QHeaderView: properly restore section data after layoutChanged()

QHeaderView is doing a complete rebuild of the sections when the layout
changed because everything could have happened. But since layoutChanged
is also called during e.g. sorting, the old data must be restored when
possible.

Task-number: QTBUG-65478
Change-Id: I088d4d843cad362b97df6dc5e0dcb9819b13547f
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

2 files changed, 73 insertions, 26 deletions

diff --git a/src/widgets/itemviews/qheaderview.cpp b/src/widgets/itemviews/qheaderview.cpp
index 0905a20812..e1aec3f4bd 100644
--- a/src/widgets/itemviews/qheaderview.cpp
+++ b/src/widgets/itemviews/qheaderview.cpp
@@ -351,7 +351,7 @@ void QHeaderView::setModel(QAbstractItemModel *model)
     if (model == this->model())
         return;
     Q_D(QHeaderView);
-    d->persistentHiddenSections.clear();
+    d->layoutChangePersistentSections.clear();
     if (d->model && d->model != QAbstractItemModelPrivate::staticEmptyModel()) {
     if (d->orientation == Qt::Horizontal) {
         QObject::disconnect(d->model, SIGNAL(columnsInserted(QModelIndex,int,int)),
@@ -2072,14 +2072,28 @@ void QHeaderViewPrivate::_q_layoutAboutToBeChanged()
         || model->columnCount(root) == 0)
         return;
 
-    if (hiddenSectionSize.count() == 0)
-        return;
+    layoutChangePersistentSections.clear();
+    layoutChangePersistentSections.reserve(std::min(10, sectionItems.count()));
+    // after layoutChanged another section can be last stretched section
+    if (stretchLastSection) {
+        const int visual = visualIndex(lastSectionLogicalIdx);
+        sectionItems[visual].size = lastSectionSize;
+    }
+    for (int i = 0; i < sectionItems.size(); ++i) {
+        const auto &s = sectionItems.at(i);
+        // only add if the section is not default and not visually moved
+        if (s.size == defaultSectionSize && !s.isHidden && s.resizeMode == globalResizeMode)
+            continue;
 
-    for (int i = 0; i < sectionItems.count(); ++i)
-        if (isVisualIndexHidden(i)) // ### note that we are using column or row 0
-            persistentHiddenSections.append(orientation == Qt::Horizontal
-                                            ? model->index(0, logicalIndex(i), root)
-                                            : model->index(logicalIndex(i), 0, root));
+        // ### note that we are using column or row 0
+        layoutChangePersistentSections.append({orientation == Qt::Horizontal
+                                                  ? model->index(0, logicalIndex(i), root)
+                                                  : model->index(logicalIndex(i), 0, root),
+                                              s});
+
+        if (layoutChangePersistentSections.size() > 1000)
+            break;
+    }
 }
 
 void QHeaderViewPrivate::_q_layoutChanged()
@@ -2087,25 +2101,57 @@ void QHeaderViewPrivate::_q_layoutChanged()
     Q_Q(QHeaderView);
     viewport->update();
 
-    const auto hiddenSections = persistentHiddenSections;
-    persistentHiddenSections.clear();
-
-    clear();
-    q->initializeSections();
-    invalidateCachedSizeHint();
+    const auto oldPersistentSections = layoutChangePersistentSections;
+    layoutChangePersistentSections.clear();
 
-    if (modelIsEmpty()) {
+    const int newCount = modelSectionCount();
+    const int oldCount = sectionItems.size();
+    if (newCount == 0) {
+        clear();
+        if (oldCount != 0)
+            emit q->sectionCountChanged(oldCount, 0);
         return;
     }
 
-    for (const auto &index : hiddenSections) {
-        if (index.isValid()) {
-            const int logical = (orientation == Qt::Horizontal
-                                 ? index.column()
-                                 : index.row());
-            q->setSectionHidden(logical, true);
+    // adjust section size
+    if (newCount != oldCount) {
+        const int min = qBound(0, oldCount, newCount - 1);
+        q->initializeSections(min, newCount - 1);
+    }
+    // reset sections
+    sectionItems.fill(SectionItem(defaultSectionSize, globalResizeMode), newCount);
+
+    // all hidden sections are in oldPersistentSections
+    hiddenSectionSize.clear();
+
+    for (const auto &item : oldPersistentSections) {
+        const auto &index = item.index;
+        if (!index.isValid())
+            continue;
+
+        const int newLogicalIndex = (orientation == Qt::Horizontal
+                                     ? index.column()
+                                     : index.row());
+        // the new visualIndices are already adjusted / reset by initializeSections()
+        const int newVisualIndex = visualIndex(newLogicalIndex);
+        auto &newSection = sectionItems[newVisualIndex];
+        newSection = item.section;
+
+        if (newSection.isHidden) {
+            // otherwise setSectionHidden will return without doing anything
+            newSection.isHidden = false;
+            q->setSectionHidden(newLogicalIndex, true);
         }
     }
+
+    recalcSectionStartPos();
+    length = headerLength();
+
+    if (stretchLastSection) {
+        // force rebuild of stretched section later on
+        lastSectionLogicalIdx = -1;
+        maybeRestorePrevLastSectionAndStretchLast();
+    }
 }
 
 /*!
diff --git a/src/widgets/itemviews/qheaderview_p.h b/src/widgets/itemviews/qheaderview_p.h
index 8fc8b88aa5..c9c2cf8493 100644
--- a/src/widgets/itemviews/qheaderview_p.h
+++ b/src/widgets/itemviews/qheaderview_p.h
@@ -231,10 +231,6 @@ public:
                 : model->rowCount(root));
     }
 
-    inline bool modelIsEmpty() const {
-        return (model->rowCount(root) == 0 || model->columnCount(root) == 0);
-    }
-
     inline void doDelayedResizeSections() {
         if (!delayedResize.isActive())
             delayedResize.start(0, q_func());
@@ -304,7 +300,6 @@ public:
     QLabel *sectionIndicator;
 #endif
     QHeaderView::ResizeMode globalResizeMode;
-    QList<QPersistentModelIndex> persistentHiddenSections;
     mutable bool sectionStartposRecalc;
     int resizeContentsPrecision;
     // header sections
@@ -335,6 +330,11 @@ public:
     };
 
     QVector<SectionItem> sectionItems;
+    struct LayoutChangeItem {
+        QPersistentModelIndex index;
+        SectionItem section;
+    };
+    QVector<LayoutChangeItem> layoutChangePersistentSections;
 
     void createSectionItems(int start, int end, int size, QHeaderView::ResizeMode mode);
     void removeSectionsFromSectionItems(int start, int end);
@@ -388,6 +388,7 @@ public:
 
 };
 Q_DECLARE_TYPEINFO(QHeaderViewPrivate::SectionItem, Q_PRIMITIVE_TYPE);
+Q_DECLARE_TYPEINFO(QHeaderViewPrivate::LayoutChangeItem, Q_MOVABLE_TYPE);
 
 QT_END_NAMESPACE