Mirror single window
Solution 1:
You can do this with xpra:
xpra start :100 --start-child=PROGRAM --bind-tcp=0.0.0.0:10000
xpra attach tcp:localhost:10000
As a bonus, it also works remotely – you can attach this window to X (or even Windows and MacOS!) on a different machine. xpra is like the console programs screen or tmux for X11.