ctrlutil.go
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
}
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
}
// 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