I know that it is possible to forward the windows of an X-windows-system application to a client's desktop via SSH when you start the application in a SSH terminal session which has X-forwarding enabled.
But is there a way to forward the windows of an application which is already running on a remote system? Most impressive would be a way to keep a windowed application running on a headless server system, so that when I ever need that application I can forward its windows on-demand to my desktop system, without the need to ever stop this application or the application being closed down because my SSH connection times out.