Eike Hein authored Aug 05, 2017 and David Edmundson committed Aug 22, 2017

Summary:
TaskGroupingProxyModel uses a simple QVector<QVector<int>> populated
with source model row indices to represent the task group tree. To
implement QAbstractItemModel::parent(), its implementation of index()
encodes row indices of the top-level vector into the internal ids of
child item model indices. This allows parent() to produce the parent
model index by simply decoding the parent row from the passed-in child
index and call index() with that row.

Top-level row indices shift up and down as the list of top-level items
changes, invalidating those internal ids. QModelIndex is not meant to
be stored, and the proxy model does take care of updating any persis-
tent model indexes with new ids, so this should be fine.

However, where it falls apart is that as internal ids are invalidated,
a QSortFilterProxyModel on top of this proxy (i.e. TasksModel) may end
up with multiple indexes with identical internal ids in its mappin...