JavaFX listener to a visible part of VBox - javafx

Question

I have a JavaFX VBox inside a BorderPane (central). The content of the VBox is calculated using some business logic and it depends on the height of the visible part of the vbox.
So basically I need a listener watching changes of the visible height of the vbox = height of the central part of the border pane.
The following code demonstrates what I have tried:
public class HelloFX extends Application {
#Override
public void start(Stage primaryStage) {
VBox vbox = new VBox();
vbox.boundsInParentProperty()
.addListener((obs, oldValue, newValue) ->
System.out.println(newValue.getHeight()));
Button button = new Button("ADD LINE");
button.setPrefHeight(25);
button.setOnAction(event ->
vbox.getChildren().add(new Label("line")));
BorderPane borderPane = new BorderPane();
borderPane.setCenter(vbox);
borderPane.setTop(button);
Scene scene = new Scene(borderPane, 100, 100);
primaryStage.setScene(scene);
primaryStage.show();
}
public static void main(String[] args) {
launch();
}
BorderPane with simple button on the top position and VBox on the central. The button click adds one line to vBox. Total scene height is 100, 25 is the button height and the rest (75) is the vBox.
I'm looking for some listener to report changes of the height of the central part of border pane. So in my example it should always print "75" no matter how many lines I have added to the vBox. The only event changing the value should be resizing the whole window. In reality once the vBox is filled my listener reports increasing height values. Apparently the height property includes the invisible part of the vbox.
EDIT
Finally I've found some solution - placing the vBox in the ScrollPane with disabled scrollbars. Then I can simply listen on the height property of the scrollpane and everything works as expected.
public class HelloFX extends Application {
#Override
public void start(Stage primaryStage) {
VBox vbox = new VBox();
ScrollPane scrollPane = new ScrollPane();
scrollPane.setVbarPolicy(ScrollPane.ScrollBarPolicy.NEVER);
scrollPane.setHbarPolicy(ScrollPane.ScrollBarPolicy.NEVER);
scrollPane.setContent(vbox);
scrollPane.heightProperty()
.addListener((obs, oldValue, newValue) ->
System.out.println(newValue));
Button button = new Button("ADD LINE");
button.setPrefHeight(25);
button.setOnAction(event ->
vbox.getChildren().add(new Label("line")));
BorderPane borderPane = new BorderPane();
borderPane.setCenter(scrollPane);
borderPane.setTop(button);
Scene scene = new Scene(borderPane, 100, 100);
primaryStage.setScene(scene);
primaryStage.show();
}
public static void main(String[] args) {
launch();
}
}