There's a comment in util/unicode (the Unicode table generator for Qt) that we map line-break class NL to BK. No rationale is given.

Acceptance critera:

• find out what effect this has, or even what the rationale was back then, and document it in the existing code comment
• the code comment has a ###, so the assumption is that we need to fix it, so create a follow-up ticket describing what to fix