Panning problem

Question

i'm modifying the code provided @ link

i'm trying to do pan without animating... so in the control_MouseMove method i did this

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void control_MouseMove(object sender, MouseEventArgs e)
        {
           
            if (this.IsMouseCaptured)
            {
               
                var physicalPoint = e.GetPosition(this);

                this.translateTransform.X = physicalPoint.X - this.ScreenStartPoint.X + this.startOffset.X;
                this.translateTransform.Y = physicalPoint.Y - this.ScreenStartPoint.Y + this.startOffset.Y;         

               
            }
        }

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the pan works when i first started the application, however, when i zoom in/out and tries to pan again, it wouldn't move, why is it so?