In the Diagram scene example, the graphics item will jump to its initial position if you move it and then right-click and choose bring to front without letting go of the left mouse button. When clicking the item again, it jumps and there is an offset beween the mouse cursor and the widget being moved.

-run the graphics view/Diagram scene Qt example
-create a square item
-left click on the square and start moving it
-right click while maintaining the left mouse button pressed: the menu appears
-move the mouse cursor on send to back an release mouse buttons
-click within the item's shape and move it : it jumps to the intial position.