ADDITIONAL SYSTEM INFORMATION :
Windows 10 Pro, 64 Bit x64, Intel Core i5-6200U CPU @ 2.30GHz 2.40 GHz
java version "1.8.0_241"
Java(TM) SE Runtime Environment (build 1.8.0_241-b07)
Java HotSpot(TM) 64-Bit Server VM (build 25.241-b07, mixed mode)

A DESCRIPTION OF THE PROBLEM :
Simple JOptionPane window needs a pixel more in width.

! Attention !

Dear Sir or Madam of Java,
I am actually a software tester and am currently refreshing my Java skills. Here I became aware of this point during a task during a tutorial. And I wanted to report this situation as a bug if it is a bug.
Thank you for your comprehension.

REGRESSION : Last worked in version 8u241

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
STEP-1: Code or create a simple showInputDialog/showMessageDialog.
STEP-2: Display the window.
STEP-3: Check the right side, the right edge of the shown simple window.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
STEP-1: A simple showInputDialog/showMessageDialog coded.
STEP-1: A simple showInputDialog/showMessageDialog is displayed.
STEP-3: MAY the right edge of the shown simple window is identical with the right edge of the titel-header bar.
ACTUAL -
STEP-1: A simple showInputDialog/showMessageDialog coded. - OK
STEP-1: A simple showInputDialog/showMessageDialog is displayed. - OK
STEP-3: The right edge of the shown simple window is NOT identical by one pixel with the right edge of the titel-header bar. - nOK

---------- BEGIN SOURCE ----------
/*
Einlesen von Benutzereingaben als Konsolenanwendung
*/

import javax.swing.JOptionPane;

public class Kreisberechnung4JOptionPane {

public static void main(String[] args) {

double radius, umfang, flaeche;
String einheit, eingabe;

eingabe = JOptionPane.showInputDialog("Gebe den Radius ein:_");
radius = Double.parseDouble(eingabe);

eingabe = JOptionPane.showInputDialog("Gebe die Einheit ein:_");
einheit = eingabe;

umfang = 2.0 * 3 * radius;
flaeche = 3 * radius * radius;

JOptionPane.showMessageDialog(null, "Umfang: " + umfang + " " + einheit
+ "\nFlaeche: " + flaeche + " " + einheit + '\u00b2');

}
}
---------- END SOURCE ----------

FREQUENCY : always