QHeaderView: Reset lastSectionLogicalIdx on clear()

Otherwise we can fail to stretch the last section when adding new
sections.

Task-number: QTBUG-52446
Change-Id: I7eb5267ac500bf4246e57c3e3a43268bb65ef1f7
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

src/widgets/itemviews/qheaderview.cpp

@@ -3465,6 +3465,7 @@ void QHeaderViewPrivate::clear()
     sectionSelected.clear();
     hiddenSectionSize.clear();
     sectionItems.clear();
+    lastSectionLogicalIdx = -1;
     invalidateCachedSizeHint();
     }
 }

tests/auto/widgets/itemviews/qheaderview/tst_qheaderview.cpp

@@ -3014,6 +3014,11 @@ void tst_QHeaderView::stretchAndRestoreLastSection()
     header.swapSections(1, 11);
     QCOMPARE(header.sectionSize(1), someOtherSectionSize);
 
+    // Clear and re-add. This triggers a different code path than seColumnCount(0)
+    m.clear();
+    m.setColumnCount(3);
+    QVERIFY(header.sectionSize(2) >= biggerSizeThanAnySection);
+
     // Test import/export of the original (not stretched) sectionSize.
     m.setColumnCount(0);
     m.setColumnCount(10);