In Metal/Ocean theme JComboBox has inconsistent border and height in comparison with JTextField and JSpinner. It puts obstacles in the way of creation of a good GUI under Metal LaF. Sample screen shots are in the attachment.

There is a test case:
============== Source Begin ===============
import javax.swing.*;
import java.awt.*;

/**
 * The test shows that under the Metal look and feel
 * JComboBox has inconsistent border and height
 * in comparison with JTextField and JSpinner.
 */
public class JComboBoxTest {
    private int FRAME_HEIGHT = 100;

    public static void main(String[] args) throws Exception {
        final JComboBoxTest test = new JComboBoxTest();
        SwingUtilities.invokeAndWait(new Runnable() {
            public void run() {
                test.setupUI();
            }
        });
    }

    private void setupUI() {
        UIManager.LookAndFeelInfo[] lafs = UIManager.getInstalledLookAndFeels();
        int yStep = FRAME_HEIGHT + FRAME_HEIGHT/2;
        int dy = -yStep * lafs.length/2;
        for (UIManager.LookAndFeelInfo laf : lafs) {
            setLookAndFeel(laf.getClassName());
            showFrame(laf.getClassName(), dy);
            dy += yStep;
        }
    }

    private void showFrame(String title, int dy) {
        JFrame frame = new JFrame(title);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new FlowLayout());

        frame.add(new JTextField("Text Field"));
        frame.add(new JSpinner(new SpinnerNumberModel(10, 1, 20, 1)));
        JComboBox combo = new JComboBox(new Object[] { "One", "Two", "Three"});
        combo.setEditable(true);
        frame.add(combo);

        frame.setSize(500, FRAME_HEIGHT);
        frame.setLocationRelativeTo(null);

        frame.setLocation(frame.getLocation().x, frame.getLocation().y + dy);
        frame.setVisible(true);
    }

    private static void setLookAndFeel(String laf){
        System.out.println("Trying to set: " + laf);
        try {
            UIManager.setLookAndFeel(laf);
            System.out.println("The laf is set: " + laf);
        } catch (Exception ex) {
            System.out.println("Cannot set the " + laf);
        }
    }
}
============== Source End ================


Testing strategy:
1. Run the test
2. Look at the frame with name "javax.swing.plaf.metal.MetalLookAndFeel". Note that the combo box is higher and has a wider border than JSpinner and JTextField.
3. Look at the frame with name "com.sun.java.swing.plaf.windows.WindowsLookAndFeel" (if it exists). There are no such problems.

Expected results:
At the frame with name "javax.swing.plaf.metal.MetalLookAndFeel" the combo box has the same height and border as JSpinner and JTextField. A good example is the frame "com.sun.java.swing.plaf.windows.WindowsLookAndFeel".

Actual results:
At the frame with name "javax.swing.plaf.metal.MetalLookAndFeel" the combo box is higher and has a wider border than JSpinner and JTextField.