When the width of content is larger than the width of accordion it is not adjusted to the content's width.

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package test.example;

import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.TitledPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
import javafx.scene.control.Accordion;

/**
 *
 * @author dm_zinkevich
 */
public class AccordionApp extends Application {

    private static int SLOT_WIDTH = 350;
    private static int SLOT_HEIGHT = 250;

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

    @Override
    public void start(Stage stage) throws Exception {
        Accordion accordion = new Accordion();
        for (int i = 0; i < 5; i++) {
            Label label = new Label("Pane " + i + " Content");
            label.setPrefSize(SLOT_WIDTH * 1.2, SLOT_HEIGHT * 1.2);
            label.setMinSize(SLOT_WIDTH * 1.2, SLOT_HEIGHT * 1.2);
            label.setMaxSize(SLOT_WIDTH * 1.2, SLOT_HEIGHT * 1.2);
            TitledPane titled_pane = new TitledPane("Pane " + i, label);
            accordion.getPanes().add(titled_pane);
        }
        accordion.setMaxSize(SLOT_WIDTH, SLOT_HEIGHT);
        accordion.setPrefSize(SLOT_WIDTH, SLOT_HEIGHT);

        Label label = new Label("stage.setTitle(System.getProperty(\"java.runtime.version\") + \"; \" + System.getProperty(\"javafx.runtime.version\"));");
        label.setPrefSize(SLOT_WIDTH * 1.2, SLOT_HEIGHT * 1.2);
        label.setMinSize(SLOT_WIDTH * 1.2, SLOT_HEIGHT * 1.2);
        label.setMaxSize(SLOT_WIDTH * 1.2, SLOT_HEIGHT * 1.2);

        VBox root = new VBox(5d);
        root.getChildren().addAll(accordion, label);

        Scene scene = new Scene(root);
        stage.setScene(scene);
        stage.setTitle(System.getProperty("java.runtime.version") + "; " + System.getProperty("javafx.runtime.version"));
        stage.show();
    }
}