问为什么小的JPopupMenu会产生视觉伪像，而大的不会？

EN

public class PopupTest {

    private static final int NUM_POPUP_ITEMS = 3;

    private JFrame frame = new JFrame();
    private JPanel panel = new JPanel();
    private TableModel model = new TableModel();
    private JTable table = new JTable();

    public static void main(String[] a) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new PopupTest();
            }
        });
    }

    public PopupTest() {
        panel.setLayout(new BorderLayout());
        panel.add(table, BorderLayout.CENTER);
        panel.setPreferredSize(new Dimension(400, 500));
        table.setModel(model);
        table.addMouseListener(new MouseAdapter() {
            @Override
            public void mousePressed(MouseEvent event) {
                popup(event);
            }
        });
        frame.setLocation(150, 150);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setContentPane(panel);
        frame.pack();
        frame.setVisible(true);
    }

    private void popup(MouseEvent e) {
        if (SwingUtilities.isRightMouseButton(e)) {
            JPopupMenu menu = new JPopupMenu();
            for (int i = 0; i < NUM_POPUP_ITEMS; i++) {
                menu.add(new JMenuItem(String.valueOf(i)));
            }
            menu.show(panel, e.getX(), e.getY());
        }
    }

    private class TableModel extends AbstractTableModel {

        private List<Double> dataList = new ArrayList<>();

        public TableModel() {
            for (int i = 0; i < 40; i++) {
                dataList.add(Math.random());
            }
        }

        @Override
        public int getRowCount() {
            return dataList.size();
        }

        @Override
        public int getColumnCount() {
            return 1;
        }

        @Override
        public Object getValueAt(int rowIndex, int columnIndex) {
            return dataList.get(rowIndex);
        }
    }
}