Comment 3 for bug 1620991

rev2045 changes the order in which Scrollbar anchors to the flickable. since r2045 it waits for the flickable to be initialized before anchoring to it.

before:
Scrollbar anchors to flickable. Flickable parent changes, engine says nothing.

now:
Flickable is initialized, its parent changes. Scrollbar waits for flickable to be initialized, then anchors to it. QML engine notices that Flickable is not a sibling/parent, and warns about failed anchoring

My opinion:
I think we should double and triplecheck if we really need the flickable parent hack, otherwise we should remove it. An alternative could be to try and change the scrollbar parent at the same as the flickable parent changes, but I'm not sure that's going to prevent the warning from showing, as there should still be a moment when Flickable and Scrollbar are anchored but not in a relationship that allows anchoring one to the other.