To reproduce place mouse over the scroll bar and turn the wheel. Expect receiving onScroll.

import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.ComboBox;
import javafx.scene.control.ScrollBar;
import javafx.scene.input.ScrollEvent;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class App extends Application {

    public static void main(String[] args) {
        launch(args);
    }
    
    @Override
    public void start(Stage stage) throws Exception {
        HBox root = new HBox(4);

        root.getChildren().add(new ComboBox());

        ScrollBar scrollBar = new ScrollBar();
        scrollBar.setOnScroll(new EventHandler<ScrollEvent>() {
            @Override
            public void handle(ScrollEvent scrollEvent) {
                System.err.println("On Scroll");
            }
        });

        root.getChildren().add(scrollBar);

        Scene scene = new Scene(root, 404, 300);
        stage.setScene(scene);
        stage.show();
    }

}