Custom popup view of QComboBox does not work correctly on Mac os X and GTK

#include <QtGui>

int main(int argc, char **argv){
  QApplication app(argc, argv);
  QTreeView *tree = new QTreeView();
  QStandardItemModel model(50, 4);
  for (int row = 0; row < 50; ++row) {
    for (int column = 0; column < 4; ++column) {
      QStandardItem *item = new QStandardItem(QString("row %0, column %1").arg(row).arg(column));
      model.setItem(row, column, item);
     }
  }
  tree->setMaximumHeight(500);
  tree->setMinimumHeight(500);
  QComboBox combo;
  combo.setView(tree);
  combo.setModel(&model);
  combo.show();
  return app.exec();
}