I don't really believe this one myself ... it's too simple to be true!

Attached is a test program. It is stripped down from something
a lot bigger, and was really intended to show problems with GridBagConstraints.

Instead, it shows a simpler problem: the Dialog does not paint on the screen
when told to do so.

However, the dialog believes it is visible, The following lines are the crux of
the code ....

// for a Dialog d ...

System.out.println(d);
d.show();
System.out.println(d);
System.out.println(d.getPeer());


They produce the output

java -cs Main
HelpAboutDialog[45,16,130x180,hidden,layout=java.awt.GridBagLayout,modeless,title=HelpAboutDialog]
HelpAboutDialog[45,16,130x180,layout=java.awt.GridBagLayout,modeless,title=HelpAboutDialog]
sun.awt.motif.MDialogPeer[HelpAboutDialog[45,16,130x180,layout=java.awt.GridBagLayout,modeless,title=HelpAboutDialog]]


which shows that the dialog has a sensible place, a sensible size, was hidden before
the call to show and is not hidden after the call to show., and has a peer.
The only problem is ... it is not showing on my screen!

xwininfo does not indicate the presence of a child of my frame in X land either.

The program was tested with
/usr/local/java/NIGHTLY/1.0betaM-3/bin/java

The program is .........
--------------------------------------------
import java.awt.Canvas;
import java.awt.Color;
import java.awt.Dialog;
import java.awt.Dimension;
import java.awt.Event;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Menu;
import java.awt.MenuBar;
import java.awt.MenuItem;
import java.awt.Rectangle;


public class Main extends Frame
{
  public static void main(String [] args) {
    new Main().show();
  }
    
  Main() {
    MenuBar mb = new MenuBar();
    Menu helpMenu = new Menu("Help");
    helpMenu.add(new HelpAbout(this));
    mb.add(helpMenu);
    mb.setHelpMenu(helpMenu);
    setMenuBar(mb);
    add("Center", new DemoCanvas(200, 100, Color.yellow));
    pack();
  }
}

class HelpAbout extends MenuItem
{
  public HelpAbout(Frame parent) {
    super("About");
    this.parent = parent;
  }

  public void postEvent(Event evt) {
    if (evt.id == Event.ACTION_EVENT && getLabel().equals(evt.arg)) {
      Dialog d = new HelpAboutDialog(parent);
      Dimension s = d.size();
      Rectangle b = parent.bounds();
      d.move(b.x + (b.width - s.width)/2, b.y + (b.height - s.height)/2);
System.out.println(d);
      d.show();
System.out.println(d);
System.out.println(d.getPeer());
    }
  }

  private Frame parent;
}


class HelpAboutDialog extends Dialog
{
  HelpAboutDialog(Frame parent) {
    super(parent, "HelpAboutDialog", false);
    setResizable(false);
    GridBagLayout gbl = new GridBagLayout();
    GridBagConstraints c = new GridBagConstraints();
    setLayout(gbl);
    c.gridwidth = GridBagConstraints.REMAINDER;
    c.anchor = GridBagConstraints.CENTER;
    c.insets.top = c.insets.bottom = c.insets.left = c.insets.right = 10;

    Color[] colors = {Color.red, Color.green, Color.blue};

    for (int i = 0; i < colors.length; i++) {
      DemoCanvas dc = new DemoCanvas(100, 30, colors[i]);
      gbl.setConstraints(dc, c);
      add(dc);
    }

    pack();
  }

  public boolean handleEvent(Event evt) {
    switch (evt.id) {
    case Event.WINDOW_DESTROY:
      dispose();
      return true;
    }

    return false;
  }
}


class DemoCanvas extends Canvas
{
  DemoCanvas(int w, int h, Color c) {

    this.c = c;
    d = new Dimension(w, h);
  }

  public void paint(Graphics g) {
    Dimension s = size();
    g.setColor(c);
    g.drawLine(0, 0, s.width, s.height);
    g.drawLine(s.width, 0, 0, s.height);
    g.drawRect(0, 0, s.width-1, s.height-1);
  }

  public Dimension minimumSize() { return d; }
  public Dimension preferredSize() { return d; }

  Color c;
  Dimension d;
}