[macos] Unified toolbar is broken in JDK 11

ADDITIONAL SYSTEM INFORMATION :
macOS Catalina
Version 10.15.3

A DESCRIPTION OF THE PROBLEM :
A unified toolbar can be achieved on macOS by setting the client property "apple.awt.brushMetalLook" to true on the root pane of a Frame. This will remove the visual appearance of the title bar.
The unified toolbar appearance is broken in Java 13.0.2 and in early access Java 14.

REGRESSION : Last worked in version 8u241

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
- Run the test program.
- Notice that some sort of title bar appears at the top.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
There should not appear a title bar like band, the window background should remain a smooth gradient.
ACTUAL -
There appears some kind of title bar at the top. The window background is no longer a smooth gradient.

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

public class AppleBrushMetal
{
    public static void main(String[] args)
    {
        SwingUtilities.invokeLater(() ->
        {
            JFrame f = new JFrame();
            JRootPane r = f.getRootPane();
            r.putClientProperty("apple.awt.brushMetalLook", true);
            f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            f.setSize(200, 200);
            f.setVisible(true);
        });
    }
}

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

FREQUENCY : always