Wheel Mouse
Press the center wheel button to display the hand symbol. Hold the wheel button down and move the mouse in the direction you want the overlay to move. Release the wheel button to stop panning.
Non-Wheel Mouse
Press the Pan Button 
to display the hand symbol. Press and hold down the Left mouse button and move the mouse in the direction you want the overlay to move. Release the button to stop panning and press the Right mouse button once to end panning. The Arrow keys on the keyboard will also pan, but they move the overlay in the opposite direction indicated on the arrow key.
on the toolbar or press the Home key on the keyboard to zoom out to the original scale and re-center the overlay on the screen.