Fix stableStringify treating shared references as circular

src/vs/base/common/objects.ts

@@ -215,19 +215,26 @@ function _stableStringify(value: unknown, seen: WeakSet<object>): string {
 		return '"[Circular]"';
 	}
 	seen.add(value as object);
+
+	let result: string;
 	if (Array.isArray(value)) {
-		return '[' + value.map(v => _stableStringify(v, seen)).join(',') + ']';
-	}
-	const keys = Object.keys(value as object).sort();
-	const parts: string[] = [];
-	for (const k of keys) {
-		const v = (value as Record<string, unknown>)[k];
-		if (v === undefined) {
-			continue;
+		result = '[' + value.map(v => _stableStringify(v, seen)).join(',') + ']';
+	} else {
+		const keys = Object.keys(value as object).sort();
+		const parts: string[] = [];
+		for (const k of keys) {
+			const v = (value as Record<string, unknown>)[k];
+			if (v === undefined) {
+				continue;
+			}
+			parts.push(JSON.stringify(k) + ':' + _stableStringify(v, seen));
 		}
-		parts.push(JSON.stringify(k) + ':' + _stableStringify(v, seen));
+		result = '{' + parts.join(',') + '}';
 	}
-	return '{' + parts.join(',') + '}';
+
+	// Track only ancestors so shared sibling references are serialized in full.
+	seen.delete(value as object);
+	return result;
 }
 
 type obj = { [key: string]: any };

src/vs/base/test/common/objects.test.ts

@@ -181,6 +181,18 @@ suite('Objects', () => {
 			objects.stableStringify(circular),
 			'{"a":1,"self":"[Circular]"}'
 		);
+
+		// A shared (non-circular) reference used in sibling branches is
+		// serialized in full and not treated as circular
+		const shared = { a: 1 };
+		assert.strictEqual(
+			objects.stableStringify({ x: shared, y: shared }),
+			'{"x":{"a":1},"y":{"a":1}}'
+		);
+		assert.strictEqual(
+			objects.stableStringify([shared, shared]),
+			'[{"a":1},{"a":1}]'
+		);
 	});
 
 	test('distinct', () => {