Created attachment 6090 
Screenshot of the issue.

As you can see on the screenshot, the small popup that comes up once the mixer button is pressed is floating over two monitors.

The same doesn't happen, if you replace e.g. the powermanager-plugin with the pulseaudio-plugin. Clicking the powermanager-plugin will pop up a small window as well, but this one keeps the boundary of the first monitor and avoids floating over two monitors

s/monitor/display/g

As you like :)

v0.2.1