[mathml] Implement the mmultiscripts element

Implement mmultiscripts element [1] and its possible
children <mprescripts> and <none>.

[1] https://mathml-refresh.github.io/mathml-core/#prescripts-and-tensor-indices-mmultiscripts

Bug: 6606
Change-Id: Ia9e48d50902546d7527e4ba063838c4a9f0d2b9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264334
Commit-Queue: Rob Buis <rbuis@igalia.com>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785551}

third_party/blink/renderer/core/css/mathml.css

@@ -47,7 +47,7 @@ math[display="block"] {
     outline: auto 1px -webkit-focus-ring-color;
 }
 
-maction, merror, mfrac, mphantom, mrow, mspace, mstyle, msub, msup, msubsup, munder, mover, munderover, msqrt, mroot
+maction, merror, mfrac, mphantom, mrow, mspace, mstyle, msub, msup, msubsup, munder, mover, munderover, msqrt, mroot, mmultiscripts, mprescripts, none
 {
     display: math;
 }

third_party/blink/renderer/core/layout/ng/mathml/ng_math_layout_utils.cc

@@ -71,6 +71,33 @@ bool IsValidMathMLFraction(const NGBlockNode& node) {
   return InFlowChildCountIs(node, 2);
 }
 
+static bool IsPrescriptDelimiter(const NGBlockNode& block_node) {
+  auto* node = block_node.GetLayoutBox()->GetNode();
+  return node && IsA<MathMLElement>(node) &&
+         node->HasTagName(mathml_names::kMprescriptsTag);
+}
+
+// Valid according to:
+// https://mathml-refresh.github.io/mathml-core/#prescripts-and-tensor-indices-mmultiscripts
+inline bool IsValidMultiscript(const NGBlockNode& node) {
+  auto child = To<NGBlockNode>(FirstChildInFlow(node));
+  if (!child || IsPrescriptDelimiter(child))
+    return false;
+  bool number_of_scripts_is_even = true;
+  while (child) {
+    child = To<NGBlockNode>(NextSiblingInFlow(child));
+    if (!child)
+      continue;
+    if (IsPrescriptDelimiter(child)) {
+      if (!number_of_scripts_is_even)
+        return false;
+      continue;
+    }
+    number_of_scripts_is_even = !number_of_scripts_is_even;
+  }
+  return number_of_scripts_is_even;
+}
+
 bool IsValidMathMLScript(const NGBlockNode& node) {
   switch (node.ScriptType()) {
     case MathScriptType::kUnder:
@@ -81,6 +108,8 @@ bool IsValidMathMLScript(const NGBlockNode& node) {
     case MathScriptType::kSubSup:
     case MathScriptType::kUnderOver:
       return InFlowChildCountIs(node, 3);
+    case MathScriptType::kMultiscripts:
+      return IsValidMultiscript(node);
     default:
       return false;
   }

third_party/blink/renderer/core/layout/ng/mathml/ng_math_scripts_layout_algorithm.h

@@ -22,23 +22,31 @@ class CORE_EXPORT NGMathScriptsLayoutAlgorithm
   explicit NGMathScriptsLayoutAlgorithm(const NGLayoutAlgorithmParams& params);
 
  private:
+  struct SubSupPair {
+    NGBlockNode sub = nullptr;
+    NGBlockNode sup = nullptr;
+  };
+
   void GatherChildren(NGBlockNode* base,
-                      NGBlockNode* sup,
-                      NGBlockNode* sub,
+                      Vector<SubSupPair>*,
+                      NGBlockNode* prescripts,
+                      unsigned* first_prescript_index,
                       NGBoxFragmentBuilder* = nullptr) const;
 
   MinMaxSizesResult ComputeMinMaxSizes(const MinMaxSizesInput&) const final;
 
   struct ChildAndMetrics {
-    STACK_ALLOCATED();
+    DISALLOW_NEW();
 
    public:
     scoped_refptr<const NGLayoutResult> result;
     LayoutUnit ascent;
     LayoutUnit descent;
     LayoutUnit inline_size;
     NGBoxStrut margins;
+    NGBlockNode node = nullptr;
   };
+  typedef Vector<ChildAndMetrics, 4> ChildrenAndMetrics;
 
   ChildAndMetrics LayoutAndGetMetrics(NGBlockNode child) const;
 
@@ -52,9 +60,10 @@ class CORE_EXPORT NGMathScriptsLayoutAlgorithm
     LayoutUnit descent;
     NGBoxStrut margins;
   };
-  VerticalMetrics GetVerticalMetrics(const ChildAndMetrics& base_metrics,
-                                     const ChildAndMetrics& sub_metrics,
-                                     const ChildAndMetrics& sup_metrics) const;
+  VerticalMetrics GetVerticalMetrics(
+      const ChildAndMetrics& base_metrics,
+      const ChildrenAndMetrics& sub_metrics,
+      const ChildrenAndMetrics& sup_metrics) const;
 
   scoped_refptr<const NGLayoutResult> Layout() final;
 };

third_party/blink/renderer/core/mathml/mathml_scripts_element.cc

@@ -17,8 +17,10 @@ static MathScriptType ScriptTypeOf(const QualifiedName& tagName) {
     return MathScriptType::kUnder;
   if (tagName == mathml_names::kMoverTag)
     return MathScriptType::kOver;
-  DCHECK(tagName == mathml_names::kMunderoverTag);
-  return MathScriptType::kUnderOver;
+  if (tagName == mathml_names::kMunderoverTag)
+    return MathScriptType::kUnderOver;
+  DCHECK_EQ(tagName, mathml_names::kMmultiscriptsTag);
+  return MathScriptType::kMultiscripts;
 }
 
 MathMLScriptsElement::MathMLScriptsElement(const QualifiedName& tagName,

third_party/blink/renderer/core/mathml/mathml_scripts_element.h

@@ -11,7 +11,15 @@ namespace blink {
 
 class Document;
 
-enum class MathScriptType { kSub, kSuper, kSubSup, kUnder, kOver, kUnderOver };
+enum class MathScriptType {
+  kSub,
+  kSuper,
+  kSubSup,
+  kMultiscripts,
+  kUnder,
+  kOver,
+  kUnderOver
+};
 
 class CORE_EXPORT MathMLScriptsElement : public MathMLElement {
  public:
@@ -39,7 +47,8 @@ struct DowncastTraits<MathMLScriptsElement> {
            mathml_element.HasTagName(mathml_names::kMunderoverTag) ||
            mathml_element.HasTagName(mathml_names::kMsubTag) ||
            mathml_element.HasTagName(mathml_names::kMsupTag) ||
-           mathml_element.HasTagName(mathml_names::kMsubsupTag);
+           mathml_element.HasTagName(mathml_names::kMsubsupTag) ||
+           mathml_element.HasTagName(mathml_names::kMmultiscriptsTag);
   }
 };
 

third_party/blink/renderer/core/mathml/mathml_tag_names.json5

@@ -59,6 +59,14 @@
       name: "msubsup",
       interfaceName: "MathMLScriptsElement",
     },
+    {
+      name: "mmultiscripts",
+      interfaceName: "MathMLScriptsElement",
+    },
+    {
+      name: "mprescripts",
+      interfaceName: "MathMLRowElement",
+    },
     {
       name: "mi",
       interfaceName: "MathMLElement",
@@ -111,5 +119,9 @@
       name: "semantics",
       interfaceName: "MathMLRowElement",
     },
+    {
+      name: "none",
+      interfaceName: "MathMLRowElement",
+    },
   ]
 }