def mousePressEvent(self, event):
        # Handle CTRL+MouseClick box behavior
        if event.modifiers() & QtCore.Qt.ControlModifier:
            itemUnderMouse = self.itemAt(event.pos().x(), event.pos().y())
            if not itemUnderMouse:
                self.modifierBoxOrigin = event.pos()
                self.boxing = True
                event.accept()
                return
        QtGui.QGraphicsView.mousePressEvent(self, event)
Special handling is needed for a drag-box that toggles selected 
dataset/ETHPy150Open andrew-gardner/dependsworkflow/depends/depends_graphics_widgets.py/GraphicsViewWidget.mousePressEvent