Fix efficiency issue with QAbstractItemView's response to dataChanged().

Before, the response to a range of cells was to refresh the entire view.
Now, it creates a rectangle that encloses the top-left and bottom-right
cells.

An equivalent change is already in the Qt 6 code, but Qt has stoppped
accepting changes to Qt 5.

Change-Id: Icbd47034ca2b8cbe134b56a47d2caff9609189a1

src/widgets/itemviews/qabstractitemview.cpp

@@ -3300,11 +3300,15 @@ void QAbstractItemView::scrollToBottom()
 
 */
 void QAbstractItemView::update(const QModelIndex &index)
+{
+    update(index, index);
+}
+void QAbstractItemView::update(const QModelIndex &topLeft, const QModelIndex &bottomRight)
 {
     Q_D(QAbstractItemView);
-    if (index.isValid()) {
-        const QRect rect = visualRect(index);
-        //this test is important for peformance reason
+    if (topLeft.isValid() && bottomRight.isValid()) {
+        const QRect rect = visualRect(topLeft).united(visualRect(bottomRight));
+        //this test is important for performance reason
         //For example in dataChanged we simply update all the cells without checking
         //it can be a major bottleneck to update rects that aren't even part of the viewport
         if (d->viewport->rect().intersects(rect))
@@ -3343,7 +3347,7 @@ void QAbstractItemView::dataChanged(const QModelIndex &topLeft, const QModelInde
     } else {
         d->updateEditorData(topLeft, bottomRight);
         if (isVisible() && !d->delayedPendingLayout)
-            d->viewport->update();
+            update(topLeft, bottomRight);
     }
 
 #ifndef QT_NO_ACCESSIBILITY

src/widgets/itemviews/qabstractitemview.h

@@ -240,6 +240,7 @@ public Q_SLOTS:
     void scrollToTop();
     void scrollToBottom();
     void update(const QModelIndex &index);
+    void update(const QModelIndex &topLeft, const QModelIndex &bottomRight);
 
 protected Q_SLOTS:
     virtual void dataChanged(const QModelIndex &topLeft, const QModelIndex &bottomRight, const QVector<int> &roles = QVector<int>());