[MU4 Issue] Mixer Button is not highlighted when mixer is open

[sammik]

Describe the bug When mixer is open, mixer button is not highlighted

To Reproduce Steps to reproduce the behavior:

1. Open Mixer (click Mixer Button, or press F10)
2. Mixer Button is not highlighted

Expected behavior I expect, button is highlighted, if Mixer is open (like Metronom button is highlighted, when is active)

Screenshots

https://user-images.githubusercontent.com/1646034/184070263-2139dfe0-b864-490a-9d98-492f9ad0f353.mp4

Platform information

• OS: [reported on Linux]