QHeaderView: Don't add new sections on no-op

When a table view adds its first row, QHeaderView::initializeSections()
is called. It initializes the vertical header view with the number of
added sections. Subsequently QHeaderView::sectionsInserted() is called
with the same amount of newly added rows/sections.
That leads to the initial amount of sections being 2x the number of
rows added in the first go. In other words, the table view will display
at least one row more than the underlying table model has.

This patch adds an OR condition to the early return check at the
beginning of QHeaderView::sectionsInserted(). The method returns early
if the number of sections equals the number of respective sections
(rows in this case) in the model.

An autotest is added in tst_QTableView::rowsInVerticalHeader().

Fixes: QTBUG-114225
Pick-to: 6.6 6.5
Change-Id: I895444f025591981965562e54e2335391db52357
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/widgets/itemviews/qheaderview.cpp

@@ -1885,8 +1885,9 @@ void QHeaderView::sectionsInserted(const QModelIndex &parent,
                                    int logicalFirst, int logicalLast)
 {
     Q_D(QHeaderView);
-    if (parent != d->root)
-        return; // we only handle changes in the root level
+    // only handle root level changes and return on no-op
+    if (parent != d->root || d->modelSectionCount() == d->sectionCount())
+        return;
     int oldCount = d->sectionCount();
 
     d->invalidateCachedSizeHint();

tests/auto/widgets/itemviews/qtableview/tst_qtableview.cpp

@@ -52,6 +52,13 @@ public:
         : QAbstractTableModel(parent), row_count(rows), column_count(columns)
     {}
 
+    void insertRows(int rows)
+    {
+        beginInsertRows(QModelIndex(), row_count, row_count + rows - 1);
+        row_count += rows;
+        endInsertRows();
+    }
+
     int rowCount(const QModelIndex& = QModelIndex()) const override
     {
         return row_count;
@@ -429,6 +436,7 @@ private slots:
     void viewOptions();
 
     void taskQTBUG_7232_AllowUserToControlSingleStep();
+    void rowsInVerticalHeader();
 
 #if QT_CONFIG(textmarkdownwriter)
     void markdownWriter();
@@ -4929,5 +4937,18 @@ void tst_QTableView::markdownWriter()
 }
 #endif
 
+void tst_QTableView::rowsInVerticalHeader()
+{
+    QtTestTableModel model(0, 2);
+    QTableView view;
+    view.setModel(&model);
+    view.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&view));
+    auto *verticalHeader = view.verticalHeader();
+    QCOMPARE(verticalHeader->count(), 0);
+    model.insertRows(2);
+    QCOMPARE(verticalHeader->count(), 2);
+}
+
 QTEST_MAIN(tst_QTableView)
 #include "tst_qtableview.moc"