The Scroll Bar in the mixer gets stuck all the way to the right
I can't 100% reproduce what is happening, but to fix this all you have to do is click the arrow at the top of a track in the mixer to once to open the extra tracks then click it again to close them.
Here is a description of one of the times the scroll bar got stuck.
1. Open a musescore file that has enough instruments to create a scroll bar at the bottom of the mixer.
2. Add a string instrument like a violin or a viola that has the arrow on the top left of the track.
3. Make sure that instrument is the last on the page so it will be farthest to the right in the mixer.
4. Click the arrow in the top left of that track, then change the patch associated with the arco, pizzicato, or tremolo patch.
5. Click the arrow again to hide the tracks.
6. The scroll bar is stuck on the right.