FULL PRODUCT VERSION :
java version "1.6.0_01"
Java(TM) SE Runtime Environment (build 1.6.0_01-b06)
Java HotSpot(TM) Client VM (build 1.6.0_01-b06, mixed mode, sharing)

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

A DESCRIPTION OF THE PROBLEM :
If the Windows taskbar is set to auto-hide, a JFrame with setDefaultLookAndFeelDecorated as true and a sized to the screen height (not maximized) will disallow the taskbar from becoming visible.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Ensure your Windows taskbar is set to auto-hide.
Create a JFrame, call setDefaultLookAndFeelDecorated(true), and set the size of the frame to the size of the screen.
Display the frame and move your mouse to the Windows taskbar.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The Windows taskbar should appear.
ACTUAL -
The Windows taskbar does not appear. If I comment out the call to setDefaultLookAndFeelDecorated(true), and move my mouse to the taskbar then the taskbar does appear.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
The following code will keep the hidden taskbar from displaying:

import java.awt.GraphicsEnvironment;
import java.awt.Rectangle;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public final class TestFrame extends JFrame
{
  public TestFrame() {
    super("Taskbar Test");

    GraphicsEnvironment env = GraphicsEnvironment.getLocalGraphicsEnvironment();
    Rectangle screenBounds = env.getMaximumWindowBounds();
    this.setBounds(0, 0, screenBounds.width, screenBounds.height);
  }

  public static void main(String[] args) {
    JFrame.setDefaultLookAndFeelDecorated(true);

    try {
      SwingUtilities.invokeLater(new Runnable() {
        public void run() {
          (new TestFrame()).setVisible(true);
        }
      });
    }
    catch (Throwable t) {
      t.printStackTrace();
      System.exit(-1);
    }
  }
}




The following code will allow the taskbar to display (JFrame.setDefaultLookAndFeelDecorated(true) is commented out):

import java.awt.GraphicsEnvironment;
import java.awt.Rectangle;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public final class TestFrame extends JFrame
{
  public TestFrame() {
    super("Taskbar Test");

    GraphicsEnvironment env = GraphicsEnvironment.getLocalGraphicsEnvironment();
    Rectangle screenBounds = env.getMaximumWindowBounds();
    this.setBounds(0, 0, screenBounds.width, screenBounds.height);
  }

  public static void main(String[] args) {
// JFrame.setDefaultLookAndFeelDecorated(true);

    try {
      SwingUtilities.invokeLater(new Runnable() {
        public void run() {
          (new TestFrame()).setVisible(true);
        }
      });
    }
    catch (Throwable t) {
      t.printStackTrace();
      System.exit(-1);
    }
  }
}




The following code also will allow the taskbar to display (frame height is set to screen height - 1):
import java.awt.GraphicsEnvironment;
import java.awt.Rectangle;

import javax.swing.JFrame;
import javax.swing.SwingUtilities;

public final class TestFrame extends JFrame
{
  public TestFrame() {
    super("Taskbar Test");

    GraphicsEnvironment env = GraphicsEnvironment.getLocalGraphicsEnvironment();
    Rectangle screenBounds = env.getMaximumWindowBounds();
    this.setBounds(0, 0, screenBounds.width, screenBounds.height - 1);
  }

  public static void main(String[] args) {
    JFrame.setDefaultLookAndFeelDecorated(true);

    try {
      SwingUtilities.invokeLater(new Runnable() {
        public void run() {
          (new TestFrame()).setVisible(true);
        }
      });
    }
    catch (Throwable t) {
      t.printStackTrace();
      System.exit(-1);
    }
  }
}
---------- END SOURCE ----------