I'd expect paintChildren to work. You should be able to trace back where
the child components are painted from. For instance using
Thread.dumpStack(); is you are not using a debugger. You should be able
to see what is happening in each method.

Some Swing painting erroneously uses setClip which will clear any user
clipping region. It's difficult to get screen corruption without
disabling double buffering, but it'll ignore your clipping region.

Tom Hawtin

1. clipping in paintChildren.
2. making sure paintChildren is called on the panel and the children are
not painted directly. In principle, there is a way to do (JComponent.isPaint
Origin), but this is package-private. The hack to do it anyway is to
- override isOptimizedDrawingEnabled to return false
- add a bogus component on top that covers every other child (at least
partially?)
This may affect paint performance, but probably not too much.

Try this: (a) clips children, (b) paint on top of the children.

import javax.swing.*;

import java.awt.*;

public class POC
   extends JComponent
{
   public POC()
   {
    add(new JComponent() { });
    add(new JTextField("XXX 2301923 91283 ")
       {
        public boolean isValidateRoot()
        {
           return false;
        }
       });
   }

   public void doLayout()
   {
    getComponent(0).setBounds(0, 0, getWidth(), getHeight());

    Dimension d = getComponent(1).getPreferredSize();

    getComponent(1).setBounds(10, 10, d.width, d.height);
   }

   public Dimension getPreferredSize()
   {
    return new Dimension(400, 400);
   }

   public boolean isOptimizedDrawingEnabled()
   {
    return false;
   }

   protected void paintChildren(Graphics g)
   {
    Shape s = g.getClip();

       {
           g.clipRect(20, 20, getWidth() - 40, getHeight() - 40);
       }
    try
    {
       super.paintChildren(g);
    }
    finally
    {
       g.setClip(s);
    }

    g.setColor(Color.red);

    Rectangle r = getComponent(1).getBounds();

    g.fillRect(r.x + r.width / 2, r.y, r.width / 2, r.height / 2);
   }

   public static void main(String[] args)
   {
    POC p = new POC();

    JFrame f = new JFrame();

    f.getContentPane().add(p);

    f.pack(); f.show();
   }
}

Christian