code: func (v *ViewPort) GetVisible() (int, int, int, int) {
  	return v.viewx, v.viewy, v.viewx + v.width - 1, v.viewy + v.height - 1
  }
docs: // GetVisible returns the upper left and lower right coordinates of the visible
  // content.  That is, it will return x1, y1, x2, y2 where the upper left cell
  // is position x1, y1, and the lower right is x2, y2.  These coordinates are
  // in the space of the content, that is the content area uses coordinate 0,0
  // as its first cell position.