code: func ItemByThumbPosition(position, itemCount, length int) int {
  	length -= 2
  	if position < 1 {
  		return -1
  	}
  	if itemCount < 1 {
  		return -1
  	}
  	if itemCount == 1 {
  		return 0
  	}
  
  	newPos := int(float32(itemCount-1)*float32(position-1)/float32(length-1) + 0.9)
  
  	if newPos < 0 {
  		newPos = 0
  	} else if newPos >= itemCount {
  		newPos = itemCount - 1
  	}
  
  	return newPos
  }
docs: // ItemByThumbPosition calculates item number by scrollbar
  // thumb position. Position - thumb position inside scrollbar,
  // itemCount - total number of items, lenght - lenght or heigth
  // of scrollbar. Return -1 if it is not possible to calculate:
  // e.g, itemCount equals zero