Question

Googling suggests that it should.

But the dragdroprobot example implementation (on the parent Robot object) suggests not:

QRectF Robot::boundingRect() const
{
    return QRectF();
}

Which is correct, or is there something more subtle going on?

Answer 1

Child items are painted directly by the scene not by the parent, and according to the documentation about boundingRect():

QGraphicsView uses this to determine whether the item requires redrawing.

So, if there is no drawing to do in the parent, there is no need to return a non-null bounding rectangle, even if the parent has child items.
And if there is some drawing in the parent, it only needs to contain its own bounding rectangle.