Does anybody know, why the Rect drawn in the paintEvent does not cover the exact area of my QWidget but instead starts at an weird offset?

Qt Code: 
  1. class ContainerWidget(QWidget):
  2.     def __init__(self):
  3.         super().__init__()
  4.         pal = QPalette()
  5.         pal.setColor(QPalette.Window, Qt.magenta)
  6.         self.setAutoFillBackground(True)
  7.         self.setPalette(pal)
  8.     def paintEvent(self, event):
  9.         opt = QStyleOption()
  10.         opt.initFrom(self)
  11.         painter = QPainter(self)
  12.         painter.setBrush( Qt.red )
  13.         painter.drawRect(self.geometry().x(), self.geometry().y(), self.geometry().width(), self.geometry().height())
  14.  
  15.  
  16. class TopWidget(QWidget):
  17.     def __init__(self):
  18.         super().__init__()
  19.         pal = QPalette()
  20.         pal.setColor(QPalette.Window, Qt.yellow)
  21.         self.setAutoFillBackground(True)
  22.         self.setPalette(pal)
  23.     def paintEvent(self, event):
  24.         painter = QPainter(self)
  25.         painter.setBrush( Qt.green )
  26.         painter.drawRect(self.x(), self.y(), self.width(), self.height())
  27.  
  28. class BottomWidget(QWidget):
  29.     def __init__(self):
  30.         super().__init__()
  31.         pal = QPalette()
  32.         pal.setColor(QPalette.Window, Qt.gray)
  33.         self.setAutoFillBackground(True)
  34.         self.setPalette(pal)
  35.     def paintEvent(self, event):
  36.         painter = QPainter(self)
  37.         painter.setBrush( Qt.blue )
  38.         painter.drawRect(self.x(), self.y(), self.width(), self.height())
  39.  
  40. def main():
  41.     app = QApplication()
  42.     containerWidget = ContainerWidget()
  43.     containerLayout = QVBoxLayout(containerWidget)
  44.     containerWidget.resize(800,600)
  45.     containerLayout.setSpacing(0)
  46.     containerLayout.setContentsMargins(0,0,0,0)
  47.  
  48.  
  49.     containerWidget.show()
  50.     sys.exit(app.exec_())
  51.  
  52. if __name__ == "__main__":
  53.     main()
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paintevent_off.jpg