I don't know Scala, but it's easy to read the example code and understand what it does. A lot of it reminds me of how well JavaFX integrates with Kotlin.
The configuration code reads very similar to how Kotlin uses apply{} to do the same things.
What I wonder, though, is how much you actually need ScalaFX to do this? Does it fall into the same category as TornadoFX does with Kotlin?
For instance, in the example code it uses some sort of extension of Application which looks like it makes the Stage setup more declarative. But, presumably you could use the standard start() routine and use the same kind of syntax to configure primaryStage. So how much value does the custom Application actually add?