QComboBox (Editable) adds item to wrong index when rootModelIndex is set

When QComboBox is editable, and using it with a hierarchical model (QStandardItemModel), and setting the root index with setRootModelIndex() to show a specific leaf of the model.
(This probably also happens with non editable QComboBox which has setRootModelIndex() other than the model root item)
Typing a new item in the editline and pressing enter adds the new item to the model, as one would expect.
However, one also would expect the item will be added to the leaf the QComboBox is showing, that is, as a child of the root index the combo box is set with - this however is NOT what happens.
The new item gets added to the root of the model it self.
I have looked at the code, the bug is in QComboBox::insertItem():

// For the common case where we are using the built in QStandardItemModel
     // construct a QStandardItem, reducing the number of expensive signals from the model
     if (QStandardItemModel *m = qobject_cast<QStandardItemModel*>(d->model)) {
         QStandardItem *item = new QStandardItem(text);//<<<<< BUG part 1
         if (!icon.isNull()) item->setData(icon, Qt::DecorationRole);
         if (userData.isValid()) item->setData(userData, Qt::UserRole);
         m->insertRow(index, item); //<<<< BUG part 2
         ++itemCount;
     }

This way of inserting a new item totally ignores the fact that QComboBox offers the ability to work with hierarchical models via setRootModelIndex(), and thus all the new items always end up as children of the model's root instead of the item combo box is using as root index.
I have patched it locally and it works well for me with the above scenario and as far as I could tell it still supports the old behavior as well, see attached patch.
Basically, the patch gets the item from the current root index the combo box has set - if its a valid one, it gets the corresponding item as parent for the new item, if not, it gets the invisibleRootItem() as parent for the new item, which is what (implicitly) happens in the original code.