Why isn't my Qt eventFilter picking up mouse events?

Question

I have basically this code to intercept certain QTreeWidget events.

MyWidget :: MyWidget ()
{
     m_tree = new QTreeWidget ();
     // ...
     m_tree -> installEventFilter (this);
}

bool MyWidget :: eventFilter (QObject * obj, QEvent * e)
{
    if (m_tree != obj)
        return QWidget :: eventFilter (obj, e);

    qDebug () << e -> type ();

    switch (e -> type ())
    {
        case QEvent :: MouseButtonPress:
        case QEvent :: MouseButtonRelease:
        case QEvent :: MouseMove:
        case QEvent :: Leave:
            qDebug () << "GOT EM";
            break;
    };

    // ...
 }

As I click and move around in the QTreeWidget, the event handler prints out e->type() for lots of events, but not the mouse events. Mouse events appear never to happen (with the exception of Leave).

Why is this happening? I should be getting move events even with mouse tracking off provided at least one button is down, and I should be getting press and release events regardless. The QTreeWidget itself behaves normally, as if no event handler is installed.