I have a dual monitor setup, and set the right screen as a primary output in System Settings > Display and Monitor. The panel and most applications respect that setting, however for some reason, when I click the audio icon in system tray, Kmix opens up on the left screen (which is not what I need). I didn't find any way to move it. Is it a bug or it's movable somehow?

That started happening not very long ago, after one of the Debian testing updates.

https://bugs.kde.org/show_bug.cgi?id=333803

Thanks, that looks like a mess. Why would anyone determine a primary display by using popups or mouse clicks?? Isn't it a queriable configuration?

I don't know that code at all (don't even have kmix installed), sorry.