QStyledItemDelegate with QComboBox doesn't work on OS X

This ComboBox delegate won't open the list of options when clicking on it instead it just changes cell focus. Issue can be seen at least on OS X.

class ItemDelegate: public QStyledItemDelegate
{
public:

ItemDelegate(QTreeWidget* parent)
: QStyledItemDelegate(parent)
{}

QWidget* createEditor(QWidget *parent, const QStyleOptionViewItem &option, const QModelIndex &index) const
{
if (index.column() == 1)

{ QComboBox *pComboBox = new QComboBox(parent); pComboBox->addItem("Hello"); pComboBox->addItem("World"); return pComboBox; }

return NULL;
}
}