Double Mouse clicks needed to invoke Event using JDK1.3.1_01

I have a window with an editable combo box and a button. When user types text or selects an item in the combo box, the button will be enabled. When the user clicks on the button, the system will pop up a message box. However, it will take two clicks for the action to take place running with JDK1.3.1_01. It was working fine in JDK 1.2.2

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

public class TestButtonAction extends JFrame implements ActionListener, DocumentListener {

    JButton btnAction = new JButton("Action");
    JComboBox cmbAction = new JComboBox();

    public TestButtonAction() {
        this.getContentPane().setLayout(null);
        this.setSize(200,100);

        // Add an editable combo box
        cmbAction.setEditable(true);
        cmbAction.setBounds(10,10,100,20);
        this.getContentPane().add(cmbAction);

        btnAction.setEnabled(false);
        btnAction.setBounds(120,10,70,20);
        this.getContentPane().add(btnAction);

        btnAction.addActionListener(this);
        cmbAction.addActionListener(this);
              ((JTextField)cmbAction.getEditor().getEditorComponent()).getDocument().addDocumentListener(this);
    }

    public void actionPerformed(ActionEvent event) {
        Object source = event.getSource();

        System.out.println("In actionPerformed():" + source);

        if (event.getSource() == cmbAction) {
            System.out.println("Action event for cmbAction");
        } else if (source == btnAction) {
            System.out.println("Action event for btnAction");
            JOptionPane.showMessageDialog(this, "Action Button clicked");
        }
    }

    public void changedUpdate(DocumentEvent evt) {
        System.out.println("Document: changedUpdate");
        resetButtons();
    }

    public void insertUpdate(DocumentEvent evt) {
        System.out.println("Document: insertUpdate");
        resetButtons();
    }

    public void removeUpdate(DocumentEvent evt) {
        System.out.println("Document: removeUpdate");
        resetButtons();
    }

    public void resetButtons() {
        int length = ((JTextField)cmbAction.getEditor().getEditorComponent()).getDocument().getLength();
        btnAction.setEnabled(length > 0);
        System.out.println("In resetButtons(): " + cmbAction.isEnabled());
    }

    public static void main(String[] args) {
        TestButtonAction tb = new TestButtonAction();
        tb.setVisible(true);
    }
}