JavaFX how to force Label to update its prefWidth after changing the text?

Pretty simple thing actually: I want my Label to update its prefWidth to fit the text into itself whenever the text changes.

Initially the Labels text is null and the prefWidth is -1.0 without setting it manually. So when the Labels text changes at runtime the prefWidth is still -1.0 and the Label displays "..." instead of the set text, because it is to small.

The Label is inside an HBox. That HBoxs prefWidth is bound to the sum of its childrens prefWidths and the HBox has enough space to grow.
Here's some code to reproduce:

public class Spielwiese extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage window) {
        Label label = new Label();
        Button button = new Button("Fill Label");
        button.setOnAction(e -> label.setText("Some Text"));
        HBox box = new HBox(label, button);
        box.prefWidthProperty().bind(label.prefWidthProperty().add(button.prefWidthProperty()));

        window.setScene(new Scene(new StackPane(box)));
        window.setWidth(800);
        window.setHeight(600);
        window.show();
    }
}

How can I get the Label to update its width to fit its text?

EDIT: The root of the Scene is a StackPane not a Pane.

It's never a good idea to try to perform layout using bindings on the min/pref/max size of a pane. The HBox already knows how to compute its preferred width, and by default does exactly what you want (the simple version is it computes it as the sum of the preferred widths of its child nodes, though in reality it's more complex, as it accounts for spacing, padding, etc.).

The default value for prefWidth for both controls and layout panes is the sentinel value Region.USE_COMPUTED_SIZE, which is just a flag to tell the node to compute its preferred size for reporting to whoever is laying it out. A label will compute its preferred size by checking the size required to display its text (and graphic), similarly for a button, and layout panes will examine the preferred sizes of their child nodes. The actual value of Region.USE_COMPUTED_SIZE is -1.0, which is why you always see that value when you call getPrefWidth().

The reason the HBox is being expanded to fill the stack pane is because that's exactly the behavior of a StackPane. From the docs:

> The stackpane will attempt to resize each child to fill its content area.

You can prevent this either by using a different root pane, which doesn't have that behavior, (e.g. a plain Pane or another HBox, etc.):

public class Spielwiese extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage window) {
        Label label = new Label();
        Button button = new Button("Fill Label");
        button.setOnAction(e -> label.setText("Some Text"));
        HBox box = new HBox(label, button);

        window.setScene(new Scene(new HBox(box)));
        window.setWidth(800);
        window.setHeight(600);
        window.show();
    }
}

or by wrapping your HBox in another pane (so the pane in which it is wrapped will fill the StackPane, but your box won't fill its parent):

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.HBox;
import javafx.scene.layout.Pane;
import javafx.scene.layout.Region;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class Spielwiese extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage window) {
        Label label = new Label();
        Button button = new Button("Fill Label");
        button.setOnAction(e -> label.setText("Some Text"));
        HBox box = new HBox(label, button);

        window.setScene(new Scene(new StackPane(new HBox(box))));
        window.setWidth(800);
        window.setHeight(600);
        window.show();
    }
}

or by forcing the box not to grow beyond its preferred size, by setting its maxWidth to use the preferred size:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.HBox;
import javafx.scene.layout.Pane;
import javafx.scene.layout.Region;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class Spielwiese extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage window) {
        Label label = new Label();
        Button button = new Button("Fill Label");
        button.setOnAction(e -> label.setText("Some Text"));
        HBox box = new HBox(label, button);
        box.setMaxWidth(Region.USE_PREF_SIZE);

        window.setScene(new Scene(new StackPane(box)));
        window.setWidth(800);
        window.setHeight(600);
        window.show();
    }
}

You didn't indicate in your question why you wanted to use a StackPane for the parent of the HBox. Since the sole functionality of a StackPane is to stack its child nodes on top of each other in z-order, if you're using a stack pane you're presumably adding other nodes on top of, or behind it. Consequently, it might actually not be necessary at all to prevent the HBox resizing (because it won't affect the stacking), but maybe it's enough just to ensure the alignment property of the HBox (and other child nodes of the StackPane) is set to the same as the StackPane (which is centered, by default). I.e. you might just need box.setAlignment(Pos.CENTER).