QGraphicsScene: stabilize stacking order of toplevel items

Removing and adding toplevel items could result in invalid stacking
order (not corresponding to insertion order).

Task-number: QTBUG-19316
Change-Id: Ia8646784a2181cfa936b101e2adaf7e7e73bb83d
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>

src/widgets/graphicsview/qgraphicsscene.cpp

@@ -392,7 +392,7 @@ void QGraphicsScenePrivate::_q_emitUpdated()
 */
 void QGraphicsScenePrivate::registerTopLevelItem(QGraphicsItem *item)
 {
-    item->d_ptr->ensureSequentialSiblingIndex();
+    ensureSequentialTopLevelSiblingIndexes();
     needSortTopLevelItems = true; // ### maybe false
     item->d_ptr->siblingIndex = topLevelItems.size();
     topLevelItems.append(item);

tests/auto/widgets/graphicsview/qgraphicsscene/tst_qgraphicsscene.cpp

@@ -242,6 +242,7 @@ private slots:
     void style();
     void sorting_data();
     void sorting();
+    void insertionOrder();
     void changedSignal_data();
     void changedSignal();
     void stickyFocus_data();
@@ -3633,6 +3634,42 @@ void tst_QGraphicsScene::sorting()
              << t_1);
 }
 
+void tst_QGraphicsScene::insertionOrder()
+{
+    QGraphicsScene scene;
+    const int numItems = 5;
+    QList<QGraphicsItem*> items;
+
+    for (int i = 0; i < numItems; ++i) {
+        QGraphicsRectItem* item = new QGraphicsRectItem(i * 20, i * 20, 200, 200);
+        item->setData(0, i);
+        items.append(item);
+        scene.addItem(item);
+    }
+
+    {
+        QList<QGraphicsItem*> itemList = scene.items();
+        QCOMPARE(itemList.count(), numItems);
+        for (int i = 0; i < itemList.count(); ++i) {
+            QCOMPARE(numItems-1-i, itemList.at(i)->data(0).toInt());
+        }
+    }
+
+    for (int i = 0; i < items.size(); ++i)
+    {
+        scene.removeItem(items.at(i));
+        scene.addItem(items.at(i));
+    }
+
+    {
+        QList<QGraphicsItem*> itemList = scene.items();
+        QCOMPARE(itemList.count(), numItems);
+        for (int i = 0; i < itemList.count(); ++i) {
+            QCOMPARE(numItems-1-i, itemList.at(i)->data(0).toInt());
+        }
+    }
+}
+
 class ChangedListener : public QObject
 {
     Q_OBJECT