Post an SSCCE.

Call
       frame.invalidate();
       frame.validate();

There is something interesting happening here.  The test code below
works fine calling setContentPane() from the EDT and following it with
validate().  If I take out the validate() statement it is messed up.  If
I call setContentPane() from the run() off the EDT it works fine.  I
don't think that is safe but it does work (at least it does on my computer).

It is a requirement that the content pane be an opaque component.  I
don't know what will happen if it isn't.

You should also look at JComponent.revalidate().  It schedules
validation of your JComponent's layout.  As it states in the docs, this
is generally not required with JComponents as any size change or layout
modification will call it automatically.

Top level containers that do not extend JComponent do need to have
validate() called when their layout is changed.

If my example here doesn't adequately describe your problem, please post
a test example of your own that shows the problem.

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

public class test6 extends JFrame implements Runnable {
    KPanel[] panels = new KPanel[2];
    int n;

    public test6() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        panels[0] = new KPanel(Color.GREEN);
        panels[1] = new KPanel(Color.BLUE);

        setContentPane(panels[0]);

        setSize(400,300);
        setVisible(true);

        new Thread(this).start();
    }

    public void run() {
        while (true) {
            ++n;
            n %= 2;
            Runnable r = new Runnable() {
                public void run() {
                    setContentPane(panels[n]);
                    validate();
                }
            };
            EventQueue.invokeLater(r);

//            setContentPane(panels[n]);

            try {
                Thread.sleep(2000);
            } catch (InterruptedException ie) { }
        }
    }

    class KPanel extends JPanel {
        final private Color color;

        public KPanel(Color color) {
            this.color = color;

            setLayout(new GridBagLayout());
            GridBagConstraints c = new GridBagConstraints();
            c.gridx = c.gridy = 0;  c.insets = new Insets(2,2,2,2);

            JButton[] b = new JButton[4];
            for (int i=0; i<b.length; i++)
                b[i] = new JButton("Button " + Integer.toString(i+1));

            add(b[0],c);
            ++c.gridx;
            add(b[1],c);
            c.gridx = 0;  ++c.gridy;
            add(b[2],c);
            ++c.gridx;
            add(b[3],c);
        }

        public void paintComponent(Graphics g) {
            g.setColor(color);
            g.fillRect(0,0,getWidth(),getHeight());
        }
    }

    public static void main(String[] args) {
        Runnable r = new Runnable() {
            public void run() {
                new test6();
            }
        };
        EventQueue.invokeLater(r);
    }
}

On Fri, 07 Sep 2007 17:24:07 -0700, A Watcher
<stocksami@earthlink.net> wrote, quoted or indirectly quoted someone
who said :

Normally you ADD JPanels to a JFrame.  Setting the contentPane is
meddling with the dark arts.