When looking at the code for QGraphicsSceneBspTree::initialize() it would seem that this line should be used

node->offset = rect.center().y(); // while index = 0.

instead of the offset line in the code below

in void QGraphicsSceneBspTree::initialize(const QRectF &rect, int depth, int index),
if (index == 0)

{ node->type = Node::Horizontal; node->offset = rect.center().x(); }