Dragging from a QAbstractItemModel drops empty string - c++

Question

I derived a custem model from QAbstractItemModel and implemented drag&drop support by
overloading a flags() method that returns Qt::ItemIsDragEnabled where applicable,
overloading supportedDropActions and
implementing mimeData() to properly return a QMimeData object with some string set for the text/plain format.
When I setup this model in a QTreeView and enable dragging via setDragEnabled(true), I can pick items from the tree view and drag them. For example, I can drag an item and drop it onto a QLineEdit, which in turn will display the string that the model returned via mimeData().
However, when I drag an item and drop it onto another application's widgets (such as notepad or simply a second instance of my application) an empty string gets dropped.
I found another clue using the Drop site example shipped with Qt: When I drag an item over it, it correctly shows that there is text/plain available and it even displays the string associated with it. Yet, when I drop the item I get an empty string again in the dropEvent()...
Sourcecode
ssce.pro:
QT += core gui widgets
TARGET = ssce
TEMPLATE = app
SOURCES += main.cpp
main.cpp:
#include <QtCore>
#include <QtWidgets>
struct Model: public QAbstractItemModel {
int rowCount(const QModelIndex &parent) const {
return parent.isValid() ? 0 : 1;
}
int columnCount(const QModelIndex &parent) const {
return 1;
}
QModelIndex index(int row, int column, const QModelIndex &parent) const {
return parent.isValid() ? QModelIndex() : createIndex(row, column, static_cast<void *>(0));
}
QModelIndex parent(const QModelIndex &child) const {
return QModelIndex();
}
Qt::ItemFlags flags(const QModelIndex &index) const {
return Qt::ItemIsEnabled | Qt::ItemIsSelectable | Qt::ItemIsDragEnabled;
}
QVariant data(const QModelIndex &index, int role) const {
return (role == Qt::DisplayRole) ? "Test item" : QVariant();
}
QStringList mimeTypes() const {
return QStringList("text/plain");
}
QMimeData *mimeData(const QModelIndexList &indexes) const {
QMimeData *d = new QMimeData();
d->setText("hello world");
return d;
}
Qt::DropActions supportedDropActions() const {
return Qt::CopyAction;
}
};
int main(int argc, char *argv[]) {
QApplication a(argc, argv);
QDialog d;
QVBoxLayout l;
QLineEdit e;
l.addWidget(&e);
QTreeView v;
Model m;
v.setModel(&m);
v.setDragEnabled(true);
l.addWidget(&v);
d.setLayout(&l);
d.show();
return a.exec();
}
Description:
Run this and you can drag the item from the tree view into the line edit, it will display hello world then. Drop the item anywhere else, including the line edit of a second instance of the sample application, and nothing will happen.