How to add transparency slider to JColorChooser with GTK look

Before Java7, if you wanted the user to select a transparent color, you had to show JColorChooser

and add your own transparency slider as the standard color picker did not have an interface for transparency / opacity.

As of Java7, the standard JColorChooser

seems to include such a slider, so I removed my own transparency slider. However, JColorChooser

when using the GTK + look and feel, this slider / custom widget is missing.

Does anyone know of an easy way to add a transparency slider to the GTK + look and feel JColorChooser

(like a system property I have to set, a key-value pair in the UI manager ...)?

Looking at this related question I am afraid no such property exists. Otherwise it could probably have been used to disable / remove the transparency slider.

Screenshots to illustrate what you have with Metal versus what you get under GTK:

Metallic appearance

GTK + appearance

Code used to take screenshots

Of course, the code must be run with JDK7 or higher

import java.awt.Color;
import java.awt.EventQueue;

import javax.swing.JColorChooser;
import javax.swing.LookAndFeel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class ColorChooserExample {
  public static void main(String[] args) {
    EventQueue.invokeLater(
        new Runnable() {
          @Override
          public void run() {
            try {
              UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
//              UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
              JColorChooser.showDialog(null, "Pick color", Color.YELLOW );
            } catch (ClassNotFoundException e) {
              throw new RuntimeException(e);
            } catch (InstantiationException e) {
              throw new RuntimeException(e);
            } catch (IllegalAccessException e) {
              throw new RuntimeException(e);
            } catch (UnsupportedLookAndFeelException e) {
              throw new RuntimeException(e);
            }
          }
        }
    );
  }
}