Using the Spacebar to Pan

To pan your scene using the Spacebar, hold down the Spacebar and left-click+drag in your scene in the direction you want the scene to move.

With the Nano-Editor On:

If the Nano-Editor is on, you'll see the preview window show up over the working window where you clicked in the scene. The scene will pan inside the preview window and when you let up on the mouse, the scene itself will change to match the preview window.
With the Nano-Editor Off:

If the Nano-Editor is off, the entire scene will move as you drag over the Pan tool button. When you let up on the mouse, the scene will stop at the same place where you panned to.