Viewport is the part of window where you see the edited picture. It does not contain anything
particulary interesting apart from picture - only scrollbars.
The most important part of viewport's howto is how you can use the mouse buttons. Shades use the following convention: Left button (the usual one) is for invoking the "action", whatever it is. If the "action" consists of two clicks (start and end), right mouse button cancels it.
Middle mouse button is used for zooming, and together with keyboard allows for much faster movement around the picture than using scrollbars and zoom buttons. If you click on some point in picture, picture view is centered at the point and zoom increases one level (2x). As you can see, there are two actions - zoom and center. By holding the Shift key while clicking you can reverse zooming (zoom out) and when using Ctrl key, zooming is not performed at all.
Zooming can be also done with keys on numpad:
Default zoom level can be adjusted in options and defaults to 1.
Mouse wheel scrolls the whole viewport. If mouse pointer is over its certain height, scrolling is performed in vertical direction (as in web browsers or word processors), when under it, scrolling is horizontal. When Shades are allowed to use custom mouse cursors (default state), cursor shows an arrow indicating scrolling direction. The threshold value for this feature can be adjusted in options - or completely switched off.