Keep the "item size hints" of moved items

It's quite expensive to re-calculate them, so we should better just move
them to the correct position, rather than throwing them away.

REVIEW: 111399

src/kitemviews/kitemlistview.cpp

@@ -1165,7 +1165,7 @@ void KItemListView::slotItemsRemoved(const KItemRangeList& itemRanges)
 
 void KItemListView::slotItemsMoved(const KItemRange& itemRange, const QList<int>& movedToIndexes)
 {
-    m_sizeHintResolver->itemsMoved(itemRange.index, itemRange.count);
+    m_sizeHintResolver->itemsMoved(itemRange, movedToIndexes);
     m_layouter->markAsDirty();
 
     if (m_controller) {

src/kitemviews/private/kitemlistsizehintresolver.cpp

@@ -114,13 +114,17 @@ void KItemListSizeHintResolver::itemsRemoved(const KItemRangeList& itemRanges)
     }
 }
 
-void KItemListSizeHintResolver::itemsMoved(int index, int count)
+void KItemListSizeHintResolver::itemsMoved(const KItemRange& range, const QList<int>& movedToIndexes)
 {
-    while (count) {
+    QVector<QSizeF> newSizeHintCache(m_sizeHintCache);
-        m_sizeHintCache[index] = QSizeF();
+
-        ++index;
+    const int movedRangeEnd = range.index + range.count;
-        --count;
+    for (int i = range.index; i < movedRangeEnd; ++i) {
+        const int newIndex = movedToIndexes.at(i);
+        newSizeHintCache[newIndex] = m_sizeHintCache.at(i);
     }
+
+    m_sizeHintCache = newSizeHintCache;
 }
 
 void KItemListSizeHintResolver::itemsChanged(int index, int count, const QSet<QByteArray>& roles)

src/kitemviews/private/kitemlistsizehintresolver.h

@@ -40,7 +40,7 @@ public:
 
     void itemsInserted(const KItemRangeList& itemRanges);
     void itemsRemoved(const KItemRangeList& itemRanges);
-    void itemsMoved(int index, int count);
+    void itemsMoved(const KItemRange& range, const QList<int>& movedToIndexes);
     void itemsChanged(int index, int count, const QSet<QByteArray>& roles);
 
     void clearCache();