It’s not necessary to inherit from DirectObject explicitly, since ShowBase already derives from it.
Yeah, this is an annoyance that most if not all new Panda users stumble over. And there will be plenty more, unless the default behaviour will be changed to not have the camera controlled by the mouse. Because that’s what disableMouse actually does, not that it will cause your mouse to stop working.
At the very least, it could be renamed to something more sensible, like “disableCameraController”.
Sorry for the rant 
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