FULL PRODUCT VERSION :
java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
ProductName: Mac OS X
ProductVersion: 10.12.4
BuildVersion: 16E195

A DESCRIPTION OF THE PROBLEM :
On macOS 10.12 the top and bottom edges of the JComboBox dropdown button are slightly higher than the top and bottom edges of the editable text field. On Mac OS X 10.11 the edges are more properly aligned.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
On macOS 10.12

javac ComboBoxDemo2.java
java ComboBoxDemo2

Observe alignment of dropdown button with editable text field, compare to Mac OS X 10.11

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
On Mac OS X 10.11 the top and bottom edges of the JComboBox dropdown button are more properly aligned with the top and bottom edges of the editable text field.
ACTUAL -
On macOS 10.12 the top and bottom edges of the JComboBox dropdown button are slightly higher than the top and bottom edges of the editable text field.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
/*
 * Copyright (c) 1995, 2008, Oracle and/or its affiliates. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *
 * - Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution.
 *
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 * contributors may be used to endorse or promote products derived
 * from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

// package components;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.*;
import java.util.*;
import java.text.SimpleDateFormat;

/* ComboBoxDemo2.java requires no other files. */
public class ComboBoxDemo2 extends JPanel
                           implements ActionListener {
    static JFrame frame;
    JLabel result;
    String currentPattern;

    public ComboBoxDemo2() {
        setLayout(new BoxLayout(this, BoxLayout.PAGE_AXIS));
        String[] patternExamples = {
                 "dd MMMMM yyyy",
                 "dd.MM.yy",
                 "MM/dd/yy",
                 "yyyy.MM.dd G 'at' hh:mm:ss z",
                 "EEE, MMM d, ''yy",
                 "h:mm a",
                 "H:mm:ss:SSS",
                 "K:mm a,z",
                 "yyyy.MMMMM.dd GGG hh:mm aaa"
                 };

        currentPattern = patternExamples[0];

        //Set up the UI for selecting a pattern.
        JLabel patternLabel1 = new JLabel("Enter the pattern string or");
        JLabel patternLabel2 = new JLabel("select one from the list:");

        JComboBox patternList = new JComboBox(patternExamples);
        patternList.setEditable(true);
        patternList.addActionListener(this);

        //Create the UI for displaying result.
        JLabel resultLabel = new JLabel("Current Date/Time",
                                        JLabel.LEADING); //== LEFT
        result = new JLabel(" ");
        result.setForeground(Color.black);
        result.setBorder(BorderFactory.createCompoundBorder(
             BorderFactory.createLineBorder(Color.black),
             BorderFactory.createEmptyBorder(5,5,5,5)
        ));

        //Lay out everything.
        JPanel patternPanel = new JPanel();
        patternPanel.setLayout(new BoxLayout(patternPanel,
                               BoxLayout.PAGE_AXIS));
        patternPanel.add(patternLabel1);
        patternPanel.add(patternLabel2);
        patternList.setAlignmentX(Component.LEFT_ALIGNMENT);
        patternPanel.add(patternList);

        JPanel resultPanel = new JPanel(new GridLayout(0, 1));
        resultPanel.add(resultLabel);
        resultPanel.add(result);

        patternPanel.setAlignmentX(Component.LEFT_ALIGNMENT);
        resultPanel.setAlignmentX(Component.LEFT_ALIGNMENT);

        add(patternPanel);
        add(Box.createRigidArea(new Dimension(0, 10)));
        add(resultPanel);

        setBorder(BorderFactory.createEmptyBorder(10,10,10,10));

        reformat();
    } //constructor

    public void actionPerformed(ActionEvent e) {
        JComboBox cb = (JComboBox)e.getSource();
        String newSelection = (String)cb.getSelectedItem();
        currentPattern = newSelection;
        reformat();
    }

    /** Formats and displays today's date. */
    public void reformat() {
        Date today = new Date();
        SimpleDateFormat formatter =
           new SimpleDateFormat(currentPattern);
        try {
            String dateString = formatter.format(today);
            result.setForeground(Color.black);
            result.setText(dateString);
        } catch (IllegalArgumentException iae) {
            result.setForeground(Color.red);
            result.setText("Error: " + iae.getMessage());
        }
    }

    /**
     * Create the GUI and show it. For thread safety,
     * this method should be invoked from the
     * event-dispatching thread.
     */
    private static void createAndShowGUI() {
        //Create and set up the window.
        JFrame frame = new JFrame("ComboBoxDemo2");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        //Create and set up the content pane.
        JComponent newContentPane = new ComboBoxDemo2();
        newContentPane.setOpaque(true); //content panes must be opaque
        frame.setContentPane(newContentPane);

        //Display the window.
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        //Schedule a job for the event-dispatching thread:
        //creating and showing this application's GUI.
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            }
        });
    }
}

---------- END SOURCE ----------