Calling grabMouse on a widget does not work when the mouse is moved outside the application window - Mac OS X

#include <QApplication> 
#include <QMainWindow> 
#include <QToolBar> 
#include <QAction> 

#include <stdio.h> 
#include <time.h> 

class test: public QMainWindow 
{ 
Q_OBJECT; 

public: 
test() :QMainWindow() 
{ 
    mToolBar = new QToolBar(this); 
    addToolBar(mToolBar); 

    mTest = new QAction(this); 
    mTest->setText("test2"); 
    connect (mTest, SIGNAL(triggered()), 
    this, SLOT(onClicked())); 
    mToolBar->addAction(mTest); 

    setMouseTracking(true); 
} 

protected: 
void mouseMoveEvent ( QMouseEvent * /* event */ ) 
{ 
    printf("mouse move event %x\n", (int)time(0)); 
} 

public slots: 
void onClicked() 
{ 
    grabMouse(); 
} 
private: 
    QAction *mTest; 
    QToolBar *mToolBar; 
}; 

int main(int argc, char *argv[]) 
{ 
    QApplication app (argc, argv); 
    test mw; 
    mw.show(); 
    return app.exec(); 
} 

#include "moc_qttest.cpp"