When transitioning to headless mode, could the display server disconnect from the local console altogether and kick back to the display manager? Then if the same user later logged in locally to the display manager, any ongoing remote session could be terminated and the display server reattached to the console. It seems like some of the needed display manager ↔ session communication is already present for user switching, and it would relieve display servers of the complexity of maintaining a single window with different inputs and outputs.