Qlabel with transparent mask does not update background?

Hallo everyone,
I'm a noob for QT and I'm trying to develop a program for my research work.

I'm trying to produce a QLabel with a transparent background and a transparent mask, in order to be able to click "through" the background of the label and see what happens.
I managed to set a transparent mask for a label but then its background is not updated, so if something happens below the label I do not see it happening.

Please, help! ?(

Here is my code:

Code:

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#include <QApplication>
#include <QtGui>
 
void delay()
{
    QTime dieTime= QTime::currentTime().addSecs(5);
    while( QTime::currentTime() < dieTime )
    QCoreApplication::processEvents(QEventLoop::AllEvents, 100);    
}
 
 int main(int argc, char* argv[])
{
  QApplication a(argc, argv);
  QMainWindow* window = new QMainWindow;
  QPixmap pixmap(40,40);
 
  //window->setWindowFlags(Qt::WindowStaysOnTopHint);
  pixmap.fill(Qt::transparent);
  QPainter p(&pixmap);
  p.setPen(Qt::blue);
  p.drawLine(5,5, 40, 40);
  p.end();
  QLabel label;
  label.setPixmap(pixmap);
  label.setMask(pixmap.mask());
  label.setWindowFlags(Qt::WindowStaysOnTopHint);
  label.setAttribute(Qt::WA_TranslucentBackground);
 // label.setAutoFillBackground(true);
  label.show();
  //QCursor::setPos(40, 40);
  delay();
 
  label.move(40,40);
  return a.exec();
}

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