fix(math): use U+2015 for underbar accent instead of U+0332 (SD-2403) (#2621)

U+0332 (combining low line) renders invisibly in some browsers when used
as a standalone MathML operator. Replace with U+2015 (horizontal bar)
which renders consistently. Also moves m:f to the "Implemented" section
in the math object registry.

Author: caio-pizzol

packages/layout-engine/painters/dom/src/features/math/converters/bar.ts

@@ -10,7 +10,7 @@ const MATHML_NS = 'http://www.w3.org/1998/Math/MathML';
  *
  * MathML output:
  *   top:           <mover> <mrow>base</mrow> <mo>&#x203E;</mo> </mover>
- *   bot (default): <munder> <mrow>base</mrow> <mo>&#x0332;</mo> </munder>
+ *   bot (default): <munder> <mrow>base</mrow> <mo>&#x2015;</mo> </munder>
  *
  * Word renders an underbar when no position is specified, so the default is "bot".
  *
@@ -35,8 +35,8 @@ export const convertBar: MathObjectConverter = (node, doc, convertChildren) => {
 
   const accent = doc.createElementNS(MATHML_NS, 'mo');
   accent.setAttribute('stretchy', 'true');
-  // U+203E = overline, U+0332 = combining low line (underbar)
-  accent.textContent = isUnder ? '\u0332' : '\u203E';
+  // U+203E = overline, U+2015 = horizontal bar (underbar)
+  accent.textContent = isUnder ? '\u2015' : '\u203E';
   wrapper.appendChild(accent);
 
   return wrapper;

packages/layout-engine/painters/dom/src/features/math/omml-to-mathml.test.ts

@@ -209,13 +209,18 @@ describe('m:bar converter', () => {
   it('renders overbar (top) as <mover> with U+203E', () => {
     const omml = {
       name: 'm:oMath',
-      elements: [{
-        name: 'm:bar',
-        elements: [
-          { name: 'm:barPr', elements: [{ name: 'm:pos', attributes: { 'm:val': 'top' } }] },
-          { name: 'm:e', elements: [{ name: 'm:r', elements: [{ name: 'm:t', elements: [{ type: 'text', text: 'x' }] }] }] },
-        ],
-      }],
+      elements: [
+        {
+          name: 'm:bar',
+          elements: [
+            { name: 'm:barPr', elements: [{ name: 'm:pos', attributes: { 'm:val': 'top' } }] },
+            {
+              name: 'm:e',
+              elements: [{ name: 'm:r', elements: [{ name: 'm:t', elements: [{ type: 'text', text: 'x' }] }] }],
+            },
+          ],
+        },
+      ],
     };
     const result = convertOmmlToMathml(omml, doc);
     expect(result).not.toBeNull();
@@ -226,42 +231,52 @@ describe('m:bar converter', () => {
     expect(mo?.textContent).toBe('\u203E');
   });
 
-  it('renders underbar (bot) as <munder> with U+0332', () => {
+  it('renders underbar (bot) as <munder> with U+2015', () => {
     const omml = {
       name: 'm:oMath',
-      elements: [{
-        name: 'm:bar',
-        elements: [
-          { name: 'm:barPr', elements: [{ name: 'm:pos', attributes: { 'm:val': 'bot' } }] },
-          { name: 'm:e', elements: [{ name: 'm:r', elements: [{ name: 'm:t', elements: [{ type: 'text', text: 'y' }] }] }] },
-        ],
-      }],
+      elements: [
+        {
+          name: 'm:bar',
+          elements: [
+            { name: 'm:barPr', elements: [{ name: 'm:pos', attributes: { 'm:val': 'bot' } }] },
+            {
+              name: 'm:e',
+              elements: [{ name: 'm:r', elements: [{ name: 'm:t', elements: [{ type: 'text', text: 'y' }] }] }],
+            },
+          ],
+        },
+      ],
     };
     const result = convertOmmlToMathml(omml, doc);
     expect(result).not.toBeNull();
     const munder = result!.querySelector('munder');
     expect(munder).not.toBeNull();
     expect(munder!.firstElementChild!.textContent).toBe('y');
     const mo = munder!.querySelector('mo');
-    expect(mo?.textContent).toBe('\u0332');
+    expect(mo?.textContent).toBe('\u2015');
   });
 
   it('defaults to underbar when m:barPr is missing (matches Word behavior)', () => {
     const omml = {
       name: 'm:oMath',
-      elements: [{
-        name: 'm:bar',
-        elements: [
-          { name: 'm:e', elements: [{ name: 'm:r', elements: [{ name: 'm:t', elements: [{ type: 'text', text: 'z' }] }] }] },
-        ],
-      }],
+      elements: [
+        {
+          name: 'm:bar',
+          elements: [
+            {
+              name: 'm:e',
+              elements: [{ name: 'm:r', elements: [{ name: 'm:t', elements: [{ type: 'text', text: 'z' }] }] }],
+            },
+          ],
+        },
+      ],
     };
     const result = convertOmmlToMathml(omml, doc);
     expect(result).not.toBeNull();
     const munder = result!.querySelector('munder');
     expect(munder).not.toBeNull();
     expect(munder!.firstElementChild!.textContent).toBe('z');
     const mo = munder!.querySelector('mo');
-    expect(mo?.textContent).toBe('\u0332');
+    expect(mo?.textContent).toBe('\u2015');
   });
 });

packages/layout-engine/painters/dom/src/features/math/omml-to-mathml.ts

@@ -31,14 +31,14 @@ const MATH_OBJECT_REGISTRY: Record<string, MathObjectConverter | null> = {
   // ── Implemented ──────────────────────────────────────────────────────────
   'm:r': convertMathRun,
   'm:bar': convertBar, // Bar (overbar/underbar)
+  'm:f': convertFraction, // Fraction (numerator/denominator)
 
   // ── Not yet implemented (community contributions welcome) ────────────────
   'm:acc': null, // Accent (diacritical mark above base)
   'm:borderBox': null, // Border box (border around math content)
   'm:box': null, // Box (invisible grouping container)
   'm:d': null, // Delimiter (parentheses, brackets, braces)
   'm:eqArr': null, // Equation array (vertical array of equations)
-  'm:f': convertFraction, // Fraction (numerator/denominator)
   'm:func': null, // Function apply (sin, cos, log, etc.)
   'm:groupChr': null, // Group character (overbrace, underbrace)
   'm:limLow': null, // Lower limit (e.g., lim)