Fix delimiter unparen syntax (#5739)

crates/typst-syntax/src/parser.rs

@@ -442,11 +442,11 @@ fn math_unparen(p: &mut Parser, m: Marker) {
         if first.text() == "(" && last.text() == ")" {
             first.convert_to_kind(SyntaxKind::LeftParen);
             last.convert_to_kind(SyntaxKind::RightParen);
-        }
-    }
-
+            // Only convert if we did have regular parens.
             node.convert_to_kind(SyntaxKind::Math);
         }
+    }
+}
 
 /// The unicode math class of a string. Only returns `Some` if `text` has
 /// exactly one unicode character or is a math shorthand string (currently just

tests/suite/math/delimited.typ

@@ -125,3 +125,11 @@ $ lr(size: #3em, |)_a^b lr(size: #3em, zws|)_a^b
 --- issue-4188-lr-corner-brackets ---
 // Test positioning of U+231C to U+231F
 $⌜a⌟⌞b⌝$ = $⌜$$a$$⌟$$⌞$$b$$⌝$
+
+--- math-lr-unparen ---
+// Test that unparen with brackets stays as an LrElem.
+#let item = $limits(sum)_i$
+$
+  1 / ([item]) quad
+  1 /  [item]
+$