QGraphicsItem Scroll to Scale Unexpected Behavior

[oruxl]

I'm making a custom QGraphicsItem class which allows the user to resize it by scrolling. To do this, I override the class's wheelEvent method and change the item's scale by the scroll delta. I also set the scale origin to be at the mouse position so that the item is always under the mouse as the user scrolls.

This works only some of the time. There is often jumping due to the scale origin not being correctly set. The scale origin is set using the mouse's position from the QWheelEvent. Is there a more reliable way to implement this functionality?

Here's the full example:

#!/usr/bin/env python3

import sys
from PyQt5 import QtWidgets, QtGui

class DragAndScaleGraphicsItem(QtWidgets.QGraphicsRectItem):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.setFlags(QtWidgets.QGraphicsItem.ItemIsMovable)
        self.setAcceptHoverEvents(True)
        self._scale = 1.0
        self._minimum_scale = 0.1
        self._maximum_scale = 4.0

    def wheelEvent(self, ev):
        steps = ev.delta() / 8.0 / 15.0
        if steps == 0:
            return

        self._scale = min(self._maximum_scale, max(self._minimum_scale, self._scale + steps))
        xform = QtGui.QTransform()
        dx = ev.pos().x()
        dy = ev.pos().y()
        xform\
            .translate(dx, dy)\
            .scale(self._scale, self._scale)\
            .translate(-dx, -dy)
        self.setTransform(xform)


def main(args):
    app = QtWidgets.QApplication(args)
    window = QtWidgets.QMainWindow()
    window.resize(800, 600)
    window.setFocus()
    window.show()

    scene = QtWidgets.QGraphicsScene()
    view = QtWidgets.QGraphicsView(scene)
    window.setCentralWidget(view)

    item = DragAndScaleGraphicsItem(0, 0, 100, 200)
    item.setX(0)
    item.setY(0)
    item.setScale(1)
    view.scene().addItem(item)

    sys.exit(app.exec_())


if __name__ == '__main__':
    main(sys.argv)