#include <QtWidgets>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QGraphicsScene scene;

    QComboBox combo;
    combo.addItem("123");
    combo.addItem("456");
    combo.addItem("789");
    // Setting style sheet causes dropdown not opening
    combo.setStyleSheet("QCombobox{}");
    scene.addWidget(&combo);

    QGraphicsView view;
    view.setScene(&scene);
    view.resize(150, 100);
    view.show();

    return app.exec();
}