isExpandableCell method
Decide whether to show the extended button.
Implementation
@override
bool isExpandableCell(PlutoCell cell) {
if (!cell.row.type.isGroup) return false;
final int checkDepth = showFirstExpandableIcon ? 0 : cell.row.depth;
return cell.row.type.isGroup &&
resolveColumnDepth(cell.column) == checkDepth;
}