How to show the UI of a DatePicker constantly. http://stackoverflow.com/questions/35650600/how-to-disable-hiding-of-combobox-popup-in-javafx8 , http://stackoverflow.com/questions/35721273/always-visible-datepicker-from-javafx .

ConstantDatePicker.java

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.DatePicker;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

/**
 * How to show the UI of a DatePicker constantly.
 *
 * http://stackoverflow.com/questions/35650600/how-to-disable-hiding-of-combobox-popup-in-javafx8
 *
 * http://stackoverflow.com/questions/35721273/always-visible-datepicker-from-javafx
 *
 * @author okr
 * @date 02.03.2016
 *
 */
public class ConstantDatePicker extends Application
{

  @Override
  public void start( final Stage primaryStage )
  {
    final DatePicker datePicker = new DatePicker();
    final StackPane root = new StackPane( datePicker );
    final Scene scene = new Scene( root, 250, 200 );
    primaryStage.setScene( scene );
    primaryStage.show();

    datePicker.setVisible( false );
    datePicker.setManaged( false );

    final com.sun.javafx.scene.control.skin.DatePickerSkin skin = (com.sun.javafx.scene.control.skin.DatePickerSkin) datePicker.getSkin();
    root.getChildren().add( skin.getPopupContent() );
  }

  /**
   * @param args ignored.
   */
  public static void main( final String[] args )
  {
    launch( args );
  }
}