How to sort messages in one thread so that the newest one is on top?

I don't think you can. I believe there is a tacit assumption that if you choose to work using a threaded view, then you are concerned with the logical progression of a conversation, and therefore would wish to see the conversation in a logical order, with a response after the message it replied to. This sort of takes us towards top-posting vs bottom-posting. Users who top-post tend to quote back the accumulated messages in their entirety, so each message is in itself a complete record of the history of the conversation, and a threaded view is of little value; the thread is within the message. Bottom-posting (adding your reply below the previous message, as done by dinosaurs such as myself), along with judicious trimming of non-relevant material is now a minority practice interest, and probably dates back to when storage and bandwidth were less plentiful and more expensive than today. Funnily enough, we all accept the bottom-posted chronological order as entirely appropriate in forums such as this one, yet are quite happy to turn it on its head in email. ;-) I have visited forums with a newest-first layout policy, and until you work out what's happening, it's very confusing. People are clearly talking about the same thing, but there's no obvious logical flow and it comes over as a mess of non-sequiturs. Then you realise that to understand a new posting you have never seen before, you need to scroll down and then work upwards through the back story. So you alternate between reading down inside a message and up between messages. With oldest first, you just read from top to bottom. More natural, I feel.