Mixer scrollbar position is not remembered

Describe the bug
When you have a score with many instruments, which requires you to scroll the mixer to see all the channels, if you change the view away from the mixer and return to it, the scrollbar position is reset to default.

To Reproduce
(the following works on a 13½ inch width display)

1. Create a score with many instruments. For example, use the "Large pit percussion" template.
2. Display the mixer and scroll to the extreme right to see the last channel.
3. Now press P to display the piano, or move to another tab (Home, Publish, Dev tools), then move the view back to the mixer.

RESULT: The scroll bar has reset to default.

Expected behavior

Scrollbar positions should be retained as long as the mixer is open.

Platform information

OS: Linux Mint 20.1, Arch.: x86_64, MuseScore version (64-bit): 4.0.0-2492766781, revision: github-musescore-musescore-80d0eb6