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 )