OK there *is* a workaround
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... but it's not perfect as it can take effort when you start a new convo.
Essentially you want to open all your chats as tabs and then switch off tabbing; convos already open will stay in their tabs. Now close any you want to split off and reopen them; they will open in their own windows.
Obviously, all new chats from this point will also have their own windows... if you want to group them I guess you'd have to repeat the process, which could mean much closing of that special chat, which could be annoying if you're webcamming...