Hi @Martin Cockshott ,
Yes, as you noticed, currently the "Move to folder" option is not available in an open email (pop-out email), so you may have to switch back to the classic Outlook or use the workarounds you mentioned above at present.
Given this, I just tried submitting it via the Help > Feedback button within Outlook. You can submit it from your end as well if possible. Hopefully this feature can be added in the near future.
Meanwhile, below is another tip about moving an email to another folder. You can probably have a go and see if it can do a little help in current situation:
Instead of closing the pop-out mail, you can just click on the main Outlook window, locate the mail in the message list, and then drag and drop the mail to the target folder.
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