Unexpected Offset in paintEvent

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?

Code:

class ContainerWidget(QWidget):
    def __init__(self):
        super().__init__()
        pal = QPalette()
        pal.setColor(QPalette.Window, Qt.magenta)
        self.setAutoFillBackground(True)
        self.setPalette(pal)
    def paintEvent(self, event):
        opt = QStyleOption()
        opt.initFrom(self)
        painter = QPainter(self)
        painter.setBrush( Qt.red )
        painter.drawRect(self.geometry().x(), self.geometry().y(), self.geometry().width(), self.geometry().height())
 
 
class TopWidget(QWidget):
    def __init__(self):
        super().__init__()
        pal = QPalette()
        pal.setColor(QPalette.Window, Qt.yellow)
        self.setAutoFillBackground(True)
        self.setPalette(pal)
    def paintEvent(self, event):
        painter = QPainter(self)
        painter.setBrush( Qt.green )
        painter.drawRect(self.x(), self.y(), self.width(), self.height())
 
class BottomWidget(QWidget):
    def __init__(self):
        super().__init__()
        pal = QPalette()
        pal.setColor(QPalette.Window, Qt.gray)
        self.setAutoFillBackground(True)
        self.setPalette(pal)
    def paintEvent(self, event):
        painter = QPainter(self)
        painter.setBrush( Qt.blue )
        painter.drawRect(self.x(), self.y(), self.width(), self.height())
 
def main():
    app = QApplication()
    containerWidget = ContainerWidget()
    containerLayout = QVBoxLayout(containerWidget)
    containerWidget.resize(800,600)
    containerLayout.setSpacing(0)
    containerLayout.setContentsMargins(0,0,0,0)
 
 
    containerWidget.show()
    sys.exit(app.exec_())
 
if __name__ == "__main__":
    main()

Attachment 13600