[georgyo]

This again is a limitation of mapping labels to IMAP, which does not understand labels.

Both the Gmail web interface and the Gmail API allow the ability to set all the labels for a message. This can effectively enable your desired functionality. But IMAP can only deal with "folders", and cannot correctly decide when to remove a single label or remove all other labels when it sees a move action.

IMAP also only deals in messages and not threads. Gmail labels also technically only apply to messages, but the web interface shows the union of all labels of a thread. This is another decision I agree with. It means that when someone explicitly adds me to a thread, the whole thread gets highlighted in my feed.

I personally really enjoy the Gmail/fastmail/proton behavior so please don't make another political campaign to make things worse again. We have enough of those.