QComboBox , QFontDialog，etc. popup not shown when used as editor for item view embedded in graphics scene

#include "mainwindow.h"
#include <QApplication>
#include <QComboBox>
#include <QGraphicsView>
#include <QStyledItemDelegate>
#include <QTableView>
#include <QtGui>

class Delegate : public QStyledItemDelegate
{
Q_OBJECT
public:
Delegate(QObject *parent = 0) : QStyledItemDelegate(parent)
{
}

QWidget* createEditor(QWidget *parent, const QStyleOptionViewItem &option, const QModelIndex &index) const

{ QComboBox *cb = new QComboBox(parent); cb->addItem("foo"); cb->addItem("bar"); cb->addItem("foobar"); return cb; }

};

#include "main.moc"
int main(int argc, char **argv)

{ QApplication app(argc, argv); QTableView *table = new QTableView; QStandardItemModel *model = new QStandardItemModel(4, 1, table); Delegate *delegate = new Delegate(table); table->setModel(model); table->setItemDelegate(delegate); QGraphicsView view; QGraphicsScene *scene = new QGraphicsScene; scene->addWidget(table); view.setScene(scene); view.show(); return app.exec(); }