Mixer changes width of inspector when undocked.

Viewing the mixer causes the inspector to increase its width even though it is undocked. The inspector only needs to change its width if the mixer is being docked. The default behaviour should be that Musescore retains the inspector width except when docking the mixer.

See video of current behavior:

OS: Windows 10 (10.0), Arch.: x86_64, MuseScore version (64-bit): 3.1.0.7011, revision: 8b1a7dc