Rendering glitches occur in QGraphicsView with background when resizeEvent scales QGraphicsItem

Question

I'm trying to scale a QGraphicsItem, (box) so that it exactly fits within a pair of lines drawn in/on the background of the QGraphicsView, as shown below

import sys
from PySide2.QtCore import Qt
from PySide2.QtGui import QPen
from PySide2.QtWidgets import QApplication , QGraphicsScene , QGraphicsView

class MyView(QGraphicsView):
    def drawBackground( self , painter , rect ):
        size = rect.size()

        # Two lines each drawn 10 units from the top and bottom of the view
        painter.drawLine( 0 , 10 , size.width() , 10 )
        painter.drawLine( 0 , size.height() - 10  , size.width() , size.height() - 10 )

    def resizeEvent( self , event ):
        size = event.size()
        box.setPos( 0 , 10 )

        # Box height = 30
        # Target height = size.height()- 20
        # (20 is total gap of lines in background)
        factor = ( size.height()- 20 )/30.0
        #print (factor * 30 ) - (size.height() - 20) # Check: prints 0.0 always
        box.setScale( factor )

if __name__=="__main__":
    app = QApplication(sys.argv)

    view = MyView()
    view.setAlignment( Qt.AlignLeft | Qt.AlignTop )
    view.setVerticalScrollBarPolicy( Qt.ScrollBarAlwaysOff )
    view.setHorizontalScrollBarPolicy( Qt.ScrollBarAlwaysOff )

    scene = QGraphicsScene()
    scene.setSceneRect( 0 , 0 , 300 , 300 )
    box = scene.addRect( 0 , 0 , 10, 30)

    pen = QPen()
    pen.setCosmetic( True )
    box.setPen( pen )

    view.setScene( scene )
    view.show()

    sys.exit(app.exec_())

The problem is that rendering glitches appear especially between the bottom of the box and the lower line in the background when the applicaion is resized. Refer to the circled portion of the screenshot below

Qt version 5.9.5, PySide2 version 5.12.1