@evanw any update on this issue?

The "legal comments" mechanism isn't designed for this use case. Legal comments are intended to be present but the specifics of where they are in the source code isn't supposed to matter. Legal comments are currently supported at the statement level but these things aren't statements (specifically a CaseBlock is a sequence of CaseClauses in JavaScript). I could "fix" what you're asking for by moving the comments into the case block so they are preserved (which is all the @preserve tag means), but that wouldn't achieve what you're trying to do.

What you're really asking for is for esbuild to support Istanbul's specific comment meta-language, which is not something that esbuild supports. You can read more about what supporting that would mean here: #578 (comment). It's very non-trivial.

I can try treating these as expression-level comments for now (which they aren't), which will attempt to preserve them as long as minification is disabled. It still won't treat them as legal comments, but perhaps that is enough for this use case.