Hi, here is a solution, and I think it works quite well in my application:

In the dialog's slot that connects with the activated() signal of the QSystemTrayIcon object, use the setParent() function as:
Qt Code: 
  1. void FtpDialog::sysTrayActivated(QSystemTrayIcon::ActivationReason reason)
  2. {
  3.     if (reason == QSystemTrayIcon::Trigger){
  4.         setParent(NULL, Qt::Window);
  5.        showNormal();
  6.         sysTrayIcon->hide();
  7.     }
  8. }
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And in the dialog's event() function:

Qt Code: 
  1. bool FtpDialog::event(QEvent *e)
  2. {
  3.     if (e->type() == QEvent::WindowStateChange) {
  4.         if (isMinimized()) {
  5.             setParent(tmp, Qt::SubWindow);
  6.             sysTrayIcon->show();
  7.             e->ignore();
  8.             qDebug()<<"event() called";
  9.         } else {
  10.             e->accept();
  11.         }
  12.  
  13.     }
  14.     return QDialog::event(e); // return with QDialog's event() call anyway.
  15. }
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Here, 'tmp' is an empty QWidget serving as a provisional 'parent' of the dialog, so that the dialog can be hidden from task-bar when minimized. In fact, you can just pass NULL as the first argument for the 'parent' widget, but in this case probably the re-stored window would exhibit slightly different with the original window size.