#include <QApplication>
#include <QWidget>
#include <QDebug>

class MyWindow : public QWidget {
protected:
    void mousePressEvent(QMouseEvent*) {
        if(mask().isEmpty()) {
            qDebug() << "Setting mask";
            setMask(QRect(0,0,200,200));
        }
        else {
            qDebug() << "clearing mask";
            clearMask();
        }
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MyWindow w;
    w.showMaximized();
    return a.exec();
}