1.I put an ObjectContainer3D object in scene, the container includes some children.
2.drag the container from original postion to somewhere,then children follow.there is no problem
3.drag the a child object of the container,then some offset is appear.
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drag3D = new Drag3D(view,container,Drag3D.PLANE_XY);//onMouseDown
drag3D.updateDrag();//onEnterFrame