Invisible mixer window
This might help someone else.
I run a twin monitor Win11 system.
After the installation of Musescore 4.4.1, the mixer would not appear no matter which method I used to invoke it. 4.4.2 wouldn't display it either.
Uninstalling Musescore did not help. What seems to have fixed the problem is an uninstall and then deletion of every folder and file that had anything to do with Musescore e.g in C:\Users...\AppData.