No SwiftUI, just plain Swift and AppKit/Cocoa. I considered SwiftUI, but at the time it was not ready and was missing a few core things I needed, like an outline view (this was 3 years ago).

Sometimes I wonder if I should have gone for a Catalyst app like Craft, for example, but again, at the time when I started Catalyst was in a very early stage. Today I'd probably give it another try. I also considered a Swift wrapper around an HTML/JS core (kind of my own slim Electron version) but decided to go for a native Mac app.