The trouble with that is, how on earth do you get the annotations to apply across revisions? Or would you be OK with them only applying to a given revision and then ... what, disappearing? Or going into a 'floating bucket' of annotations for the entire file?

I'd be fine with them sticking to particular lines of code[0] and disappearing if those lines were changed. This is not something I'd like to commit into codebase, it's for exploration / preparing work.

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[0] - "smart stick" can be implemented by remembering not just the line in file, but also the contents of that line and a (fuzzy) context.

Interesting. In these terms, it could probably just be a totally separate, general-purpose 'text file annotation' tool with git support (but not a requirement).

Sticky comment is actually another form of git blame, and will be available in future versions of GitPlex.

Upsource has sticky comments that survive code changes well.