App stays on disconnected screen
1) Move the MuseScore app on a secondary screen
2) Exist MuseScore
3) Disconnect the secondary screen
4) Launch MuseScore
It starts on the secondary screen and is not visible / controllable until you reconnect a 2nd screen.
Note: I use multiple desktops on my Windows 10 laptop and this happened to me when MuseScore was launched on my desktop #3