FULL PRODUCT VERSION :
java version "1.6.0_07"
Java(TM) SE Runtime Environment (build 1.6.0_07-b06)
Java HotSpot(TM) Server VM (build 10.0-b23, mixed mode)


ADDITIONAL OS VERSION INFORMATION :
Linux cy 2.6.26-1-686 #1 SMP Sat Jan 10 18:29:31 UTC 2009 i686 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
When a JTabbedPane is used with the WRAP_TAB_LAYOUT policy, its layout manager is TabbedPaneLayout.
TabbedPaneLayout.preferredTabAreaHeight determines the row count (local variable "rows") which is incremented when the width of the current tab row exceeds the acceptable width of the tabbed pane. At least, this is what I can guess, as it is not what is done. The row count is increased in the following "if":
if (x != 0 && x + tabWidth > width) {
    rows++;
    x = 0;
}

In that boolean expression, "width" is the width of the widest component among all tabs. This is *not* the maximum width of a row.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Runs an application that shows a main frame containing a JScrollPane with an embedded JTabbedPane.
The attached source code is the best way to reproduce the bug.

------------
Launch the application and make sure the scroll pane's scrollbar is not yet visible. Make the main frame just as high as needed to hide the scrollbar.
Click anywhere in the main frame to add tabs.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The visibility of the vertical scroll bar should not change when adding tabs, as long as the main frame is wide enough to enable the tabbed pane to lay out its tab in a single row.
ACTUAL -
When adding enough tabs, the vertical scroll bar appears. This is because the JScrollPane's decision to make the scrollbar visible or not is based on its managed component's preferred height. Each time a tab is added, the tabbed pane's preferred height increases from approximately the height of a tab.
This bug is not visible if the JTabbedPane is not contained by a layout that takes the preferred size into account.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.Dimension;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;

import javax.swing.BoxLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTabbedPane;


public class Program
{
/**
* Program main entry point.
* @param inArguments Command line arguments.
*/
public static void main(final String[] inArguments)
{
MainFrame frame = new MainFrame();
frame.setVisible(true);
}
}

class MainFrame extends JFrame
{
public MainFrame()
{
this.setSize(new Dimension(500, 120));

JPanel panel = new JPanel();
panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));
JTabbedPane tabbedPane = new JTabbedPane(JTabbedPane.TOP, JTabbedPane.WRAP_TAB_LAYOUT);
tabbedPane.addTab("Tab1", new Panel());
panel.add(tabbedPane);
tabbedPane.addMouseListener(new MouseListener());

JScrollPane scrollPane = new JScrollPane(panel);
this.add(scrollPane);
}
}

class MouseListener extends MouseAdapter
{
@Override
public void mouseClicked(MouseEvent e)
{
JTabbedPane tabbedPane = (JTabbedPane)e.getSource();
tabbedPane.addTab("Tab" + (tabbedPane.getTabCount() + 1), new Panel());
}
}

class Panel extends JPanel
{
public Panel()
{
this.setMinimumSize(new Dimension(150, 200));
this.add(new JLabel("Click to add tabs."));
}
}


---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
There is no bug if the SCROLL_TAB_LAYOUT policy is used. In that case, the tabbed pane uses a TabbedPaneScrollLayout that implements a different behaviour.