I have come to the conclusion that there's a little quirk I think people should know about, or that should be fixed if it can be.
If you are not signed in, and you pick on a Reply button, the Sign-in thingie will come up, because it will require you to sign in before you can post a Reply. After you sign in, you will be able to fill in your Reply, but here's the quirk: It seems that if you picked Reply at a message somewhere down the line in a given thread [not the initial post], your message will nevertheless appear to be in reply to the original post, and not in reply to the message on whose Reply button you picked!
I haven't tested this extensively, but it is suggested by a lot of "in reply to" situations I've noticed. Sometimes I can't quite tell which of several possible messages in a thread a given message is supposed to be in reply to, and sometimes what I think of the message, or whether I want to Reply to it myself, would be affected by knowing which message it's replying to. It's harder to figure out if the possible ones are Replies themselves, not messages from the Original Poster, yet the message you're looking at claims to be in reply to the OP, which makes it look like it can't be in reply to any of the ones you think it might be. I now think, in reality, it only means the person who wrote the message you're looking at was not yet signed in when they picked on a Reply button.
So if you're not signed in but you're going to pick on Reply to answer something, sign in first!