Changed the list of xml:ids so that it's implemented with a true model based on QStringListModel, proxied through a QSortFilterProxyModel, enabling regular expression filtering. This is based on an example in Blanchette and Summerfield (pp. 250ff). The next step would be to replace the QTreeWidget with a QTreeView and build a similar model there, but I'm not quite sure how I would keep the two models in sync; the thing I don't yet understand is how, or even whether, you can have two distinct models which derive from the same underlying model, or one model which is simultaneously a QStringListModel and a custom tree model.