fix(math): use stretchable U+203E for both over and underbar accents (SD-2408) (#2639)

U+2015 (horizontal bar) doesn't stretch in MathML, so underbars on
compound expressions like x+y only appeared under a single letter.
Use U+203E (overline) for both — it's stretchable and munder/mover
controls the position.

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>&#x2015;</mo> </munder>
+ *   bot (default): <munder> <mrow>base</mrow> <mo>&#x203E;</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+2015 = horizontal bar (underbar)
-  accent.textContent = isUnder ? '\u2015' : '\u203E';
+  // U+203E = overline (stretchable in MathML, used for both over and under)
+  accent.textContent = '\u203E';
   wrapper.appendChild(accent);
 
   return wrapper;

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

@@ -235,7 +235,7 @@ describe('m:bar converter', () => {
     expect(mo?.textContent).toBe('\u203E');
   });
 
-  it('renders underbar (bot) as <munder> with U+2015', () => {
+  it('renders underbar (bot) as <munder> with U+203E', () => {
     const omml = {
       name: 'm:oMath',
       elements: [
@@ -257,7 +257,7 @@ describe('m:bar converter', () => {
     expect(munder).not.toBeNull();
     expect(munder!.firstElementChild!.textContent).toBe('y');
     const mo = munder!.querySelector('mo');
-    expect(mo?.textContent).toBe('\u2015');
+    expect(mo?.textContent).toBe('\u203E');
   });
 
   it('defaults to underbar when m:barPr is missing (matches Word behavior)', () => {
@@ -281,6 +281,6 @@ describe('m:bar converter', () => {
     expect(munder).not.toBeNull();
     expect(munder!.firstElementChild!.textContent).toBe('z');
     const mo = munder!.querySelector('mo');
-    expect(mo?.textContent).toBe('\u2015');
+    expect(mo?.textContent).toBe('\u203E');
   });
 });