Java SE (Java Platform, Standard Edition)

Hello,

I'm working on an application with a drag and drop window system. One Application window with 10+ inner windows draggable over the screen.
Now, I'm trying to implement a "popup" mechanism for inside of the inner windows. For example when clicking a button in a window, that the window turns black and a configuration panel appears on top.
This works great but I have some problems with JButtons underneath the popup panel. I active the popup by clicking a button, the window turns dark and the popup is shown where the button was clicked.
When I move the mouse out of the button (which is supposed to be in normal panel with a higher zorder than the popup) it results that the button turns visible through the popup.

To be more specific I've prepared a little test which demonstrates my problem:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

@SuppressWarnings("serial")
public class ButtonOverlapTest extends JPanel {

	private JPanel glasspane;

	public ButtonOverlapTest() {
		setPreferredSize(new Dimension(400,400));
		setLayout(null);
		
		JButton btnOpen = new JButton("open");
		btnOpen.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				showPopup();
			}
		});
		
		JPanel pnlCenter = new JPanel();
		pnlCenter.setSize(getPreferredSize());
		pnlCenter.setBackground(Color.red);
		pnlCenter.add(btnOpen);
		
		add(pnlCenter);
	}
	
	public void showPopup() {
		JButton btnClose = new JButton("close");
		btnClose.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				removePopup();
			}
		});
		JPanel popup = new JPanel();
		popup.setLayout(new BorderLayout());
		popup.setBackground(Color.white);
		popup.setPreferredSize(new Dimension(150,150));
		popup.add(btnClose, BorderLayout.SOUTH);
		
		glasspane = new JPanel();
		glasspane.setOpaque(true);
		glasspane.setSize(getPreferredSize());
		glasspane.setBackground(new Color(0,0,0,100));
		glasspane.add(popup);
		
		Component c = getComponent(0);
		add(glasspane);
		setComponentZOrder(c, 1);
		setComponentZOrder(glasspane, 0);
		validate();
		repaint();
	}
	
	public void removePopup() {
		remove(glasspane);
		validate();
		repaint();
	}
	
	public static void main(String[] args) {
		
		JFrame frame = new JFrame();
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.getContentPane().add(new ButtonOverlapTest());
		frame.pack();
		frame.setVisible(true);
	}
}

A simple frame with a red JPanel in it with a Button. When I click the button, my "popup" appears occulting the open button.
Perfect until I move the mouse out of the open button which is invisible. Once out of it bounds, it get's visible again.

Where is my fault?

Thanks in advance.