Non-modal JDialog is not painted and blocks the GUI

Question

I have developed a GUI that's basically a JFrame with a JDesktopPane.
The user can, via a menu item, pop up a JDialog that contains some JLists and then select some value from it. Once he/she has done the selection and clicks on OK, the dialog disappears (data processing is then done in the main GUI) and comes back once a specific event has happened. The user then selects other data and so on, until he/she clicks on Cancel, which definitely disposes of the JDialog.
The graphics of the JDialog are build in the class constructor, which does a pack() but does not make the dialog visible yet. The dialog appears only when doSelection() is called.
     /** Called the first time when user selects the menu item, and then
     when a specific event has happened. */
     public Data[] doSelection() {
          dialog.setVisible(true);
          // ... Code that reacts to user's input. Basically, the ActionListener
          // added to the buttons retrieves the user's selection and calls
          // dialog.setVisible(false) if OK is clicked, or calls dialog.dispose()
          // if Cancel is clicked.
     }Now, everything works fine if the JDialog is modal, but if I make it non-modal only the window decorations of the JDialog are painted, and the control doesn't return to the main GUI. Calling doLayout() or repaint() on the doSelection() has no effect. How can this be fixed?
I hope I have been able to explain the problem satisfactorily, I could not create a suitable SSCCEE to show you. Thanks in advance for any hint.

Answer 1

Ok, I've taken some time to think about this problem and I've modified the code a bit.
Now the dialog shows itself and is responsive (i.e. its JLists can be operated), but the Ok button does not close the dialog as I'd want. I believe that I'm messing up things about threading, and the operations in actionPerformed() should be carried out in another thread, if possible.
Thanks in advance for any hint / suggestion / comment / insult.
     private Data[] selection;
     /** Constructor */
     public MyDialog() {
          // ... Here is the code that builds the dialog...
          dialog.setModal(false);
          dialog.pack();
          // Note that the dialog is not visible yet
     public Data[] doSelection() {
          operatorAnswer = NONE_YET;
          dialog.setVisible(true);          
          while (operatorAnswer == NONE_YET) {
               try {
                    wait();
               } catch (InterruptedException e) { }
          return (operatorAnswer == OK ? selection : null);
     public void actionPerformed(ActionEvent evt) {
          if (okButton.equals(evt.getSource())) {
               operatorAnswer = OK;
               retrieveSelection();
               dialog.setVisible(false);
          else if (cancelButton.equals(evt.getSource())) {               
               operatorAnswer = CANCEL;
               dialog.dispose();
     private void retrieveSelection() {
          // ... Here is the code that retrieves selected data from the dialog's JLists
          // and stores it in the "selection" private array...
          notifyAll();
     }