Maybe others can benefit from my misfortune if I share it here. I duplicated the problem, so I think it wasn’t just a burp.
If you have cleared your cookies, or otherwise haven’t logged in, and compose a post in the handy window at the bottom, then if you submit it, you will encounter a screen asking you to log in. If you click “back” to retrieve your post, it will be gone. If you continue forward, it will be gone, and you will be told that you haven’t selected a forum. You’ll have to go around the world to get back where you were, and you’ll have to write a new post.
Usually, I compose my posts in Notepad or paste them there before submitting. This happened to me on one of the rare times I didn’t. Murphy’s law, I suppose. I hope this is the right forum. I didn’t intend a rant, but merely a board related public service.