code: func ThumbPosition(itemNo, itemCount, length int) int {
  	length -= 2
  	if itemNo < 0 {
  		return -1
  	}
  	if itemNo >= itemCount-1 {
  		return length - 1
  	}
  
  	if length < 4 {
  		return 0
  	}
  
  	ydiff := int(float32(itemNo) / float32(itemCount-1.0) * float32(length-1))
  	return ydiff
  }
docs: // ThumbPosition returns a scrollbar thumb position depending
  // on currently active item(itemNo), total number of items
  // (itemCount), and length/height of the scrollbar(length)
  // including arrows. Returns position in interval of (1..lenght-2)
  // or -1 if the thumb is not visible