I have only used the FX2D renderer in size() and can’t add more to this topic.
I did some testing and this works with Processing 3.5.4 but not with Processing 4.0b8:
// Add this for Processing 4:
//import processing.javafx.*;
// Code from example here:
// https://www.geeksforgeeks.org/javafx-button-with-examples/
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
void setup()
{
size(640, 480, FX2D);
background(209);
stroke(255);
Stage s = new Stage();
s.setTitle("creating buttons");
// create a button
Button b = new Button("button");
// create a stack pane
StackPane r = new StackPane();
// add button
r.getChildren().add(b);
// create a scene
Scene sc = new Scene(r, 200, 200);
// set the scene
s.setScene(sc);
s.show();
}
void draw()
{
}
:)