Code including an HBox spaces correctly when its class is statically included within the Stage.
When the exact class is 'insert' ed dynamically into the Stage - there is no spacing in the HBox - all nodes are overlayed in the first spot

we have this as a test example MainSceneGroup can extend Group or CustomNode - it still fails the same

public class MainSceneGroup extends Group{
    init {
        content = [
            Rectangle {
                width: 200,
                height: 200
                fill: Color.BLACK
            }
            HBox {
                spacing: 20
                content: [
                    Rectangle {
                        width: 20,
                        height: 20
                        fill: Color.WHITE
                    }
                    Rectangle {
                        width: 20,
                        height: 20
                        fill: Color.WHITE
                    }
                    Rectangle {
                        width: 20,
                        height: 20
                        fill: Color.WHITE
                    }
                ]
            }
        ]
    }
}



Now with a simple Main.fx

public def myStage = Stage {
    title: "HBox Test"
    resizable: false
    scene: Scene {
        width: 400
        height: 400
        content: MainSceneGroup{
        }
    }
};



It works like a charm - All three White Rectangles are nicely spaced

Now in my app - I followed the nice example of the WhiteOut Game and liked the idea of a transition screen. Splash --> Main Scene
So I started like this
I put an insert into the main Scene -
None of my HBox or VBox ever spaced themselves correctly - they always overlayed themselves !

So - back to the test
Swap the Main.fx above with this one

public def myStage = Stage {
    title: "HBox Test"
    resizable: false
    scene: Scene {
        width: 400
        height: 400
    }
};
 
function run() {
   var mainScene = MainSceneGroup {};
   insert mainScene into Main.myStage.scene.content;
}



Now - All the HBox white rectangles are on top of each other - no spacing