Sometimes I'd like to discuss some ideas related to a question without including them in the comments. For (1) it gets too long and (2) often the discussion is quite specific and probably isn't relevant to others who come across the question.
Having the automatic "move to chat" feature when comments get too long is a great idea, but I think SO users should be able to pre-empt this and start a chat for a question, and maybe an answer too, manually.
For others who then come across this chat through another means than the question it'd be nice for them to know the context of the chat. I'd like to have an automatic explicit link between the chat and question created in such cases.
