mjhjmtu

So I have set up X11 forwarding between two computers such that I can open a gui program in the server (in terminal) and it starts in the client. I followed the variant A in this answer: https://askubuntu.com/a/207189/794249

Now I would like to know if there is a way to forward a single window in a multi-window program running in server to the client (for instance, in gimp floating mode).

What I am exactly trying to accomplish here is a sort of dual monitor effect where a laggy client laptop can be a sort of preview monitor (which does not need to be realtime). Used in conjunction with mouse sharing program, I imagine it might work.

I don't know much about the internals of X and things like that so if you think that this approach is not suitable for what I am trying to accomplish, then any suggestions are welcome.

Not easily: The client (gimp) opens a connection to a specific X server using the DISPLAY variable/options, and then asks the X server to open windows etc. Which means all windows will "belong" to the same X server, you can't just switch a window between different X servers (one local, one remote). To make a client use several X servers at once, that has to be baked into the client.

I'm not sure about your exact use case (Do you want to use the laptop as a laggy second screen in a "multi-screen" setup on your main computer? Do you want to graphically access applications on different computer on an application-by-application basis? Something different?), but have a look at Xpra to see if that could do what you want.

There exist programs that can do what you propose, xbattle is one example, source should be available. but it is written from the start to be multi-headed. Trying to retrofit that onto an existing application could be tricky.