display-math newline preservation (Option A ruling 2026-09-27): inline spans restore all six debris chars, display spans keep real newlines — multi-line equations never corrupted; \right/\rho CR-debris still restores; sloppy \neq-in-display accepted as the rarest slice

src/core/math-delims.ts

@@ -229,6 +229,16 @@ export function transformMathDelimiters(markdown: string): string {
  * balanced `\(…\)` / `\[…\]` spans are touched — the same scanner rules as
  * transformMathDelimiters (escaped delimiters are literal, unbalanced
  * delimiters pass through untouched).
+ *
+ * ASYMMETRIC NEWLINE RULE (owner ruling 2026-09-27, Option A): inline spans
+ * restore all six debris chars — real newlines/tabs inside INLINE math
+ * essentially never occur (options and inline math are single-line by
+ * nature). Display spans restore five and KEEP real newlines — multi-line
+ * display math (aligned environments, equation systems) is common, and a
+ * real line break is byte-identical on the wire to sloppy `\neq`/`\nu`/
+ * `\neg`/`\nabla` (JSON `\n` resolves to a newline either way). Corrupting
+ * legitimate content is strictly worse than failing to repair that rarest
+ * slice, so display-span n-command sloppiness stays broken by design.
  */
 const ESCAPE_DEBRIS: Record<string, string> = {
 	"\b": "\\b",
@@ -241,6 +251,11 @@ const ESCAPE_DEBRIS: Record<string, string> = {
 
 const DEBRIS_RE = /[\b\t\n\v\f\r]/;
 
+/** Display spans keep real newlines (multi-line equations) — debris there is
+ *  everything except \n (a lone real \r never occurs in authored text, and
+ *  `\right`/`\rho`/`\rangle` corruption is common in display math). */
+const DISPLAY_DEBRIS_RE = /[\b\t\v\f\r]/;
+
 export function restoreMathEscapes(text: string): string {
 	if (!DEBRIS_RE.test(text)) return text;
 	const out: string[] = [];
@@ -256,7 +271,8 @@ export function restoreMathEscapes(text: string): string {
 				const closeAt = findClose(text, i + open.length, close);
 				if (closeAt !== -1) {
 					const inner = text.slice(i + open.length, closeAt);
-					out.push(open + inner.replace(DEBRIS_RE, (c) => ESCAPE_DEBRIS[c]!) + close);
+					const debris = isDisplay ? DISPLAY_DEBRIS_RE : DEBRIS_RE;
+					out.push(open + inner.replace(debris, (c) => ESCAPE_DEBRIS[c]!) + close);
 					i = closeAt + close.length;
 					continue;
 				}

tests/tool-parts.test.ts

@@ -216,3 +216,53 @@ describe("LaTeX-escape restoration (the JSON-debris fix, owner ruling 2026-09-26
 		assert.equal(parsed.question, "\\(\\x0corall");
 	});
 });
+
+describe("display-math newline rule (Option A, owner ruling 2026-09-27)", () => {
+	// TS literal below: \\[ is \[, \\forall is intact \forall, \n is a REAL newline, \\\\ is the LaTeX \\ line break.
+	const MULTILINE = "Compare:\n\\[\n\\forall x, & f(x) = x^2 \\\\\n& g(x) = x^3\n\\]";
+
+	it("real newlines inside display math are kept — multi-line equations pass through untouched", () => {
+		const parsed = parseQuizInput({
+			question: MULTILINE,
+			options: [{ label: "f grows slower" }, { label: "g grows slower" }],
+			correctAnswer: "f grows slower",
+			explanation: "",
+		});
+		assert.ok(parsed);
+		assert.equal(parsed.question, MULTILINE);
+	});
+
+	it("non-newline debris inside display math still restores (the \\right CR case)", () => {
+		const parsed = parseQuizInput({
+			question: "Evaluate \\[\\left( a \right)\\] carefully.",
+			options: [{ label: "a" }, { label: "not a" }],
+			correctAnswer: "a",
+			explanation: "",
+		});
+		assert.ok(parsed);
+		assert.equal(parsed.question, "Evaluate \\[\\left( a \\right)\\] carefully.");
+	});
+
+	it("sloppy \\neq in display math stays broken — the accepted cost (newline kept, byte-identical to a real line break)", () => {
+		const debris = "Is \\[p \neq q\\] valid?";
+		const parsed = parseQuizInput({
+			question: debris,
+			options: [{ label: "yes" }, { label: "no" }],
+			correctAnswer: "no",
+			explanation: "",
+		});
+		assert.ok(parsed);
+		assert.equal(parsed.question, debris);
+	});
+
+	it("inline spans still restore newline debris (inline math is single-line by nature)", () => {
+		const parsed = parseQuizInput({
+			question: "Is \\(p \neq q\\) valid?",
+			options: [{ label: "yes" }, { label: "no" }],
+			correctAnswer: "no",
+			explanation: "",
+		});
+		assert.ok(parsed);
+		assert.equal(parsed.question, "Is \\(p \\neq q\\) valid?");
+	});
+});