Use case:
If A uses Windows it is very convenient to log into a running session e.g. via VNC. But if A uses Linux, AFAIK, this is not possible. Using VNC requires a "vncserver"-session, which is a separate session. You could get screen captures from remote by querying the X-server, but you cannot press buttons on the screen.
Is there some workaround for this?
There is x11vnc: "x11vnc allows one to view remotely and interact with real X displays (i.e. a display corresponding to a physical monitor, keyboard, and mouse) with any VNC viewer. It has built-in SSL encryption and authentication, UNIX account and password support, server-side scaling, single port HTTPS and VNC, mDNS service advertising, and TightVNC and UltraVNC file-transfer".
It could be used with existing X11 session, without need to start one under "xvncserver".