To reproduce:

1. Right-click on the scroll bar at empty space between knob and arrow.
2. See that knob moves.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.ScrollBar;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class BugScrollBar extends Application {

    @Override
    public void start(Stage stage) {

        HBox root = new HBox(20d);
        
        ScrollBar scrollBar = new ScrollBar();
        root.getChildren().add(scrollBar);
        
        Scene scene = new Scene(root, 300, 250);

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

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

}