code: public void show(double viewportOffset) {
          if(viewportOffset < 0) {
              navigator.scrollCurrentPositionBy(viewportOffset);
          } else if(viewportOffset > sizeTracker.getViewportLength()) {
              navigator.scrollCurrentPositionBy(viewportOffset - sizeTracker.getViewportLength());
          } else {
              // do nothing, offset already in the viewport
          }
      }
docs: Forces the viewport to acts as though it scrolled from 0 to {@code viewportOffset}). <em>Note:</em> the
  viewport makes an educated guess as to which cell is actually at {@code viewportOffset} if the viewport's
  entire content was completely rendered.
  
  @param viewportOffset See {@link OrientationHelper} and its implementations for explanation on what the offset
  means based on which implementation is used.