#include <QtWidgets/QApplication>
#include <QtWidgets/QMainWindow>
#include <QtWidgets/QWidget>
#include <QtGui/QCursor>

class MouseArea : public QWidget
{
public:
    MouseArea(QWidget *parent)
        : QWidget(parent)
        , m_cursor_hidden(false)
    {}

   void mousePressEvent(QMouseEvent *) override
   {
       if (m_cursor_hidden == false) {
           m_cursor_hidden = true;
           QGuiApplication::setOverrideCursor( QCursor(Qt::BlankCursor) );
       } else {
           m_cursor_hidden = false;
           QGuiApplication::restoreOverrideCursor();
       }
   }

private:
   bool m_cursor_hidden;
};


class MainWindow : public QMainWindow
{
public:
    MainWindow(QWidget *parent = nullptr)
        : QMainWindow(parent)
        , m_area(this)
    {
        m_area.resize( this->size() );
    }

private:
    MouseArea m_area;
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.setFixedSize({600, 600});
    w.show();

    return a.exec();
}