Skip over a line-box which is empty. These don't have any baselines which
     // should be added.
-    if (line_box->Children().IsEmpty())
+    if (line_box->IsEmptyLineBox())
       return false;
 
     LayoutUnit offset =

third_party/blink/renderer/core/layout/ng/ng_physical_fragment.cc

@@ -253,9 +253,9 @@ NGPhysicalFragment::NGPhysicalFragment(NGFragmentBuilder* builder,
       break_token_(std::move(builder->break_token_)),
       type_(type),
       sub_type_(sub_type),
+      style_variant_((unsigned)builder->style_variant_),
       is_fieldset_container_(false),
-      is_old_layout_root_(false),
-      style_variant_((unsigned)builder->style_variant_) {}
+      is_old_layout_root_(false) {}
 
 NGPhysicalFragment::NGPhysicalFragment(LayoutObject* layout_object,
                                        NGStyleVariant style_variant,
@@ -268,9 +268,9 @@ NGPhysicalFragment::NGPhysicalFragment(LayoutObject* layout_object,
       break_token_(std::move(break_token)),
       type_(type),
       sub_type_(sub_type),
+      style_variant_((unsigned)style_variant),
       is_fieldset_container_(false),
-      is_old_layout_root_(false),
-      style_variant_((unsigned)style_variant) {}
+      is_old_layout_root_(false) {}
 
 // Keep the implementation of the destructor here, to avoid dependencies on
 // ComputedStyle in the header file.

third_party/blink/renderer/core/layout/ng/ng_physical_fragment.h

@@ -249,10 +249,7 @@ class CORE_EXPORT NGPhysicalFragment
   scoped_refptr<NGBreakToken> break_token_;
 
   const unsigned type_ : 2;      // NGFragmentType
-  const unsigned sub_type_ : 3;  // Union of NGBoxType and NGTextType
-  unsigned is_fieldset_container_ : 1;
-  unsigned is_old_layout_root_ : 1;
-  unsigned border_edge_ : 4;  // NGBorderEdges::Physical
+  const unsigned sub_type_ : 3;  // NGBoxType, NGTextType, or NGLineBoxType
   const unsigned style_variant_ : 2;  // NGStyleVariant
 
   // The following bitfield is only to be used by NGPhysicalLineBoxFragment
@@ -262,6 +259,9 @@ class CORE_EXPORT NGPhysicalFragment
   // The following bitfield is only to be used by NGPhysicalBoxFragment (it's
   // defined here to save memory, since that class has no bitfields).
   unsigned children_inline_ : 1;
+  unsigned is_fieldset_container_ : 1;
+  unsigned is_old_layout_root_ : 1;
+  unsigned border_edge_ : 4;  // NGBorderEdges::Physical
 
   // The following bitfield is only to be used by NGPhysicalTextFragment (it's
   // defined here to save memory, since that class has no bitfields).