Then you push a button, but what if the person won't chat with you?
What is the point of the feature?
Comments over time are generally removed. To learn more about comments reference: "How do comments work?"
Comments are temporary "Post-It" notes left on a question or answer.
You should not expect them to be around forever: Once a clarification
has been made, an edit added to the post to include new information,
or the issue in the comment is otherwise resolved, it is subject to
deletion. In reality, many obsolete or chatty comments remain
untouched due to the high volume of comments posted, but this does not
mean that they can't or shouldn't be deleted in the future.
The purpose of the comment system is not to have a full discussion which is why it will post a chatroom for you to take your discussion.
In regards to the situation I believe you may have mis-understood the question and made an answer too quickly. It seems it was a mistake, we are all human (well some of us) and I see you removed your answer so it's not a big deal.
On the issue of tagging, this isn't a bug, a bug tag is used only when an issue exists so please dont use the bug tag unless you have a site issue.
There needs to be an "undo" because I think I made a mistake with
turning a comment thread to a chat. Or at least a warning that you
can't undo it.
No, there doesn't need to be an undo. You actually did what you were supposed to do. Your took what was a discussion to the right place and finished debating or whatever to resolve your debate/discussion/event. I also see you want the chatroom to be removed. Over time the system will automatically hide it from everyone so you wont have to worry.
Hope this helps.