While using Phonon::VideoWidget and QMainWindow, ActivationChange event is not received when the phone is locked for the first time. However when the phone is unlocked, multiple number of ActivationChange events are received.
Also, from the second time onwards, the events are received correctly.

Attached is an app (Modified version of TestStackWidget_2 from QTBUG-13761) which reproduces this issue.

Install the application on an N8, launch the app, click on screen 2. When in screen 2, lock the phone. Look for the qdebug stmt - "Received the activation change event". This log is printed everytime the app receives a QWidget::changeEvent with event type QEvent::ActivationChange