无法在场景中正确定位 QGraphicsRectItem

我一辈子都想不通，但我把它归结为一个自成一体的问题。

我正在尝试做的是在QGraphicsScene中选择的项目周围绘制一个QGraphicsRectItem。绘制矩形后，可以以将所有项目一起移动的方式移动矩形。我已经研究了QGraphicsItemGroup，并决定在我的实际用例中不可行。

问题：我已经能够完成上述所有内容，除了我无法正确定位矩形项目，即它的大小合适，并且通过移动它移动所有项目，但它没有与所选项目的统一边界矩形对齐。我试图将所有内容都保留在场景坐标中，所以我不确定为什么会有偏移。

为什么似乎存在偏移，如何缓解？

这是可以通过按住 ctrl 单击或橡皮筋选择来测试的可运行代码（我知道这是大量的代码，但相关部分已注释）。

#####The line where the position of the rect item is set is marked like this#####
from PyQt4.QtGui import *
from PyQt4.QtCore import *
import sys

class DiagramScene(QGraphicsScene):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.selBox = None
        self.selectionChanged.connect(self.onSelectionChange)
    @pyqtSlot()
    def onSelectionChange(self):
        count = 0
        items = self.selectedItems()
        # Get bounding rect of all selected Items
        for item in self.selectedItems():
            if count == 0:
                rect = item.mapRectToScene(item.boundingRect())
            else:
                rect = rect.unite(item.mapRectToScene(item.boundingRect()))
            count += 1
        if count > 0:
            if self.selBox:
                # Update selBox if items are selected and already exists
                self.selBox.setRect(rect)
                self.selBox.items = items
            else:
                # Instantiate selBox if items are selected and does not already exist
                self.selBox = DiagramSelBox(rect, items)
                ##### Set position of selBox to topLeft corner of united rect #####
                self.selBox.setPos(rect.topLeft())
                self.addItem(self.selBox)
        elif self.selBox:
            # Remove selBox from scene if no items are selected and box is drawn
            self.removeItem(self.selBox)
            del self.selBox
            self.selBox = None

class DiagramSelBox(QGraphicsRectItem):
    def __init__(self, bounds, items, parent=None, scene=None):
        super().__init__(bounds, parent, scene)
        self.setFlag(QGraphicsItem.ItemIsSelectable, True)
        self.pressPos = None
        self.items = items
    def paint(self, painter, option, widget=None):
        pen = QPen(Qt.DashLine)
        painter.setPen(pen)
        painter.drawRect(self.rect())
    def mousePressEvent(self, e):
        # Get original position of selBox when clicked
        self.pressPos = self.pos()
        # mouseEvent is not passed on to scene so item selection
        # does not change
    def mouseMoveEvent(self, e):
        super().mouseMoveEvent(e)
        if self.pressPos:
            # Move selBox is original position is set
            newPos = self.mapToScene(e.pos()) - self.rect().center()
            self.setPos(newPos)
    def mouseReleaseEvent(self, e):
        # Update position of all selected items
        change = self.scenePos() - self.pressPos
        for item in self.items:
            item.moveBy(change.x(), change.y())
        super().mouseReleaseEvent(e)

if __name__ == "__main__":
    app = QApplication(sys.argv)
    view = QGraphicsView()
    view.setDragMode(QGraphicsView.RubberBandDrag)
    scene = DiagramScene()
    scene.setSceneRect(0, 0, 500, 500)
    rect1 = scene.addRect(20, 20, 100, 50)
    rect2 = scene.addRect(80, 80, 100, 50)
    rect3 = scene.addRect(140, 140, 100, 50)
    rect1.setFlag(QGraphicsItem.ItemIsSelectable, True)
    rect2.setFlag(QGraphicsItem.ItemIsSelectable, True)
    rect3.setFlag(QGraphicsItem.ItemIsSelectable, True)
    view.setScene(scene)
    view.show()
    sys.exit(app.exec_())