#include <QMainWindow>
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QMainWindow w;
    w.setWindowFlags(Qt::WindowTransparentForInput | Qt::WindowStaysOnTopHint);
    w.setWindowOpacity(0.3);
    w.showMaximized();
    return a.exec();
}