FULL PRODUCT VERSION :
Java(TM) SE Runtime Environment (build 1.6.0-beta2-b77)

A DESCRIPTION OF THE PROBLEM :
JTabbedPane layout appears to result in unneccessary layout of non-visible Components. The JTabbedPane should mark Components as valid if are not the selected Component. If a JTabbedPane has many Components and it is resized it will result in the layout of all its Components. For hidden Components that have expensive layouts this can result in a slowing down of the resize/layout process.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run this test and resize the window. See that the ExpensiveLayoutManager attached to the non-visible Component, sleeps the resize process by 5 seconds.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;


public class JTabbedPane4260723 implements Runnable, ActionListener{

    Random ran = new Random();
    JTabbedPane jtp;
    public void run(){
jtp = new JTabbedPane();
        for(int i = 0; i < 10; i ++){
            JPanel jp = new JPanel(){
public void paint(Graphics g){
                        Dimension size = getSize();
                        int x = size.width/2;
                        int y= size.height/2;
                        g.setColor(Color.RED);
                        g.fillRect(x,y, size.width/4, size.height/4);
                    }
};
            if(i == 5) jp.setLayout(new ExpensiveLayoutManager());
            jp.setName(String.valueOf(i));
jtp.addTab(String.valueOf(i), jp);
        }
        JFrame jf = new JFrame();
        JButton jb = new JButton("Random Switch");
        jb.addActionListener(this);
        jf.add(jb, BorderLayout.SOUTH);
        jf.add(jtp);
        jf.setSize(new Dimension(300,300));
        jf.setVisible(true);


    }

    public void actionPerformed(ActionEvent e){
int i = ran.nextInt(10);
        jtp.setSelectedIndex(i);

    }

    public static class ExpensiveLayoutManager implements LayoutManager{

        public void addLayoutComponent(String name, Component comp) {}
        public void layoutContainer(Container parent) {
            try{
System.out.println("SLEEPING....");
                Thread.sleep(5000);
            }
            catch(Exception x){}

        }

        public Dimension minimumLayoutSize(Container parent){ return new Dimension(200,200); }
        public Dimension preferredLayoutSize(Container parent){
            return new Dimension(400,400);
        }
        public void removeLayoutComponent(Component comp) {}


    }

    public static void main(String ... args){
        if(args.length > 0){
            try{
int i = Integer.parseInt(args[0]);
                if(i == 0) UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                else if(i == 1) UIManager.setLookAndFeel("com.sun.java.swing.plaf.motif.MotifLookAndFeel");

            }catch(Exception x){}
        }
        SwingUtilities.invokeLater(new JTabbedPane4260723());
    }

}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Non-visible widgets should not engage in layout
ACTUAL -
they perform layout

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;


public class JTabbedPane4260723 implements Runnable, ActionListener{

    Random ran = new Random();
    JTabbedPane jtp;
    public void run(){
jtp = new JTabbedPane();
        for(int i = 0; i < 10; i ++){
            JPanel jp = new JPanel(){
public void paint(Graphics g){
                        Dimension size = getSize();
                        int x = size.width/2;
                        int y= size.height/2;
                        g.setColor(Color.RED);
                        g.fillRect(x,y, size.width/4, size.height/4);
                    }
};
            if(i == 5) jp.setLayout(new ExpensiveLayoutManager());
            jp.setName(String.valueOf(i));
jtp.addTab(String.valueOf(i), jp);
        }
        JFrame jf = new JFrame();
        JButton jb = new JButton("Random Switch");
        jb.addActionListener(this);
        jf.add(jb, BorderLayout.SOUTH);
        jf.add(jtp);
        jf.setSize(new Dimension(300,300));
        jf.setVisible(true);


    }

    public void actionPerformed(ActionEvent e){
int i = ran.nextInt(10);
        jtp.setSelectedIndex(i);

    }

    public static class ExpensiveLayoutManager implements LayoutManager{

        public void addLayoutComponent(String name, Component comp) {}
        public void layoutContainer(Container parent) {
            try{
System.out.println("SLEEPING....");
                Thread.sleep(5000);
            }
            catch(Exception x){}

        }

        public Dimension minimumLayoutSize(Container parent){ return new Dimension(200,200); }
        public Dimension preferredLayoutSize(Container parent){
            return new Dimension(400,400);
        }
        public void removeLayoutComponent(Component comp) {}


    }

    public static void main(String ... args){
        if(args.length > 0){
            try{
int i = Integer.parseInt(args[0]);
                if(i == 0) UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                else if(i == 1) UIManager.setLookAndFeel("com.sun.java.swing.plaf.motif.MotifLookAndFeel");

            }catch(Exception x){}
        }
        SwingUtilities.invokeLater(new JTabbedPane4260723());
    }

}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
None at this moment