On the Qt5 branch (qml2) with the new picking implementation, the scaling of the viewport defeats picking in some cases. Putting a line into viewport.cpp after the picking fbo render to save the fbo out to an image file shows the result - the pick fbo does not match the screen display.

The symptoms visible are (for example) that the tea_service example you cannot pick the teaspoons, even though you can pick all the other larger objects.