Insert note without advance
One of the most annoying things I find with Musescore is the auto advance when you get to the edge of the screen.
If you're just entering a single note with the mouse this is fine. However when you want to enter a chord you create the first note, and it vanishes off the side of the screen. So you scroll back, and add another note, and immediately it vanishes off the side of the screen again.
You need to have a way to enter a note while at the same time not advancing so the note you're working on doesn't keep getting shoved out of view. Maybe holding SHIFT while clicking to insert the note, as you would to enter notes of a chord using the keyboard-entry method.