Get the height of a node in JavaFX (generate a layout pass)

Question

How to get the height or prefer height of a node in JavaFX, I have 3 VBox and I want to add nodes to the the most freer panel, example:

           Childrens      Total Height of the children's(Sum)
VBoxA          5                     890
VBoxB          4                     610
VBoxC          2                     720

in this case, the most freer is the VBoxB, I calculate the most freer pane with this method:

private int getFreerColumnIndex() {
    if(columns.isEmpty())
        return -1;

    int columnIndex = 0;
    int minHeight = 0;
    for(int i = 0; i < columns.size(); i++) {
        int height = 0;
        for(Node n : columns.get(i).getChildren()) {
            height += n.getBoundsInLocal().getHeight();
        }

        if(i == 0) {
            minHeight = height;
        } else if(height < minHeight) {
            minHeight = height;
            columnIndex = i;
        }

        if(height == 0)
            break;
    }

    return columnIndex;
}

This method only works if I add 1 element at the time. But if I add more elements at the time:

for (int i = 0; i < 10; i++) {
    SomeNode r1 = new SomeNode ();
    myPane.addElement(r1);
}

the method getFreerColumnIndex return the same index. This is because the new nodes dont have heigth in local yet.
So this line:

height += n.getBoundsInLocal().getHeight(); 

will return 0 with the new nodes.

Anyone knows how to get the heigth of a node ?

---

Extra:

SomeNode extends of Node

Method addElement() at myPane:

public void addElement(final Node element) {
     index = getFreerColumnIndex();
     columns.get(index).getChildren().add(element);
}

---

Extra 2:

suppose we have 3 vbox: Before:

 A      B      C
 |      |      |
        |      |
        |

Run:

for (int i = 0; i < 10; i++) {
    SomeNode r1 = new SomeNode ();
    myPane.addElement(r1);                      
}

After:

 A      B      C
 |      |      |
 |      |      |
 |      |
 |
 |
 |
 |
 |
 |
 |
 |

Correct:

 A      B      C
 |      |      |
 |      |      |
 |      |      |
 |      |      |
 |      |      |
 |

| = Some node

Answer 1

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.stage.Stage;
import java.io.Closeable;

public class Sizing  extends Application {

  @Override
  public void start(Stage stage) {
    var label = new Label();
    var labelHeight = label.getHeight();
    System.out.println(labelHeight);

    try (var s = new SizingSandbox(label)) {
      labelHeight = label.getHeight();
    }

    System.out.println(labelHeight);
  }

  public static void main(String[] args)  { launch(args); }
}


class SizingSandbox extends Group  implements Closeable {

  public SizingSandbox(Node... nodes) {
    new Scene(this);
    getChildren().addAll(nodes);
    layItOut();
  }

  @Override
  public void close() {
    try {
      getChildren().removeAll();
    } catch (Exception e) { }
  }

  private void layItOut() {
    applyCss();
    layout();
  }
}

// output:
// 0.0
// 17.0