[mathml] Fix mo-form-dynamic.html

When an mrow programmatically has a child added or removed, any
direct operator children may need to recompute the lspace/rspace
values in case no form value was given, since the fallback logic
to compute the form value depends on the previous and next sibling [1].
Add the method CheckFormAfterSiblingChange to implement this update logic.

Bug: 6606

[1] https://mathml-refresh.github.io/mathml-core/#dfn-algorithm-for-determining-the-form-of-an-embellished-operator

Change-Id: I8c641fda6c3d32a263430aba474c409dc3e56fd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2467957
Commit-Queue: Rob Buis <rbuis@igalia.com>
Reviewed-by: Frédéric Wang <fwang@igalia.com>
Cr-Commit-Position: refs/heads/master@{#816588}

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

@@ -254,6 +254,13 @@ bool MathMLOperatorElement::HasBooleanProperty(OperatorPropertyFlag flag) {
   return properties_.flags & flag;
 }
 
+void MathMLOperatorElement::CheckFormAfterSiblingChange() {
+  if (properties_.dictionary_category !=
+          MathMLOperatorDictionaryCategory::kUndefined &&
+      !FastHasAttribute(mathml_names::kFormAttr))
+    SetOperatorFormDirty();
+}
+
 void MathMLOperatorElement::SetOperatorFormDirty() {
   properties_.dictionary_category =
       MathMLOperatorDictionaryCategory::kUndefined;

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

@@ -44,6 +44,8 @@ class CORE_EXPORT MathMLOperatorElement final : public MathMLElement {
   double DefaultLeadingSpace();
   double DefaultTrailingSpace();
 
+  void CheckFormAfterSiblingChange();
+
  private:
   base::Optional<OperatorContent> operator_content_;
   // Operator properties calculated from dictionary and attributes.

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

@@ -5,6 +5,7 @@
 #include "third_party/blink/renderer/core/mathml/mathml_row_element.h"
 
 #include "third_party/blink/renderer/core/layout/ng/mathml/layout_ng_mathml_block.h"
+#include "third_party/blink/renderer/core/mathml/mathml_operator_element.h"
 #include "third_party/blink/renderer/platform/runtime_enabled_features.h"
 
 namespace blink {
@@ -22,4 +23,18 @@ LayoutObject* MathMLRowElement::CreateLayoutObject(const ComputedStyle& style,
   return new LayoutNGMathMLBlock(this);
 }
 
+void MathMLRowElement::ChildrenChanged(const ChildrenChange& change) {
+  if (change.by_parser == ChildrenChangeSource::kAPI) {
+    for (auto* child = firstChild(); child; child = child->nextSibling()) {
+      if (child->HasTagName(mathml_names::kMoTag)) {
+        // TODO(crbug.com/1124298): make this work for embellished operators.
+        static_cast<MathMLOperatorElement*>(child)
+            ->CheckFormAfterSiblingChange();
+      }
+    }
+  }
+
+  MathMLElement::ChildrenChanged(change);
+}
+
 }  // namespace blink

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

@@ -16,6 +16,8 @@ class CORE_EXPORT MathMLRowElement : public MathMLElement {
  public:
   explicit MathMLRowElement(const QualifiedName&, Document&);
 
+  void ChildrenChanged(const ChildrenChange&) override;
+
  private:
   LayoutObject* CreateLayoutObject(const ComputedStyle&,
                                    LegacyLayout legacy) override;

third_party/blink/web_tests/TestExpectations

@@ -1228,7 +1228,6 @@ crbug.com/6606 external/wpt/mathml/presentation-markup/mrow/legacy-mrow-like-ele
 crbug.com/6606 external/wpt/mathml/presentation-markup/operators/embellished-operator-001.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/operators/embellished-operator-002.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/operators/mo-axis-height-1.html [ Failure ]
-crbug.com/6606 external/wpt/mathml/presentation-markup/operators/mo-form-dynamic.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/scripts/cramped-001.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/spaces/space-like-001.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/presentation-markup/spaces/space-like-002.html [ Failure ]