#include <QApplication>
#include <QBoxLayout>
#include <QLineEdit>
#include <QPushButton>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::Floor);

    QWidget w;
    QVBoxLayout* l = new QVBoxLayout(&w);
    QPushButton* btn = new QPushButton("Toggle Fullscreen");
    QLineEdit* le = new QLineEdit(QString(QT_VERSION_STR));
    l->addWidget(btn);
    l->addWidget(le);

    QObject::connect(btn, &QPushButton::clicked, &w, [&w](){
        if (!w.isFullScreen())
            w.showFullScreen();
        else
            w.showNormal();
    });

    w.show();
    return a.exec();
}