Demonstrating ChoiceBox focus behavior in JavaFx. https://stackoverflow.com/questions/35869386/javafx-automatically-expand-choicebox-when-on-focus

ChoiceBoxFocusApp.java

package com.isp.stackoverflow;

import java.util.Arrays;

import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.scene.Scene;
import javafx.scene.control.ChoiceBox;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.GridPane;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

/**
 * https://stackoverflow.com/questions/35869386/javafx-automatically-expand-choicebox-when-on-focus
 *
 * @author okr
 * @date 09.03.2016
 *
 */
public class ChoiceBoxFocusApp extends Application
{
  /**
   * @param args ignored.
   */
  public static void main( final String[] args )
  {
    launch( args );
  }

  @Override
  public void start( final Stage primaryStage )
  {
    final StackPane root = new StackPane();
    final GridPane pane = new GridPane();
    final ChoiceBox<String> box1 = new ChoiceBox<>();
    box1.getItems().addAll( "1", "2", "3" );
    final ChoiceBox<String> box2 = new ChoiceBox<>();
    box2.getItems().addAll( "a", "b", "c" );

    for ( final ChoiceBox<String> choiceBox : Arrays.asList( box1, box2 ) )
    {
      final ChangeListener<? super Boolean> showHideBox = ( __, ___, isFocused ) ->
      {
        if ( isFocused.booleanValue() )
        {
          choiceBox.show();
        }
        else
        {
          choiceBox.hide();
        }
      };
      choiceBox.focusedProperty().addListener( showHideBox );
      choiceBox.addEventFilter( MouseEvent.MOUSE_RELEASED, release ->
      {
        release.consume();
        choiceBox.requestFocus();
      } );
    }

    pane.add( box1, 0, 0 );
    pane.add( box2, 1, 0 );
    root.getChildren().add( pane );
    primaryStage.setScene( new Scene( root ) );
    primaryStage.show();
  }
}