What to do with a partially answered question?

Question

If a question has an answer that partially answers the question, but not fully what should I do?

Options

1. Leave an answer that only answers the unanswered portion of the question. Con: May make for a short answer.
2. Leave an answer that fully answers the question. Con: Duplicates the other answer.
3. Leave a comment on the answer or question. Con: Unless user chooses to incorporate into their question it won't have good visibility and might be deleted.
4. Edit answer. Con: Substantially changes answer which might offend user or get get rejected.

Answer 1

If you know the rest and there is already a partial answer, add an answer that answers the whole thing. If your source for the part that is already covered is the other answer, make sure you credit them and link to it. We prefer full answers when we can get them. So adding a more complete answer to an already answered question is both supported and encouraged.

I actually literally just did this. On this question, the first answer was OK, but didn't look at any nuance in the issue at all.

Answer 2

What to do?

Answer it!

We don't discourage duplicating answers, because they're never actually exact duplicates. Differences in phrasing, organisation, or reasoning can make two answers that seem like duplicates quite different, and the votes will show the difference. The only kind of duplication we discourage is when it's so short that it can't possible add anything useful over the existing answers. (Those usually don't get deleted, but they do get downvoted into obscurity.)

So in this case, submitting your own answer that's complete (2) is one of the preferred courses of action, as it has no actual con and will probably be a better answer. You can leave a comment instead (3) if you like, but you're right that the author may ignore it; if they do you can leave your own answer, if you remember to return to that question.

Editing the answer (4) is sometimes appropriate, but that's rare and you'll generally know it when you see it. Most often a new answer is better so the reputation goes to the person who actually did the work, not just whoever posted an incomplete answer first.

The least helpful is (1), because then the information is split across the answers. At which point, a third person would be justified in coming along and writing a complete answer to solve that division.