QGraphicsView is a subclass of QAbstractScrollArea which is the cause of these behaviors.

In the first case, the QAbstractScrollArea adds itself as a event filter to the MyGraphicsView when setViewport() is called. The QAbstractScrollArea s event filter captures the mouse event, first sends it through viewportEvent(), and then to the QWidget event handling which propagates to the MyGraphicsView mouse event handlers. Only after this is the QAbstractScrollArea s event filter finished and MyFilter gets to run.

In the second case, key events are delivered only to the MyGraphicsView because in setViewport() the QAbstractScrollArea sets itself as the focus proxy. If the focus proxy is reset with the following code, the key events will be delivered.

w.viewport()->setFocusProxy(0);

An alternative is to install the event filter on both the graphics view and its viewport, but modify the filter to only process key events from one object and mouse events from the other.

Change MyFilter.h

  QObject *keyObj;
  QObject *mouseObj;

public:
  MyFilter(QObject *keyObj, QObject *mouseObj, QObject *parent = NULL);

Change MyFilter.cpp

MyFilter::MyFilter(QObject *keyObj, QObject *mouseObj, QObject *parent /*= NULL*/ ) : QObject(parent), keyObj(keyObj), mouseObj(mouseObj)

and

if (obj == keyObj && e->type() == QEvent::KeyPress)
{
    qDebug()<<"Key Event recieved by MyFilter";
}
else if (obj == mouseObj && e->type() == QEvent::MouseButtonPress)
{
    qDebug()<<"Mouse Event recieved by MyFilter";
}

Change main.cpp

MyFilter *filter = new MyFilter(&w, w.viewport(), &w);

// Use this line to install to the viewport
w.viewport()->installEventFilter(filter);

//Use this line to install to MyGraphicsView
w.installEventFilter(filter);

How about to try not using filter but reimplement necessary QEvent handlers at MyGraphicsView like here:

void MyGraphicsView::mousePressEvent(QMouseEvent* pe)
{
if (pe->buttons() & Qt::LeftButton)
{
    this->setCursor(Qt::CrossCursor);
    zoomOrigin = pe->pos();
    rubberBand = new QRubberBand(QRubberBand::Rectangle, this);
    rubberBand->setGeometry(QRect(zoomOrigin, QSize(0,0)));
    rubberBand->show();
}
if (pe->buttons() & Qt::MidButton)
{
    panOrigin = pe->pos();
        this->setCursor(Qt::ClosedHandCursor);
}
}