Say you have a Community on Instance A, a User-B from Instance B posts to that community, a User-C on Instance C comments on that post, and User D on Instance D is viewing that post, and its comments from Instance D. How should user D go about sharing a comment from that post? Should User D share the comment from Instance D, Instance A, or Instance C?
I imagine the most “ideal” outcome would be “whatever link the default webui shows as the fediverse link (the colorful icon on all posts / comments)”, as that’s the “canonical” link (technically, the ActivityPub ID, which is I think required to be a link as per the spec) of that specific post or comment.
It’d be the most accurate in terms of visibility (could be deleted by OP and the delete might not reach your instance), latest edits, vote counts, and replies for that specific thing, that said it’s definitely not as convenient as just copying the link in your URL bar and pasting it to wherever.
Also depending on the exact software running on an instance, that link may not point to a human readable page, but that’s a pretty rare edge case that even the most incomplete real life implementations I’m aware of handle reasonably well (even if it ends up doing a redirect)