package javaapplication3;

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

public class JavaApplication3 extends Application {

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

    @Override
    public void start(Stage stage) throws Exception {
        
        ComboBox<String> cmb = new ComboBox<>();
        cmb.setEditable(true);
        
        TextField tf = new TextField();

         tf.textProperty().bind(cmb.getEditor().textProperty());

        HBox root = new HBox(10d);
        
        root.getChildren().addAll(cmb, tf);
        Scene scen = new Scene(root, 300, 200);
        stage.setScene(scen);
        stage.show();
    }
}

Steps to reproduce:
1. Type 123456789 in ComboBox text field.
2. Click on ComboBox and press end.
3. You will notice that last digits are hidden by the drop down button