[codeapprove]

Actually you can’t comment on lines that haven’t been changed if they’re too far from the changed lines! It’s a weird limitation of GitHubs data model.

(source: that’s one of the features that people like most about the review tool I created, CodeApprove)

[dllthomas]

Annotations can be added by "checks" anywhere, though, I believe.

I like to use that to surface cross-references (annotating anchors whose references appear in changed or context lines, and references whose anchors do).