FULL PRODUCT VERSION :
java version "1.8.0_20"
Java(TM) SE Runtime Environment (build 1.8.0_20-b26)
Java HotSpot(TM) 64-Bit Server VM (build 25.20-b23, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Mac 10.9.5

A DESCRIPTION OF THE PROBLEM :
In Java 7 and below, the Enter key event in a editable JComboBox is forwarded to trigger the default button action. In Java 8, default button is not triggered. Java 8 behavior is expected to be the same as Java 7.

REGRESSION. Last worked in version 7u55

ADDITIONAL REGRESSION INFORMATION:
java version "1.7.0_55"
Java(TM) SE Runtime Environment (build 1.7.0_55-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.55-b03, mixed mode)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Have a JComboBox. Make it editable.
2. Have a JButton in the same panel / frame. Make it the default button.
3. With focus on the JComboBox, press Enter key.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
In Java 7 and below, the Enter key event is forwarded to trigger the default button action. In Java 8, default button is not triggered. Java 8 behavior is expected to be the same as Java 7.
ACTUAL -
When I press Enter key, default button is not triggered.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.SwingUtilities;

public class MyFrame extends JFrame {

    private static MyFrame me;

    public MyFrame() {
        Container c = getContentPane();
        c.setLayout(new BorderLayout());

        // Press Enter key with focus on this component:
        JComboBox jcb = new JComboBox(new String[]{"Option: One", "Option: Two"});
        jcb.setEditable(true);
        c.add(jcb, BorderLayout.CENTER);

        JButton jb = new JButton("Ok");
        jb.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                JOptionPane.showMessageDialog(me, "Button pressed!");
            }
        });
        SwingUtilities.getRootPane(c).setDefaultButton(jb);
        c.add(jb, BorderLayout.EAST);

        pack();
        setVisible(true);
    }

    public static void main(String[] arg) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                me = new MyFrame();
            }
        });
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
The only workaround I can think of is having separate code paths for Java 8 and for earlier Java versions. For Java 8, I will have to manually capture the Enter key event and forward it to the button action.