QHeaderView: fix inconsistent saved state, ignored during restore

This happens because QTreeView disconnects QHeaderView's
_q_layoutChanged slot (!). So the stretching of the last section,
done in _q_layoutAboutToBeChanged, invalidated total length, but didn't
recalculate it (since that's done in _q_layoutChanged). As a result,
length was inconsistent, and saveState would save that, and
restoreState() would early-return, not doing anything.

Let's just ensure length is always consistent, so we don't depend on the
complex issue of whether _q_layoutChanged should be called or not.

This an adapted backport of 4a04eea4f4 from 5.11, the unittest shows that
c0e45ae851 is missing in this branch though.

Change-Id: I4137a19e0a6fdf820dd53fb55e858d1d04a2c113
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

1 files changed, 53 insertions, 0 deletions

diff --git a/tests/auto/widgets/itemviews/qtreeview/tst_qtreeview.cpp b/tests/auto/widgets/itemviews/qtreeview/tst_qtreeview.cpp
index dfcaa9b5b9..6ecee806de 100644
--- a/tests/auto/widgets/itemviews/qtreeview/tst_qtreeview.cpp
+++ b/tests/auto/widgets/itemviews/qtreeview/tst_qtreeview.cpp
@@ -162,6 +162,7 @@ private slots:
     void renderToPixmap();
     void styleOptionViewItem();
     void keyboardNavigationWithDisabled();
+    void saveRestoreState();
 
     // task-specific tests:
     void task174627_moveLeftToRoot();
@@ -4038,6 +4039,58 @@ 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(1); // ## this is wrong, it's 2 in qtbase-5.11
+    QVERIFY(view.header()->isSectionHidden(1)); // ## this is wrong, it's 2 in qtbase-5.11
+    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