ADDITIONAL SYSTEM INFORMATION :
(Tested with multiple versions of Java from 8 through 14).

scrappy:Documents jimdouglas$ jdk-14.jdk/Contents/Home/bin/java -version
openjdk version "14-ea" 2020-03-17
OpenJDK Runtime Environment (build 14-ea+1-1)
OpenJDK 64-Bit Server VM (build 14-ea+1-1, mixed mode, sharing)
scrappy:Documents jimdouglas$ sw_vers
ProductName:	Mac OS X
ProductVersion:	10.14.5
BuildVersion:	18F132
scrappy:Documents jimdouglas$ 

A DESCRIPTION OF THE PROBLEM :
When opening a JComboBox component, it briefly shows the contents of the previously opened JComboBox. 

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
The best way to see this is to capture a video of the interaction (CMD+SHIFT+5), spend 10-15 seconds cycling through the JComboBox components, then step through the video frame by frame using the right and left arrow keys.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
When opening a JComboBox, I'd expect to see the contents of that specific list, with no rendering artifacts.
ACTUAL -
This effect is extremely subtle, but reasonably consistent; most of the time, opening the JComboBox very briefly flashes the contents of the previously opened list before switching over to the correct list.

---------- BEGIN SOURCE ----------
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ComboBox extends JPanel
{
    public ComboBox()
    {
        String[] x1 = { "a", "b", "c", "d", "e", "f", "g", "h", "i", "j" };
        String[] x2 = { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" };
        String[] x3 = { "k", "l", "m", "n", "o", "p", "q", "r", "s", "t" };
 
        JComboBox cb1 = new JComboBox(x1);
        JComboBox cb2 = new JComboBox(x2);
        JComboBox cb3 = new JComboBox(x3);
 
        add(cb1);
        add(cb2);
        add(cb3);

        setBorder(BorderFactory.createEmptyBorder(20,20,20,20));
    }
 
    public static void main(String[] args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                System.out.println(UIManager.getLookAndFeel());
                JFrame frame = new JFrame("ComboBox");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                JComponent newContentPane = new ComboBox();
                newContentPane.setOpaque(true);
                frame.setContentPane(newContentPane);
                frame.pack();
                frame.setVisible(true);
            }
        });
    }
}

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

CUSTOMER SUBMITTED WORKAROUND :
None.

FREQUENCY : always