Please run the provided test
Notice that when mouse slowly enters either frames it becomes selected
Move the mouse cursor from one frame *title bar* to another frame's *title bar*
If you do it several times back and forth you noticed that frames are out of sync -
 the mouse is over one frame, but the other one is still selected
You shold move the mouse quickly to reproduce it

Expected behavour:
If mouse is over the internal frame, it should be selected

Current behavour:
If you move the mouse quickly they become out of sync

JDK 6.0

import javax.swing.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.beans.PropertyVetoException;

public class JifProblem {
    public static void main(String[] args) throws Exception {
        final JFrame frame = new JFrame("JInternalFrame problem");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        JDesktopPane pane = new JDesktopPane();
        
        JInternalFrame f1 = getInternalFrame();
        f1.setBounds(15, 10, 300, 50);
        pane.add(f1);

        JInternalFrame f2 = getInternalFrame();
        f2.setBounds(330, 10, 300, 50);
        pane.add(f2);
        
        frame.add(pane);
        frame.setSize(650, 150);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    static private JInternalFrame getInternalFrame() {
        final JInternalFrame frame = new JInternalFrame("First frame");

        frame.setResizable(true);
        frame.addMouseListener(new MouseAdapter() {
            public void mouseEntered(MouseEvent e) {
                try {
                    frame.setSelected(true);
                } catch (PropertyVetoException e1) {
                    e1.printStackTrace();
                }
            }
        });
        frame.setVisible(true);
        return frame;
    }
}