ADDITIONAL SYSTEM INFORMATION :
Windows 10; Ubuntu 16.04; Ubuntu 18.04
openjdk version "11.0.6" 2020-01-14
OpenJDK Runtime Environment AdoptOpenJDK (build 11.0.6+10)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 11.0.6+10, mixed mode)


A DESCRIPTION OF THE PROBLEM :
Issue with JOptionPane.showMessageDialog using a JTextPane (or JEditorPane).
An additional vertical white line appears at the right of the dialog box.

REGRESSION : Last worked in version 8

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
See 'Source code for an executable test case'

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Graphical result, the message box without extra white line.
ACTUAL -
Ugly message box.

---------- BEGIN SOURCE ----------
import javax.swing.BorderFactory;
import javax.swing.JOptionPane;
import javax.swing.JTextPane;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;
import javax.swing.UIManager.LookAndFeelInfo;
import javax.swing.text.html.HTMLEditorKit;
import javax.swing.text.html.StyleSheet;

public class TestMessageBox {

public static void main(String[] args) {
try {
for (LookAndFeelInfo info : UIManager.getInstalledLookAndFeels()) {
if ("Nimbus".equals(info.getName())) {
UIManager.setLookAndFeel(info.getClassName());
break;
}
}

int color = UIManager.getColor("Panel.background").getRGB() & 0x00FFFFFF;
final String colorBackground = String.format("#%06x", Integer.valueOf(color));

JTextPane pane = new JTextPane();
pane.setContentType("text/html");
pane.setEditable(false);
pane.setBorder(BorderFactory.createEmptyBorder());

HTMLEditorKit htmlEditorKit = (HTMLEditorKit)pane.getEditorKit();
StyleSheet styleSheet = htmlEditorKit.getStyleSheet();
styleSheet.addRule("ul{list-style-type: none; margin:4px 0px 12px 20px");
styleSheet.addRule("body{background-color: " + colorBackground + "; padding: 0px");
styleSheet.addRule("li{padding:2px;}");

pane.setText("Riri<br/>Fifi<br/>Loulou");

SwingUtilities.invokeLater(() -> JOptionPane.showMessageDialog(null, pane, "Information", JOptionPane.INFORMATION_MESSAGE));
}
catch (Exception e) {
e.printStackTrace();
}
}
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
import java.awt.Color;

import javax.swing.BorderFactory;
import javax.swing.JOptionPane;
import javax.swing.JTextPane;
import javax.swing.SwingUtilities;
import javax.swing.UIDefaults;
import javax.swing.UIManager;
import javax.swing.UIManager.LookAndFeelInfo;
import javax.swing.plaf.ColorUIResource;
import javax.swing.text.html.HTMLEditorKit;
import javax.swing.text.html.StyleSheet;

public class TestMessageBox3 {

public static void main(String[] args) {
try {
for (LookAndFeelInfo info : UIManager.getInstalledLookAndFeels()) {
if ("Nimbus".equals(info.getName())) {
UIManager.setLookAndFeel(info.getClassName());
break;
}
}

Color bgColor = UIManager.getColor("Panel.background");

UIDefaults defaults = new UIDefaults();
defaults.put("TextPane.background", new ColorUIResource(bgColor));
defaults.put("TextPane[Enabled].backgroundPainter", bgColor);

JTextPane pane = new JTextPane();

pane.setContentType("text/html");
pane.putClientProperty("Nimbus.Overrides", defaults);
pane.putClientProperty("Nimbus.Overrides.InheritDefaults", Boolean.TRUE);
pane.setEditable(false);
pane.setBorder(BorderFactory.createEmptyBorder());

HTMLEditorKit htmlEditorKit = (HTMLEditorKit)pane.getEditorKit();
StyleSheet styleSheet = htmlEditorKit.getStyleSheet();
styleSheet.addRule("ul{list-style-type: none; margin:4px 0px 12px 20px");
styleSheet.addRule("body{padding: 0px;}");
styleSheet.addRule("li{padding:2px;}");

pane.setText("Riri<br/>Fifi<br/>Loulou");

SwingUtilities.invokeLater(() -> JOptionPane.showMessageDialog(null, pane, "Information", JOptionPane.INFORMATION_MESSAGE));
}
catch (Exception e) {
e.printStackTrace();
}
}
}

FREQUENCY : always