QWindowContainer: Don't embed a QWidget

Embedding a QWidget in a window container (via its windowHandle())
may cause crashes, e.g. during drag & drop and when the application
goes out of scope.

If a QWidget->windowHandle() is attempted to be embedded in a window
container, return the pointer to the widget instead of creating a
container.

Add an autotest.

Update documentation.

[ChangeLog][QtWidgets][QWindowContainer] If createWindowContainer()
is called with a QWidgetWindow argument, return pointer to the
widget instead of new container.

Fixes: QTBUG-119113
Change-Id: Id052a03be13adce05bbd025d86270d265dfb662e
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit b9ee6d3b2e465eb70ba43ea62d2ada5327a138c8)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/kernel/qwindowcontainer.cpp

@@ -3,6 +3,7 @@
 
 #include "qwindowcontainer_p.h"
 #include "qwidget_p.h"
+#include "qwidgetwindow_p.h"
 #include <QtGui/qwindow.h>
 #include <QtGui/private/qguiapplication_p.h>
 #include <qpa/qplatformintegration.h>
@@ -164,11 +165,31 @@ public:
     application can greatly hurt the overall performance of the
     application.
 
+    \li Since 6.7, if \a window belongs to a widget (that is, \a window
+    was received from calling \l windowHandle()), no container will be
+    created. Instead, this function will return the widget itself, after
+    being reparented to \l parent. Since no container will be created,
+    \a flags will be ignored. In other words, if \a window belongs to
+    a widget, consider just reparenting that widget to \a parent instead
+    of using this function.
+
     \endlist
  */
 
 QWidget *QWidget::createWindowContainer(QWindow *window, QWidget *parent, Qt::WindowFlags flags)
 {
+    // Embedding a QWidget in a window container doesn't make sense,
+    // and has various issues in practice, so just return the widget
+    // itself.
+    if (auto *widgetWindow = qobject_cast<QWidgetWindow *>(window)) {
+        QWidget *widget = widgetWindow->widget();
+        if (flags != Qt::WindowFlags()) {
+            qWarning() << window << "refers to a widget:" << widget
+                       << "WindowFlags" << flags << "will be ignored.";
+        }
+        widget->setParent(parent);
+        return widget;
+    }
     return new QWindowContainer(window, parent, flags);
 }
 

tests/auto/widgets/kernel/qwindowcontainer/tst_qwindowcontainer.cpp

@@ -57,6 +57,7 @@ private slots:
     void testDockWidget();
     void testNativeContainerParent();
     void testPlatformSurfaceEvent();
+    void embedWidgetWindow();
     void cleanup();
 
 private:
@@ -410,6 +411,39 @@ void tst_QWindowContainer::testPlatformSurfaceEvent()
     QVERIFY(ok);
 }
 
+void tst_QWindowContainer::embedWidgetWindow()
+{
+    {
+        QWidget parent;
+        QWidget *widget = new QWidget;
+        widget->show();
+        QVERIFY(QTest::qWaitForWindowExposed(widget));
+        QVERIFY(widget->windowHandle());
+        QPointer<QWindow> widgetWindow = widget->windowHandle();
+        auto *container = QWidget::createWindowContainer(widgetWindow, &parent);
+        QCOMPARE(container, widget);
+        QCOMPARE(widget->parent(), &parent);
+        delete widget;
+        QTRY_VERIFY(widgetWindow.isNull());
+    }
+
+    QPointer<QWidget> widget = new QWidget;
+    QPointer<QWindow> widgetWindow;
+    {
+        QWidget parent;
+        widget->show();
+        QVERIFY(QTest::qWaitForWindowExposed(widget));
+        QVERIFY(widget->windowHandle());
+        widgetWindow = widget->windowHandle();
+        auto *container = QWidget::createWindowContainer(widgetWindow, &parent);
+        QCOMPARE(container, widget);
+        QCOMPARE(widget->parent(), &parent);
+    }
+    QTRY_VERIFY(widget.isNull());
+    QTRY_VERIFY(widgetWindow.isNull());
+
+}
+
 QTEST_MAIN(tst_QWindowContainer)
 
 #include "tst_qwindowcontainer.moc"