feat: multiline serialization to MathML

Author: arnog

CHANGELOG.md

@@ -50,6 +50,13 @@
 
 ### Resolved Issues
 
+- Fixed MathML serialization for multiline mathfields. Previously, serializing
+  multiline mathfields (such as `\displaylines{\\ }`) to MathML would result in
+  invalid MathML with syntax errors and console warnings. The fix corrects a
+  syntax error in array delimiter serialization and properly handles line break
+  (`\\`) atoms in multiline environments by skipping them (since line breaks are
+  represented by the `<mtr>` table structure in MathML, not by explicit line
+  break atoms).
 - **#2146** Fixed nested subscript and superscript creation when using physical
   keyboard shortcuts (`Shift+-`, `Ctrl+-`). Previously, typing `a_b_` would
   highlight `b` and reuse the outer subscript instead of creating a subscript of

src/formats/atom-to-math-ml.ts

@@ -724,6 +724,16 @@ function atomToMathML(atom: Atom, options: { generateID?: boolean }): string {
   switch (atom.type) {
     case 'first':
       break; // Nothing to do
+    case 'error':
+      // Line breaks (\\) in multiline environments are represented by the table structure
+      // in MathML, so we don't need to output anything for them
+      if (atom.command === '\\\\') break;
+      // For other error atoms, wrap in merror
+      result = `<merror${makeID(atom.id, options)}>${toMathML(
+        atom.body,
+        options
+      )}</merror>`;
+      break;
     case 'group':
     case 'root':
       result = toMathML(atom.body, options);
@@ -772,7 +782,7 @@ function atomToMathML(atom: Atom, options: { generateID?: boolean }): string {
         (arrayAtom.rightDelim && arrayAtom.rightDelim !== '.')
       ) {
         if (arrayAtom.rightDelim && arrayAtom.rightDelim !== '.')
-          result += `'<mo>${SPECIAL_DELIMS[arrayAtom.rightDelim!] || arrayAtom.rightDelim}</mo>`;
+          result += `<mo>${SPECIAL_DELIMS[arrayAtom.rightDelim!] || arrayAtom.rightDelim}</mo>`;
 
         result += '</mrow>';
       }