ADDITIONAL SYSTEM INFORMATION :
Windows 10 Home
JDK 15.0.1

A DESCRIPTION OF THE PROBLEM :
When using System L&F with extremely basic Swing programs (including the How to use Menus trail), JMenu gives inconsistent results on different machines.

Seems to work fine on Mac, but have seen inconsistent results on Windows 10 and Windows 7; two people with Windows 10 may experience different results, although using the same OS.

Tested with Java 8 to 15, occurs on all releases.

Please refer to this Stack Overflow post: https://stackoverflow.com/questions/65387329/why-does-jmenu-not-render-correctly-when-using-system-lf

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the program
Use menu before resizing to see (some-what) proper results
Resize the window

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Proper native graphics, consistent with other Windows applications (no highlight bleeding, no inconsistencies when resizing the window)
ACTUAL -
Highlight of "New..." bleeds into "Report" item, Z-Indexing of components seem to be ignored

---------- BEGIN SOURCE ----------
import javax.swing.*;

public class Demo {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(Demo::launchUI);
    }

    private static void launchUI() {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException e) {
            e.printStackTrace();
        }

        JFrame frame = new JFrame();
        JMenuItem report = new JMenuItem("Report");
        JMenu newMenu = new JMenu("New...");
        JMenu fileMenu = new JMenu("File");
        JMenuBar bar = new JMenuBar();

        newMenu.add(report);
        fileMenu.add(newMenu);
        bar.add(fileMenu);
        frame.setJMenuBar(bar);

        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
- Do not use System L&F (not much of a work-around; native feel is required)

FREQUENCY : always