Please run the following simple test:

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

public class Test {
    static JFrame frame;
    static JComboBox choice;
    public static void main(String args[]) {
        try {
            EventQueue.invokeAndWait(new Runnable() {
                public void run() {
                    frame = new JFrame("frame");
                    choice = new JComboBox<String>();
                    for (int i = 1; i < 5; ++i){
                        choice.addItem("item-0"+i);
                    }

                    frame.setBounds(100, 100, 300, 300);
                    frame.add(choice);
                    frame.setVisible(true);
                }
            });
        } catch(Exception ex) {}
    }
}

when HiDPI emulation is disabled, the choice could be changed; otherwise can not.

Mac OS X 10.10 + latest client build (Nov. 17)