Not really a solution to literal "grabbing windows of programs already running", but still…
On X11, it is also known as screen for X11: it allows you to run programs, usually on a remote host, direct their display to your local machine, and then to disconnect from these programs and reconnect from the same or another machine, without losing any state.
Xpra also allows forwarding of sound, clipboard and printing services.
Sessions can be accessed over SSH, or password protected over plain TCP sockets with or without SSL.
It has to be installed, configured and started beforehand, just like
tmux for console applications. You have to run your program via
xpra in the first place, then you can do the magic of (re)connecting from different machines.
xpra is available as
xpra package at least in Debian and Ubuntu. I have used it few times. The performance was better than for plain X forwarding.
Examples are here. The following are from
man xpra on my Kubuntu:
xpra start :7
xpra server using display number
firefox running inside the
xpra server. Run this on the host where
xpra was started or in terminal forwarded by
xpra. No window will appear until you attach with
You run the above examples on your A. Then you can attach directly from A:
xpra attach :7
Attach to the
xpra server that is using local display number
:7. Any apps running on that server will appear on your screen.
or from your B:
xpra attach ssh:foo@frodo:7
ssh to attach to the
xpra server that is running on machine
frodo as user
foo and using display
:7. Any apps running on that server will appear on your local screen.