Opening Musescore 3.6 creates blank popup in front of program with no way to move or close it.
Just installed on a brand new Dell Optiplex 3080 w/Windows 10. Uninstalled and reinstalled, still the same. Changed display resolution way down and it disappeared, then changed it back and popup reappears. Disappears in 
Comments
Try pressing Esc
Problem solved. Renegade Inspector Panel. Some of the panels open as undocked if they were closed as undocked. If I had opened a score and selected a note I would have seen it was the Inspector Panel. Since the top was above the screen it was blank.
Solution: (keep trying display settings)
Changing display from default of 1920x1080 to 1600x900 showed the top of the panel allowing me to close it.
I don't recall using undocked panels so I'm not sure of the cause.