Fix crash when dragging an invalid item in the header view

Dragging an invalid item in the header view leads to a crash. It's
better to safeguard before operating anything w.r.t. to the dragged
item.

This patch fixes that issue by validating the invalid item before
performing drag operation. Also added an test case to verify the same.

Fixes: QTBUG-127782
Pick-to: 6.8
Change-Id: I95398c7410d6fb6427ec06ff4b7da42472bc6d18
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
Reviewed-by: MohammadHossein Qanbari <mohammad.qanbari@qt.io>

src/quick/items/qquicktableview.cpp

@@ -7307,6 +7307,8 @@ void QQuickTableViewSectionDragHandler::handleEventPoint(QPointerEvent *event, Q
                     // Grab the image for dragging header
                     const QPoint cell = tableView->cellAtPosition(point.position(), true);
                     auto *item = tableView->itemAtCell(cell);
+                    if (!item)
+                        break;
                     if (m_drag.isNull()) {
                         m_drag = new QDrag(item);
                         connect(m_drag.data(), &QDrag::actionChanged, this,

tests/auto/quickcontrols/qquickheaderview/data/reorderHeader.qml

@@ -0,0 +1,39 @@
+// Copyright (C) 2023 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only
+
+import QtQuick
+import QtQuick.Controls
+import TestTableModelWithHeader
+
+ApplicationWindow {
+    width: 400
+    height: 400
+
+    property alias headerView: headerView
+
+    Column {
+        HorizontalHeaderView {
+            id: headerView
+            objectName: "horizontalHeader"
+            syncView: tableView
+            textRole: "customRole"
+            movableColumns: true
+        }
+        TableView {
+            id: tableView
+            width: 100
+            height: 100
+            model: TestTableModelWithHeader {
+                rowCount: 2
+                columnCount: 2
+            }
+            delegate: Label {
+                text: customRole
+                leftPadding: 10
+                rightPadding: 10
+
+                required property string customRole
+            }
+        }
+    }
+}

tests/auto/quickcontrols/qquickheaderview/tst_qquickheaderview.cpp

@@ -226,10 +226,9 @@ private slots:
     void listModel();
 
     void resizableHandlerBlockingEvents();
-
     void headerData();
-
     void warnMissingDefaultRole();
+    void dragInvalidItemDuringReorder();
 
 private:
     QQmlEngine *engine;
@@ -459,6 +458,37 @@ void tst_QQuickHeaderView::warnMissingDefaultRole()
     QVERIFY(QTest::qWaitForWindowExposed(window));
 }
 
+void tst_QQuickHeaderView::dragInvalidItemDuringReorder()
+{
+    QQuickApplicationHelper helper(this, QStringLiteral("reorderHeader.qml"));
+    QVERIFY2(helper.errorMessage.isEmpty(), helper.errorMessage);
+    QQuickWindow *window = helper.window;
+    window->show();
+    QVERIFY(QTest::qWaitForWindowExposed(window));
+
+    auto hhv = window->findChild<QQuickHorizontalHeaderView *>("horizontalHeader");
+    QVERIFY(hhv);
+
+    const auto item = hhv->itemAtIndex(hhv->index(0, 0));
+    QQuickWindow *itemWindow = item->window();
+
+    QSignalSpy columnMovedSpy(hhv, SIGNAL(columnMoved(int, int, int)));
+    QVERIFY(columnMovedSpy.isValid());
+
+    const QPoint localPos = QPoint(item->width() - 5, item->height() - 5);
+    const QPoint startPos = itemWindow->contentItem()->mapFromItem(item, localPos).toPoint();
+    const QPoint startDragDist = QPoint(0, qApp->styleHints()->startDragDistance() + 1);
+    const QPoint dragLength(0, 100);
+
+    QTest::mousePress(itemWindow, Qt::LeftButton, Qt::NoModifier, startPos);
+    QTest::mouseMove(itemWindow, startPos + startDragDist);
+    QTest::mouseMove(itemWindow, startPos + dragLength);
+    QTest::mouseRelease(itemWindow, Qt::LeftButton, Qt::NoModifier, startPos + dragLength);
+
+    QVERIFY(!QQuickTest::qIsPolishScheduled(item));
+    QCOMPARE(columnMovedSpy.size(), 0);
+}
+
 QTEST_MAIN(tst_QQuickHeaderView)
 
 #include "tst_qquickheaderview.moc"