QHeaderView: Preserve settings on layoutChange with empty model

Do not clear the settings of QHeaderView during layoutChange when the
model is empty and the section count did not change. This will not work
when a section is moved or a section is replaced with a new one during
layoutChange. But since layoutChanged is also called on sorting, this
patch ensures that the settings are not cleared in this case.
This restores the behavior to the same as before 5.9.4.

Task-number: QTBUG-66444
Task-number: QTBUG-65478
Change-Id: I39989cfd45b42e58f49d18ec014d3a941cadb6c9
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/src/widgets/itemviews/qheaderview.cpp b/src/widgets/itemviews/qheaderview.cpp
index edef2e9bf8..b0359de3ea 100644
--- a/src/widgets/itemviews/qheaderview.cpp
+++ b/src/widgets/itemviews/qheaderview.cpp
@@ -2205,6 +2205,30 @@ void QHeaderViewPrivate::_q_sectionsChanged()
         return;
     }
 
+    bool hasPersistantIndexes = false;
+    for (const auto &item : oldPersistentSections) {
+        if (item.index.isValid()) {
+            hasPersistantIndexes = true;
+            break;
+        }
+    }
+
+    // Though far from perfect we here try to retain earlier/existing behavior
+    // ### See QHeaderViewPrivate::_q_layoutAboutToBeChanged()
+    // When we don't have valid hasPersistantIndexes it can be due to
+    // - all sections are default sections
+    // - the row/column 0 which is used for persistent indexes is gone
+    // - all non-default sections were removed
+    // case one is trivial, in case two we assume nothing else changed (it's the best
+    // guess we can do - everything else can not be handled correctly for now)
+    // case three can not be handled correctly with layoutChanged - removeSections
+    // should be used instead for this
+    if (!hasPersistantIndexes) {
+        if (oldCount != newCount)
+            q->initializeSections();
+        return;
+    }
+
     // adjust section size
     if (newCount != oldCount) {
         const int min = qBound(0, oldCount, newCount - 1);