Pretty cool! I have a Korg nanoKONTROL2 set up w/ MIDI Mixer to control the system volume, but was trying to get the transport control buttons to control Spotify. The plug in is installed and working (volume works), but none of the buttons (e.g. name: "Spotify: Play/Pause Button") don't show up as an assignment choice in MIDI Mixer when I create a button/group for the play (start) button. I do see errors in the dev console. I can't figure out if they are related to the buttons or not.
DevTools failed to load source map: Could not parse content for file:///C:/Users/User/AppData/Roaming/midi-mixer-app/plugins/ca.jaggernaut.midi-mixer.spotify/lib/index.js.map: Unexpected end of JSON input
DevTools failed to load source map: Could not parse content for file:///C:/Users/User/AppData/Roaming/midi-mixer-app/plugins/ca.jaggernaut.midi-mixer.spotify/lib/spotify-login.js.map: Unexpected end of JSON input
Any ideas of what the problem could be that I don't see any button assignment options?
(both MIDI mixer and your plugin are the latest versions: v2.7.3 and v1.1.3 respectively)