Attach X-forwarded window to both displays?

Question

I know I can start an application on the main display of a linux computer over ssh with something like :

DISPLAY=localhost:0 firefox http://nyan.cat

I know I can open windows on my laptop over ssh if I connect to the computer via

ssh -X user@computer

But is there a way to open a Window on both displays ?
Or at least, to move a window opened on main display to my laptop display over ssh (without restarting the program) ?

And if plain ssh cannot, is there something which can ?

Answer 1

No, one X11 window can be only shown on one X11 display.

And no, moving in runtime is generally not possible (in theory it could be done, but only if the program implemented that itself).

You could, however, run xpra instead of plain X11 forwarding; it allows detaching X11 programs in a similar way to SCREEN or tmux, and reattaching them to a different display.