I have created a JavaFX application that needs to run as an overlay. Because of different requirements we’re starting it with Platform.startup(). The first stage that I create is a splash screen like so (pseudo-Kotlin follows):

class SplashScreen : Stage(StageStyle.TRANSPARENT) {
    init {
        isAlwaysOnTop = true

        // create root, configure, etc.

        val scene = Scene(root, w, h)
        scene.fill = Color.TRANSPARENT

        this.scene = scene
    }
}

fun main() {

    System.setProperty("apple.awt.UIElement", "true") // https://developer.apple.com/documentation/bundleresources/information_property_list/lsuielement
    Platform.setImplicitExit(false) // Due to app requirements
    Platform.startup {
        instance = SplashScreen().apply { show() }
    }
}

The problem with that code is that JavaFX seems to be ignoring the "apple.awt.UIElement" property, and creates a menu and adds it to the application switch pane in macOS.

On the other hand, if I call Platform.startup() inside a new thread it picks up the property without any issue and starts as required.

fun main() {

    System.setProperty("apple.awt.UIElement", "true") // https://developer.apple.com/documentation/bundleresources/information_property_list/lsuielement
    Thread {
        Platform.setImplicitExit(false) // Due to app requirements
        Platform.startup {
            instance = SplashScreen().apply { show() }
        }
    }.start()
}

Why is this behaving that way? I would expect that the property should be picked up by JavaFX right after defining it.