Hi,
I very well know that QMainWindow is top level/parent window. That's what initially I did, when classes "RaptorLrsGUI" and "RaptorCoreGUI" are derived from QWidget and QStackedWidget is used , I don't see anything on screen, it goes blank. The below code is exactly as per suggested and it does not work.
Also I have changed the code to remove showMaximized on widget and use single shot timer.

Thanks for your patience..:-)

Qt Code: 
  1. Base::Base(QWidget *parent) : QMainWindow(parent, Qt::FramelessWindowHint)
  2. {
  3.        qDebug() << "Base CTOR called\n";
  4.  
  5.        QPalette pal = palette();  // retrieves the default palette for the widget, as defined by the app's style
  6.        pal.setColor( QPalette::Window, Qt::white );
  7.        setPalette( pal );
  8.  
  9.        setAutoFillBackground( true );
  10.  
  11.        //Show the first screen, loading raptor status       
  12.        lrsGUI = new RaptorLrsGUI;                                                   //RaptorLrsGUI derived from QWidget
  13.        lrsGUI->setlabel(QString("Loading Raptor Status..."));
  14.        coreGUI = new RaptorCoreGUI;                                             //RaptorCoreGUI derived from QWidget
  15.  
  16.        stackedWidget = new QStackedWidget();       
  17.        stackedWidget->addWidget(lrsGUI);
  18.        stackedWidget->addWidget(coreGUI);
  19.  
  20.        stackedWidget->setCurrentIndex(0);       
  21. }
  22.  
  23. Base::~Base()
  24. {
  25.     qDebug() << "Main Thread: destroying base objects \n";
  26.     delete lrsGUI;
  27.     delete coreGUI;
  28.     delete stackedWidget;
  29. }
  30.  
  31.  
  32. void Base::rcvInitData()
  33. {
  34.     qDebug()<< "Main Thread (CALLBACK): Display second screen\n";
  35.     stackedWidget->setCurrentIndex(1);
  36. }
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Rest of the code remain same as earlier.