Locking the Mirror Tool's Position

How do I get the Mirror app to stay with one window, and not display whatever window I'm currently working in?

When viewing a window in the Mirror app, tapping on the screen will display series of buttons, one of them looking like a lock. Tapping this lock button will keep the Mirror app's focus on this window, following wherever you move it on screen, whether it's deselected, behind other windows, or even partway off the screen.