My wife's machine uses HDMI for audio-output to an amplifier. Windows considers this monitor #3. Is there a way to prevent any window from ever being moved to this "display"? (I assume that means if a window is moved there, it'll be moved to a different monitor.)

She lost a window the other day, took me awhile to figure out what happened...

That's useful for new windows, but she accidentally moved an existing window (I assume with the "next window" shortcut key built into Windows, not sure).

Thanks though, I'll at least set up what you provided, better than nothing!