ADDITIONAL SYSTEM INFORMATION :
Windows 10, AdoptOpenJDK 15.0.1.9-hotspot

A DESCRIPTION OF THE PROBLEM :
We use internalFrame.setOpaque(true); and lnf without transparent shadows to speedup JDesktopPane redraw with many internal frames. When Windows scale is set to 125%, when we move or resize the internal frame, the artefacts are sometimes leaved uncleared: lines or dots from the right and bottom border of the frame. This happens always (on every app launch), but to see it someone need to move/resize the internal frame for some time to get luck. It seems that JDesktopPane does not clear/redraw one pixel on the right/bottom border due to some rounding effect with 125% scale. Tested with Windows (system) look and feel and also with FlatLaf.
Maybe it is related to https://bugs.openjdk.java.net/browse/JDK-8253530


---------- BEGIN SOURCE ----------
package desktoppanebug;

import java.awt.Color;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.JDesktopPane;
import javax.swing.JFrame;
import javax.swing.JInternalFrame;
import javax.swing.UIManager;

public class DesktopPaneBug {
    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception ex) {
            ex.printStackTrace();
        }

        JDesktopPane desktopPane = new JDesktopPane();
        desktopPane.setBackground(Color.WHITE);

        JInternalFrame frame = new JInternalFrame("InternalFrame");
        frame.setOpaque(true); // This is the main cause

        frame.setLocation(10, 10);
        frame.setSize(200, 100);
        frame.setResizable(true);
        frame.setVisible(true);

        desktopPane.add(frame);

        JFrame mainFrame = new JFrame("JInternalFrame 125% scale bug");
        mainFrame.getContentPane().add(desktopPane);
        mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainFrame.setSize(800, 500);
        mainFrame.setVisible(true);
    }
}

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

FREQUENCY : always