headerWidthForRange function
Get the width of the rows headers given the actual range, this adjusts the
size of the header to show larger numbers.
The starting value is kRowHeaderWidth.
Implementation
double headerWidthForRange(Range range) {
final length = range.end.toString().length;
final digitOverflow = math.max(length - 3, 0);
return kRowHeaderWidth + digitOverflow * 8;
}