For sake of completeness here is the answer from Qt developers (in case someone reads this and has the same issue):
http://lists.qt-project.org/pipermail/development/2018-April/032630.html

Unfortunately the reason why it was done in that way is still a mystery but it seems that the hook code can be safely removed

Thanks for sharing! All the best with your project

For the recored, filed QTBUG-53845

QContactManager is part of QtMobility, it is not part of standard Qt.
Do you have QtMobility?
http://doc-snapshots.qt.io/qt-mobility/qcontactmanager.html

Hi and welcome to devnet,

It will only follow it's parent widget if you put it in a layout that you set on the said parent widget. Otherwise it's up to you to handle the positioning and size of the child widget.

Hi @White_Rabbit, and welcome to the Qt Dev Net!

I don't know if there's a way to change the display format of the editor. You can ask the Qt Creator engineers directly (and request the feature to set the editor's format) at the Qt Creator Mailing List: http://lists.qt-project.org/mailman/listinfo/qt-creator . Subscribe to the list and post there.