How to mirror an application window in a second display? Is there any way to mirror an application window onto a second display?

Specific use case: I want to show a terminal window in a second screen (i.e. projector), while still working on that same window on my laptop display. Complete display mirror wouldn't work because I want to keep working with multiple apps opened but project only the terminal window.
You can use an application like tmux within which you can run your first terminal session (on first display), and then open a different terminal window on the second display and use tmux attach-session to mirror the session that is already running on the first display. This should allow you to run and view commands from both the displays.

Note that this technique is only useful for mirroring the terminal and no other GUI program.