feat(math): implement m:bar overbar/underbar converter

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

@@ -0,0 +1,43 @@
+import type { MathObjectConverter } from '../types.js';
+
+const MATHML_NS = 'http://www.w3.org/1998/Math/MathML';
+
+/**
+ * Convert m:bar (overbar/underbar) to MathML <mover> or <munder>.
+ *
+ * OMML structure:
+ *   m:bar → m:barPr (optional: m:pos@m:val="top"|"bot"), m:e (base expression)
+ *
+ * MathML output:
+ *   top:           <mover> <mrow>base</mrow> <mo>&#x203E;</mo> </mover>
+ *   bot (default): <munder> <mrow>base</mrow> <mo>&#x0332;</mo> </munder>
+ *
+ * Word renders an underbar when no position is specified, so the default is "bot".
+ *
+ * @spec ECMA-376 §22.1.2.7
+ */
+export const convertBar: MathObjectConverter = (node, doc, convertChildren) => {
+  const elements = node.elements ?? [];
+
+  const barPr = elements.find((e) => e.name === 'm:barPr');
+  const pos = barPr?.elements?.find((e) => e.name === 'm:pos');
+  const posVal = pos?.attributes?.['m:val'];
+  const isUnder = posVal !== 'top';
+
+  const base = elements.find((e) => e.name === 'm:e');
+
+  const wrapper = doc.createElementNS(MATHML_NS, isUnder ? 'munder' : 'mover');
+
+  const baseContent = convertChildren(base?.elements ?? []);
+  const mrow = doc.createElementNS(MATHML_NS, 'mrow');
+  mrow.appendChild(baseContent);
+  wrapper.appendChild(mrow);
+
+  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';
+  wrapper.appendChild(accent);
+
+  return wrapper;
+};

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

@@ -8,3 +8,4 @@
  */
 export { convertMathRun } from './math-run.js';
 export { convertFraction } from './fraction.js';
+export { convertBar } from './bar.js';

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

@@ -204,3 +204,64 @@ describe('convertOmmlToMathml', () => {
     expect(children.some((c) => c.localName === 'mn')).toBe(true); // 1
   });
 });
+
+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' }] }] }] },
+        ],
+      }],
+    };
+    const result = convertOmmlToMathml(omml, doc);
+    expect(result).not.toBeNull();
+    const mover = result!.querySelector('mover');
+    expect(mover).not.toBeNull();
+    expect(mover!.firstElementChild!.textContent).toBe('x');
+    const mo = mover!.querySelector('mo');
+    expect(mo?.textContent).toBe('\u203E');
+  });
+
+  it('renders underbar (bot) as <munder> with U+0332', () => {
+    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' }] }] }] },
+        ],
+      }],
+    };
+    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');
+  });
+
+  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' }] }] }] },
+        ],
+      }],
+    };
+    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');
+  });
+});

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

@@ -10,7 +10,7 @@
  */
 
 import type { OmmlJsonNode, MathObjectConverter } from './types.js';
-import { convertMathRun, convertFraction } from './converters/index.js';
+import { convertMathRun, convertFraction, convertBar } from './converters/index.js';
 
 export const MATHML_NS = 'http://www.w3.org/1998/Math/MathML';
 
@@ -30,10 +30,10 @@ export const MATHML_NS = 'http://www.w3.org/1998/Math/MathML';
 const MATH_OBJECT_REGISTRY: Record<string, MathObjectConverter | null> = {
   // ── Implemented ──────────────────────────────────────────────────────────
   'm:r': convertMathRun,
+  'm:bar': convertBar, // Bar (overbar/underbar)
 
   // ── Not yet implemented (community contributions welcome) ────────────────
   'm:acc': null, // Accent (diacritical mark above base)
-  'm:bar': null, // Bar (overbar/underbar)
   'm:borderBox': null, // Border box (border around math content)
   'm:box': null, // Box (invisible grouping container)
   'm:d': null, // Delimiter (parentheses, brackets, braces)