FULL PRODUCT VERSION :
java version "1.6.0_04"
Java(TM) SE Runtime Environment (build 1.6.0_04-b12)
Java HotSpot(TM) Client VM (build 10.0-b19, mixed mode, sharing)


ADDITIONAL OS VERSION INFORMATION :
Linux hp-pav1 2.6.22-14-generic #1 SMP Fri Feb 1 04:59:50 UTC 2008 i686 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
The calculation of the preferred size seems to be wrong when tabLayoutPolicy is set to JTabbedPane.SCROLL_TAB_LAYOUT. It misses for sure 4 pixels in height. Could be that the space needed for the components to scroll over the tabs were not taken into account correctly when calculation of the preferred size. The size needed when laying out the final components takes up more space so the actual height for the tab is reduced. I have added an simple example showing the problem without the 4 pixel correction and also an example where the correction is added to show how it can be worked around.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Execute the attached testcase


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
A JFrame showing a tab containing a JPanel with size 300,300 with scrollbars not shown, because pack has been called.
ACTUAL -
A JFrame showing a tab containing a JPanel with size 300,300 with scrollbars shown even though pack has been called.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
As described above this is the code for which scrollbars are shown:

public class TabbedPaneTest {

        public static void main(String[] args) {
                JFrame frame = new JFrame("Some Frame");
                JTabbedPane pane = new JTabbedPane();
                pane.setTabLayoutPolicy(JTabbedPane.SCROLL_TAB_LAYOUT);
                pane.add(new JScrollPane(new JPanel(){
                        public Dimension getPreferredSize(){
                                return new Dimension(300,300);
                        }
                }),"Some Title");
                frame.add(pane);
                frame.pack();
                frame.setVisible(true);
        }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
This code shows a workaround where preferred size (height) is corrected only when pane.setTabLayoutPolicy(JTabbedPane.SCROLL_TAB_LAYOUT) is set. This line can be commented out to see that in case tabLayoutPolicy is used no correction is needed (no scrollbars are shown then).

public class TabbedPaneTest {

        public static void main(String[] args) {
                JFrame frame = new JFrame("Some Frame");
                JTabbedPane pane = new JTabbedPane(){
                        public Dimension getPreferredSize(){
                                if (JTabbedPane.SCROLL_TAB_LAYOUT==getTabLayoutPolicy()){
                                        Dimension dim = super.getPreferredSize();
                                        dim.height +=4;
                                        return dim;
                                }else{
                                        return super.getPreferredSize();
                                }
                        }
                };
                pane.setTabLayoutPolicy(JTabbedPane.SCROLL_TAB_LAYOUT);
                pane.add(new JScrollPane(new JPanel(){
                        public Dimension getPreferredSize(){
                                return new Dimension(300,300);
                        }
                }),"Title 1");
                frame.add(pane);
                frame.pack();
                frame.setVisible(true);
        }
}