QHeaderView: fix inconsistent saved state, ignored during restore

The code that updates a section size must also update length,
otherwise saveState can end up saving inconsistent state, and
restoreState() goes to an early-return, not doing anything.

The actual bug was fixed meanwhile because _q_sectionsChanged is called
again, which recalculates length. I still see this only as a safety
measure, every other code path that changes section sizes updates length
right away.

Change-Id: I6cc16261692d93b3640afafef600a5bdff8dca0c
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

src/widgets/itemviews/qheaderview.cpp

@@ -2191,7 +2191,11 @@ void QHeaderViewPrivate::_q_sectionsAboutToBeChanged(const QList<QPersistentMode
     if (stretchLastSection && lastSectionLogicalIdx >= 0 && lastSectionLogicalIdx < sectionItems.count()) {
         const int visual = visualIndex(lastSectionLogicalIdx);
         if (visual >= 0 && visual < sectionItems.size()) {
-            sectionItems[visual].size = lastSectionSize;
+            auto &itemRef = sectionItems[visual];
+            if (itemRef.size != lastSectionSize) {
+                length += lastSectionSize - itemRef.size;
+                itemRef.size = lastSectionSize;
+            }
         }
     }
     for (int i = 0; i < sectionItems.size(); ++i) {

tests/auto/widgets/itemviews/qtreeview/tst_qtreeview.cpp

@@ -162,6 +162,7 @@ private slots:
     void renderToPixmap();
     void styleOptionViewItem();
     void keyboardNavigationWithDisabled();
+    void saveRestoreState();
 
     void statusTip_data();
     void statusTip();
@@ -4076,6 +4077,58 @@ void tst_QTreeView::keyboardNavigationWithDisabled()
     QCOMPARE(view.currentIndex(), model.index(6, 0));
 }
 
+class RemoveColumnOne : public QSortFilterProxyModel
+{
+public:
+    bool filterAcceptsColumn(int source_column, const QModelIndex &) const override
+    {
+        if (m_removeColumn)
+            return source_column != 1;
+        return true;
+    }
+    void removeColumn()
+    {
+        m_removeColumn = true;
+        invalidate();
+    }
+private:
+    bool m_removeColumn = false;
+};
+
+
+void tst_QTreeView::saveRestoreState()
+{
+    QStandardItemModel model;
+    for (int i = 0; i < 100; i++) {
+        QList<QStandardItem *> items;
+        items << new QStandardItem(QLatin1String("item ") + QString::number(i)) << new QStandardItem(QStringLiteral("hidden by proxy")) << new QStandardItem(QStringLiteral("hidden by user"));
+        model.appendRow(items);
+    }
+    QCOMPARE(model.columnCount(), 3);
+
+    RemoveColumnOne proxy;
+    proxy.setSourceModel(&model);
+    QCOMPARE(proxy.columnCount(), 3);
+
+    QTreeView view;
+    view.setModel(&proxy);
+    view.resize(500, 500);
+    view.show();
+    view.header()->hideSection(2);
+    QVERIFY(view.header()->isSectionHidden(2));
+    proxy.removeColumn();
+    QCOMPARE(proxy.columnCount(), 2);
+    QVERIFY(view.header()->isSectionHidden(1));
+    const QByteArray data = view.header()->saveState();
+
+    QTreeView view2;
+    view2.setModel(&proxy);
+    view2.resize(500, 500);
+    view2.show();
+    view2.header()->restoreState(data);
+    QVERIFY(view2.header()->isSectionHidden(1));
+}
+
 class Model_11466 : public QAbstractItemModel
 {
     Q_OBJECT