             }
 
             if areAnySelected && areAnyNotSelected {
-                return .mixed
+                return .mixed   // TODO This doesn't seem to actually work for the sources group?
             }
         }
 

        guard let item = item else { fatalError() }

        let number = object as! NSNumber
        var newState = NSCell.StateValue(rawValue: number.intValue)
        // It doesn't make sense to switch from off to mixed, so go directly to on
        if newState == NSCell.StateValue.mixed {
            newState = NSCell.StateValue.on
        }

        let sources: [SMInputStreamSource]

        (cell as! NSCell).backgroundStyle = .light
    }

    private func buttonStateForInputSources(_ sources: [SMInputStreamSource]) -> NSControl.StateValue {
        let selectedSources = midiDocument.selectedInputSources

        var areAnySelected = false
