Nested parentheses in email (header) syntax highlighting are not detected. Namely, gray is used only from the first opening to the first closing parentheses instead of from the outer opening and closing parentheses.

(In email headers, parentheses indicate a comment. These comments can contain parenthetical material as well, leading to nested parentheses.)

A real-life example header where this issue shows:
---
Received: from redacted.outbound.protection.example.com
 (mail-redacted.outbound.protection.example.com [0.0.0.0])
 (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits))
 (No client certificate requested)
 by mx2.example.org (Postfix) with ESMTPS for <someone@example.org>;
 Thu, 13 Aug 2020 06:35:02 -0400 (EDT)
---
The last closing parenthesis on line three of the example is black instead of gray.

(Beware Bug 398717.)