If you are showing popup on mouse press and continue drag w/o releasing mouse button,
then menu item under mouse is not selected.

Here is a test to reproduce the problem:

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

public class SwingMenus {
    static JFrame frame;
    static JButton button;
    static JPopupMenu menu;
    static class Listener extends MouseAdapter {
        public void mousePressed(MouseEvent e) {
            if (e.isPopupTrigger()) {
                menu.show(button, e.getX(), e.getY());
            }
        }

        public void mouseReleased(MouseEvent e) {
            if (e.isPopupTrigger()) {
                menu.show(button, e.getX(), e.getY());
            }
        }
    }
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable()
            {
                public void run() {
                    frame = new JFrame("test frame");
                    button = new JButton("test button");
                    menu = new JPopupMenu("test menu");
                    menu.add(new JMenuItem("item 1"));
                    menu.add(new JMenuItem("item 2"));
                    JMenu submenu1 = new JMenu("submenu 1");
                    menu.add(submenu1);
                    submenu1.add(new JMenuItem("item 1.1"));
                    submenu1.add(new JMenuItem("item 1.2"));
                    JMenu submenu2 = new JMenu("submenu 2");
                    menu.add(submenu2);
                    submenu2.add(new JMenuItem("item 2.1"));
                    submenu2.add(new JMenuItem("item 2.2"));
                    JMenu submenu21 = new JMenu("submenu 2");
                    submenu2.add(submenu21);
                    submenu21.add(new JMenuItem("item 2.1"));
                    submenu21.add(new JMenuItem("item 2.2"));
                    menu.add(new JSeparator());
                    menu.add(new JMenuItem("item 3"));
                    menu.add(new JMenuItem("item 4"));
                    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                    frame.setSize(200, 200);
                    frame.add(menu);
                    frame.add(button);
                    button.addMouseListener(new Listener());
                    frame.setVisible(true);
                }
            });
    }
}
I cannot reproduce the bug in 1.5.0_06.
I can reproduce it in 1.6b65, but only with modified test case:

---- Begin of source ---
import java.awt.event.*;
import javax.swing.*;

public class SwingMenus {
    static JFrame frame;
    static JButton button;
    static JPopupMenu menu;

    static class Listener extends MouseAdapter {
        public void mousePressed(MouseEvent e) {
            menu.show(button, e.getX(), e.getY());
        }

        public void mouseReleased(MouseEvent e) {
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                frame = new JFrame("test frame");
                button = new JButton("test button");
                menu = new JPopupMenu("test menu");
                menu.add(new JMenuItem("item 1"));
                menu.add(new JMenuItem("item 2"));
                JMenu submenu1 = new JMenu("submenu 1");
                menu.add(submenu1);
                submenu1.add(new JMenuItem("item 1.1"));
                submenu1.add(new JMenuItem("item 1.2"));
                JMenu submenu2 = new JMenu("submenu 2");
                menu.add(submenu2);
                submenu2.add(new JMenuItem("item 2.1"));
                submenu2.add(new JMenuItem("item 2.2"));
                JMenu submenu21 = new JMenu("submenu 2");
                submenu2.add(submenu21);
                submenu21.add(new JMenuItem("item 2.1"));
                submenu21.add(new JMenuItem("item 2.2"));
                menu.add(new JSeparator());
                menu.add(new JMenuItem("item 3"));
                menu.add(new JMenuItem("item 4"));
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setSize(200, 200);
                frame.add(menu);
                frame.add(button);
                button.addMouseListener(new Listener());
                frame.setVisible(true);
            }
        });
    }
}
--- End of source ---

It is regression.