This is great, thank you! I'll review this in more detail over the weekend, but at a glace it's a solid start.

Ideally, we would want to match the USB connection with the display ID in Windows (e.g. \\?\DISPLAY#GSM7707#5&6646027&1&UID4355). I don't have a Studio Display to test with, so I'll need some help figuring out how to identify them.

Ideally, we would want to match the USB connection with the display ID in Windows (e.g. \\?\DISPLAY#GSM7707#5&6646027&1&UID4355)

Can you explain what the individual values are in that string? I'm not sure what they would correspond to in the USB protocol. Or why the code sometimes uses key and sometimes id?

It seems id can be a string or number in a few code paths, but I only ever see it set to a string. key appears to be the last segment of the display id. I think both are meant to uniquely identify the display. There are a few things that need hwid (from which id is constructed) in order to send ddc messages. I think id and key might be redundant? All cases could be served by the serial number.

'5&2930019f&1&UID4357': {
  id: '\\\\?\\DISPLAY#APPAE3A#5&2930019f&1&UID4357',
  key: '5&2930019f&1&UID4357',
  num: 2,
  brightness: 50,
  brightnessMax: 100,
  brightnessRaw: 50,
  type: 'none',
  connector: 'displayport_external',
  min: 0,
  max: 100,
  hwid: [
    '\\\\?\\DISPLAY',
    'APPAE3A',
    '5&2930019f&1&UID4357',
    '{e6f07b5f-ee97-4a90-b076-33f57bf4eaa7}'
  ],
  name: 'StudioDisplay',
  serial: '2034519119',
  features: {
    luminance: false,
    brightness: false,
    contrast: false,
    powerState: false,
    volume: false
  },
  brightnessType: 0,
  brightnessValues: [ 50, 100 ]
},
'00008030-001E41402628802E': {
  id: '00008030-001E41402628802E',
  key: '00008030-001E41402628802E',
  num: 3,
  brightness: 100,
  brightnessMax: 100,
  brightnessRaw: 50,
  type: 'studio-display',
  connector: 'unknown',
  min: 0,
  max: 100,
  hwid: [],
  name: 'Apple Studio Display',
  serial: '00008030-001E41402628802E'
}

I dug through WMI and the USB interface trying to associate them. But the monitor's information doesn't match the USB interfaces information, so there doesn't seem to be a way. I could just guess and assign monitors that have the name: StudioDisplay and hwid[1]: APPAE3A values to a USB Studio Display (I'll need to overwrite the serial number so I know which one to talk to to get/set brightness). But that might get tricky if the user happens to have multiple studio displays. What happens if the lists return in a different order?

I think it'd be safer to stick with the serial number as key/id/serial approach.

Took me a little longer than expected to dig into this, but I think this is good enough to merge in. I'll make some additions/adjustments when I work it into v1.16.0. Apple Studio Display users won't get access to all of the Twinkle Tray features, but that's better than not having it work at all.

Thanks again for the PR! I'll tag you in the v1.16.0 beta issue when I have a build ready, if you don't mind testing.