On MacOs QScrollArea::setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn) does not always display the vertical scroll bar

#include <QApplication>
#include <QDialog>
#include <QLayout>
#include <QScrollArea>
#include <QSize>
#include <QWidget>
#include <QLabel>
#include <QPushButton>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QDialog dialog;
    auto dialogLayout = new QVBoxLayout(&dialog);
    QWidget* mainWidget = new QWidget;
    QVBoxLayout* mainLayout = new QVBoxLayout(mainWidget);
    dialogLayout->addWidget(mainWidget);
    QWidget* topWidget = new QWidget;
    QScrollArea* topScrollArea = new QScrollArea;
    topScrollArea->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn);
    topScrollArea->setWidget(topWidget);
    topScrollArea->setWidgetResizable(true);
    mainLayout->addWidget(topScrollArea);
    topScrollArea->setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOn);
    QVBoxLayout *topLayout = new QVBoxLayout();
    topWidget->setLayout(topLayout);
    for (int i = 0; i < 10; i++){
        QPushButton *tempButton = new QPushButton(QString("%1").arg(i));
        tempButton->setMinimumHeight(50);
        topLayout->addWidget(tempButton);
    }
    dialog.show();
    return a.exec();
}