Fix a bug in the initialization (and debug-output) of BSP trees

On a horizontal split the current rectangle is split along the y-axis,
creating two children with the dimensions [x, y, width, height / 2]
and [x, y + height / 2, width, height / 2] respectively. When the
BSP tree is initialized, the type of the root node is set to
"Horizontal". However, the offset of the root node is wrongly
initialized with a split along the x-axis instead of the y-axis.
This leads to wrong behavior on QGraphicsScenes with a non square
aspect ratio. E.g on a QGraphicsScene with an apsect ratio favoring
the y-axis, every item between yItem = sceneWidth/2 and
yItem = sceneHeight/2 will be added to the wrong leaf.

[ChangeLog][QWidgets][QGraphicsScene] Fixed a bug in the initialization
of BSP trees to increase the performance of QGraphicsScenes with non
quadratic scene rectangles.

Fixes: QTBUG-87174
Pick-to: 5.15
Change-Id: I360033e94e22eb961f820278993754d10bfc1e45
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/widgets/graphicsview/qgraphicsscene_bsp.cpp b/src/widgets/graphicsview/qgraphicsscene_bsp.cpp
index 75eb50a3ac..9b05428c0a 100644
--- a/src/widgets/graphicsview/qgraphicsscene_bsp.cpp
+++ b/src/widgets/graphicsview/qgraphicsscene_bsp.cpp
@@ -190,7 +190,7 @@ void QGraphicsSceneBspTree::initialize(const QRectF &rect, int depth, int index)
     Node *node = &nodes[index];
     if (index == 0) {
         node->type = Node::Horizontal;
-        node->offset = rect.center().x();
+        node->offset = rect.center().y();
     }
 
     if (depth) {
@@ -272,7 +272,7 @@ QRectF QGraphicsSceneBspTree::rectForIndex(int index) const
     QRectF rect = rectForIndex(parentIdx);
     const Node *parent = &nodes.at(parentIdx);
 
-    if (parent->type == Node::Horizontal) {
+    if (parent->type == Node::Vertical) {
         if (index & 1)
             rect.setRight(parent->offset);
         else