Just for completeness, the rowCount() was still faulty. Here's the corrected version:
Qt Code: 
  1. int EnumItemModel::rowCount(const QModelIndex &parent) const
  2. {
  3.     if (parent.column() > 0)
  4.         return 0;
  5.     if (!parent.isValid())  // return total top level items
  6.         return enumTypes.count();
  7.     long parentRowPlusOne = (long)parent.internalPointer();
  8.     if (!parentRowPlusOne)  // type name, e.g. "Color"
  9.         return enumTypes[parent.row()].second.count();
  10.     // leaf item, e.g. "Red"
  11.     return 0;
  12. }
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