FULL PRODUCT VERSION :
java version "1.6.0_13"
Java(TM) SE Runtime Environment (build 1.6.0_13-b03)
Java HotSpot(TM) Client VM (build 11.3-b02, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Versione 5.1.2600]

Microsoft Windows [Version 5.2.3790]

Linux ws-devel8 2.6.27-11-generic #1 SMP Wed Apr 1 20:57:48 UTC 2009 i686 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
With Java 1.6.0_13 if you have an applet run through Java Web Start and you call:

getAppletContext().showDocument(myURL, "_blank");

the browser is not launched and the URL is not opened.
This problem sometimes happens with 1.6.0_12, too, although we have an applet in which this works correctly in 1.6.0_12, but does not work in 1.6.0_13.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. compile the attached code
2. create a JAR and sign it
3. create the following JNLP and put it into a web server, together with the JAR, adapting the codebase and jar href attributes values as appropriate:

<jnlp spec="1.5+" codebase="http://localhost:8080/test/" version="1.0">
  <information>
    <title>Test</title>
    <vendor>Test</vendor>
    <homepage href="http://www.sun.com"/>
  </information>
  <security>
    <all-permissions/>
  </security>
  <update check="timeout" policy="always"/>
  <resources>
    <j2se href="http://java.sun.com/products/autodl/j2se" version="1.6.0_13"/>
    <j2se version="1.6"/>
    <j2se version="1.5"/>
    <jar href="http://localhost:8080/test/test.jar" download="eager" main="false"/>
  </resources>
  <applet-desc name="Test" main-class="test.MyApplet" width="400" height="200">
  </applet-desc>
</jnlp>

4. connect to that jnlp using any browser
5. after the application has started, click on the "click" button

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
A new browser page is opened, pointing to http://www.sun.com

This happens correctly if you have any of the following tested Java version installed:

1.5.0_17
1.6.0_01
1.6.0_03
1.6.0_05
1.6.0_10
ACTUAL -
Nothing happens.

This is always reproducible with 1.6.0_13.

The attached test case reproduces the problem also with 1.6.0_12. However, we have a production applet in which the described method works correctly with 1.6.0_12, but does not work with 1.6.0_13.

The problem has been reproduced under Windows XP, Windows XP x64 Edition and Linux Ubuntu 8.10.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
No error messages are given. No exceptions are thrown.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
package test;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.net.URL;

import javax.swing.JApplet;
import javax.swing.JButton;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class MyApplet extends JApplet
{

	@Override
	public void init()
	{
		SwingUtilities.invokeLater(new Runnable()
			{
				public void run()
				{
					JPanel p = new JPanel();
					JButton b = new JButton("click");
					b.addActionListener(new ActionListener()
						{
							public void actionPerformed(ActionEvent e)
							{
								try
								{
									getAppletContext()
									  .showDocument(
									  new URL("http://www.sun.com"),
									  "_blank");
								}
								catch(Exception e1)
								{
									throw new RuntimeException(e1);
								}
							}
						});
					p.add(b);
					add(p);
				}
			});
	}
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Uninstall JRE 1.6.0_13.

SUPPORT :
YES

Release Regression From : 6u12
The above release value was the last known release where this 
bug was not reproducible. Since then there has been a regression.