Keeps a viewport's scroll offsets inside the content: from 0 to the content's size at the zoom less the viewport's. In a paged mode (SinglePage, TwoUp) the range along the strip widens, when the first or last spread is shorter than the viewport, to the offsets that centre it, so the first and last spreads can be shown on their own.
public Viewport Clamp(in Viewport viewport)
The viewport.