Can’t find my way out. I can toggle back to editing mode but every time I want to use the mixing board it would come back in full screen mode. (Ardour 6.3).
Just tested here on 6.3 (Linux). Deselecting View -> Maximize Mixer Space while in the editor window works for me.
It sounds like your mixer window might be detached too. If you don’t want that go to Window -> Mixer -> Attach.