getHeaderTop method
Implementation
double getHeaderTop(int next, int headerIndex, TextSpan span) {
if (next == 0 && headerIndex == 0) return 0;
if (next == 0) return 0;
int length = map[headers[next - 1]]!.length;
double rawLayer = length / crossAxisCount;
int layer = rawLayer.floor();
bool isBig = length > (layer * crossAxisCount);
double itemsHeight = (isBig ? layer + 1 : layer) * boxSize;
next--;
double bound =
headerSize + itemsHeight + getHeaderTop(next, headerIndex, span);
return bound;
}