App to reproduce:

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

public class Main extends Application {

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

    @Override
    public void start(Stage stage) throws Exception {
        HBox pane = new HBox();
        final ComboBox<String> combo = new ComboBox<String>();
        combo.setEditable(true);

        pane.getChildren().addAll(combo, new Button("Button"));

        Scene scene = new Scene(pane, 100, 100);
        stage.setScene(scene);
        stage.show();
    }
}