MIDI input menu has no "None" option
In MuseScore 3,
As long as a MIDI input device is available, MuseScore will grab it. There is no way to say "I don't wan't any MIDI input device"; the MIDI Input control in "Preferences" has no "None"/blank option like the MIDI Output menu has.
You might ask, "why is this problem? Just don't supply any input.", but if you have midi loopbacks set up, it is a big problem, and you have to set up extra ones just to placate this bug.
There is a subtlety here, too. Because of this bug (feature?), a person attaching a midi input device, say, a keyboard, doesn't have to use Preferences to select it -- MS "grabs" it, and if it didn't do so, there would be an additional step in using a midi keyboard not now necessary, i.e., to select it in Preferences.
Well, if that's a problem, then one might presume that it can continue grabbing midi input devices as available, for that reason alone, but one should be able to say "no, I wish you hadn't grabbed that", and select "none", but that creates a problem on restart if it grabs a midi device that is a loopback that, as very likely, has been chosen as MIDI output. The clean solution is to require explicit selection of a MIDI Input device and stopping this grabbing behavior, with default "none". Maybe it should simply avoid choosing the same input device as output device (as it warns you about now), but you still need a "none" option if it grabs it for input before you set it up as output.
So this is controversial.