Once in a while, when one is venturing out of one's area of expertise, one may post a question and find it answered quickly and completely in the comments section. Perhaps someone has linked to a well-known open problem that the question is equivalent to, or it is a short but crucial hint that one did not think of, or maybe even a counterexample showing how the assumptions are false. The point is that the question was not necessarily a poor one, but it has been resolved completely and ought to be marked as answered. Sadly, there is no way to do so. And the StackExchange software will consider it an unanswered question and will keep bumping it up to the front page periodically.
What should be done in such a case?
As far as I can tell, the best the asker can do is to request the comment author to post their comment as an answer, so it can be accepted. However, if the request is not heeded, the question remains "unanswered". What is the best thing to do then? Leave it alone? Post the comment as your own answer and accept it? Delete the question?