#include <QApplication>
#include <QMainWindow>
#include <QDebug>

int main(int argc, char *argv[])
{
  //  QApplication::setAttribute(Qt::AA_EnableHighDpiScaling); // does not matter
  QApplication app(argc, argv);

  // Show child on the left screen
  QWidget *widget = new QWidget(nullptr, Qt::Dialog);
  widget->setGeometry(-1700, 200, 600, 400);
  widget->setWindowTitle("child");

  // Turn of the left (secondary) screen. The widget is no longer visible
  widget->show();

  app.exec();
}