The current Harmattan Qt Quick components application template implementation is broken. It actually creates two instances of QDeclarativeView: one from the booster cache in the QmlApplicationViewer::create method and the other implicitly, as the base class instance gets created.
See more detailed description in here: https://projects.maemo.org/bugzilla/show_bug.cgi?id=289079
In addition to being ineffective and not really getting the benefit of the booster, two instances causes problems when you want to do more advanced things with the QDeclarativeView instance, e.g. create your own event filters (see e.g. the event filter described in the Harmattan SDK target release notes at http://www.developer.nokia.com/info/sw.nokia.com/id/f3013263-1fbc-4cae-9f01-b4aa8e702eb4/Qt_SDK_Harmattan_Target_Release_Notes.html).
There are at least two ways to fix this:
1) Easy and probably the recommended one: drop the QmlApplicationViewer::create method and do not take the QDeclarativeView from the booster cache at all.
2) Change the implementation of QmlApplicationViewer so that it is not inherited from QDeclarativeView.