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, 5 insertions, 1 deletions

diff --git a/src/widgets/itemviews/qheaderview.cpp b/src/widgets/itemviews/qheaderview.cpp
index 27da7a16e5..d5ab9f3208 100644
--- a/src/widgets/itemviews/qheaderview.cpp
+++ b/src/widgets/itemviews/qheaderview.cpp
@@ -2078,7 +2078,11 @@ void QHeaderViewPrivate::_q_layoutAboutToBeChanged()
     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) {