It is a regression in new API.

Run the following sample:
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import javafx.stage.*;
import javafx.scene.*;
import javafx.scene.shape.*;
import javafx.scene.paint.*;

import java.lang.Math;
import javafx.animation.Timeline;
import javafx.animation.KeyFrame;

class Particle{
    public var x:Number;
    public var y:Number;
}

class BrownianMotion extends CustomNode{

    public var particles: Particle[];

    public override function create (): Node {
        return Group{
            content:
                for (particle in particles)
                Circle{
                    centerX: bind particle.x
                    centerY: bind particle.y
                    radius: 2
                    fill: Color.BLACK
                }
        }
    }

    public function run(){
        var dx = 6.0;
        for (particle in particles){
            particle.x += Math.random() * dx - dx / 2;
            particle.y += Math.random() * dx - dx / 2;
        }


    }

}

var d = 400;
var N = 50;

var brownianMotion = BrownianMotion{
    particles:
        for (i in [1..N])
        Particle{
            x: Math.random() * d
            y: Math.random() * d
        }
}

Timeline {
    repeatCount: Timeline.INDEFINITE
    keyFrames : [
        KeyFrame {
            time : 16ms
            action: function() {
                brownianMotion.run();
            }
        }
    ]
}.play();

Frame{
    title: "Brownian Motion"
    width: d
    height: d

    closeAction: function () { java.lang.System.exit(0); }

    scene: Scene{
        content: brownianMotion
    }
    visible: true
}
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The frame hangs.