Window transparency throws IAE if called before pack or setSize. The test case below throws IAE. However, if pack is moved before the call to setWindowOpaque, all is fine.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class TPTest extends JFrame {
    
    public TPTest() {
        super("TPTest");
        
        add(new JButton("Hi!"));
    }

    private static void createAndShowGUI(String[] args) {
        TPTest test = new TPTest();
        test.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        //test.setSize(400, 400);
        //test.setLocationRelativeTo(null);
        //test.setVisible(true);
        test.setUndecorated(true);
        com.sun.awt.AWTUtilities.setWindowOpaque(test, false);
        test.pack();
        test.setVisible(true);
    }
    
    public static void main(final String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI(args);
            }
        });
    }
}

IAE:

Exception in thread "AWT-EventQueue-0" java.lang.IllegalArgumentException: Width (0) and height (0) cannot be <= 0
at java.awt.image.DirectColorModel.createCompatibleWritableRaster(DirectColorModel.java:999)
at sun.awt.image.SunVolatileImage.getBackupImage(SunVolatileImage.java:225)
at sun.awt.image.VolatileSurfaceManager.getBackupSurface(VolatileSurfaceManager.java:252)
at sun.awt.image.VolatileSurfaceManager.initialize(VolatileSurfaceManager.java:108)
at sun.awt.image.SunVolatileImage.<init>(SunVolatileImage.java:72)
at sun.awt.image.SunVolatileImage.<init>(SunVolatileImage.java:101)
at java.awt.GraphicsConfiguration.createCompatibleVolatileImage(GraphicsConfiguration.java:284)
at java.awt.GraphicsConfiguration.createCompatibleVolatileImage(GraphicsConfiguration.java:218)
at sun.awt.windows.TranslucentWindowPainter$VIWindowPainter.getBackBuffer(TranslucentWindowPainter.java:279)
at sun.awt.windows.TranslucentWindowPainter.updateWindow(TranslucentWindowPainter.java:131)
at sun.awt.windows.WWindowPeer.updateWindow(WWindowPeer.java:652)
at sun.awt.windows.WWindowPeer.setOpaque(WWindowPeer.java:639)
at java.awt.Window.applyOpaque(Window.java:3299)
at java.awt.Window.addNotify(Window.java:652)
at java.awt.Frame.addNotify(Frame.java:469)
at java.awt.Window.pack(Window.java:686)
at TPTest.createAndShowGUI(TPTest.java:21)
at TPTest.access$000(TPTest.java:5)
at TPTest$1.run(TPTest.java:28)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)