Default JPanel rendered unstyled in JOptionPane

[ArthurKwasny]

The default JPanel from the javax.swing package won't render with its applied style, if it is displayed in a JOptionPane / JDialog (magenta / red panel).
If using an anonymous or named sub class of JPanel, the panel will be rendered without any issues (green / blue panel).

In a JFrame all panels are rendered as expected (top window):

It does not matter, how styling is applied, e.g. setting background via JPanel.setBackground(Color) will also not render the correct background color.

Tested with FlatLaf 3.2.5, Adoptium JDK 21 on Windows 10.

Test code:

import java.awt.Color;

import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.WindowConstants;

import com.formdev.flatlaf.FlatClientProperties;
import com.formdev.flatlaf.FlatLightLaf;

public class OptionPaneBug {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            FlatLightLaf.setup();

            // correct rendering:
            JFrame frame = new JFrame();
            frame.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
            frame.setContentPane(createPanel());
            frame.pack();
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);

            // fails to render main / r panel with its applied style:
            JOptionPane.showMessageDialog(frame, createPanel());
        });
    }

    static JPanel createPanel() {
        JPanel panel = new JPanel();
        panel.setBackground(Color.MAGENTA);

        // default class
        JPanel r = new JPanel();
        r.putClientProperty(FlatClientProperties.STYLE,
                "background: #ff0000; border: 12,12,12,12;");
        panel.add(r);

        // anonymous sub class
        JPanel g = new JPanel() {};
        g.putClientProperty(FlatClientProperties.STYLE,
                "background: #00ff00; border: 12,12,12,12;");
        panel.add(g);

        // named sub class
        JPanel b = new TestPanel();
        b.putClientProperty(FlatClientProperties.STYLE,
                "background: #0000ff; border: 12,12,12,12;");
        panel.add(b);

        return panel;
    }
}

and

public class TestPanel extends javax.swing.JPanel {}

[DevCharly]

This affects only JOptionPane (but not JDialog).

fixed in latest 3.3-SNAPSHOT: https://github.com/JFormDesigner/FlatLaf#snapshots

Thanks for reporting