#include <QApplication>
#include <QGuiApplication>

#include <QStyle>
#include <QProxyStyle>

#include <QFileDialog>


class MyProxyStyle : public QProxyStyle
{ public:
    virtual int pixelMetric(PixelMetric metric, const QStyleOption *option = nullptr, const QWidget *widget = nullptr) const override;
};


int MyProxyStyle::pixelMetric(PixelMetric metric, const QStyleOption *option, const QWidget *widget) const
{ if(metric==QStyle::PM_SmallIconSize)
    return 50; // some large icon size

  return QProxyStyle::pixelMetric(metric,option,widget);
}


int main(int argc, char *argv[])
{ // non-integer device-pixel ratio causes problems
//  QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::PassThrough); // looks good at 1.5 device pixel ratio
  QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::RoundPreferFloor); // drive icons incorrect at 1.5 device pixel ratio

  // application
  QApplication app(argc,argv);

  // use style to increase default small icon size
  QApplication::setStyle(new MyProxyStyle());

  // tested on 4K screen under Windows 10 with a device pixel ratio 1.5
  // select "My Computer" (icon in combo box looks correct) and then select one
  // of the drives (icon in combo box is now distorted). folder icons are okay.
  QFileDialog filedialog;
// not relevant   filedialog.setOption(QFileDialog::DontUseNativeDialog,true);
  filedialog.show();

  return QApplication::exec();
}