QDockWidget: Use resizer without native window decoration

975c0bdf73 has disabled usage of a resizer
on Windows. This has caused a regression, when a dock widget was
rendered frameless.

Use a resizer in that case.

Fixes: QTBUG-120694
Task-number: QTBUG-102196
Pick-to: 6.7 6.6 6.5 6.2
Change-Id: I9fbcb1a8b1d8869995093bc1da7524ec81e9f02d
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/widgets/widgets/qdockwidget.cpp

@@ -883,15 +883,15 @@ Qt::DockWidgetArea QDockWidgetPrivate::toDockWidgetArea(QInternal::DockPosition
 
 void QDockWidgetPrivate::setResizerActive(bool active)
 {
-#ifdef Q_OS_WINDOWS
-    Q_UNUSED(active);
-#else
     Q_Q(QDockWidget);
+    const auto *dwLayout = qobject_cast<QDockWidgetLayout *>(layout);
+    if (dwLayout->nativeWindowDeco(q->isFloating()))
+        return;
+
     if (active && !resizer)
         resizer = new QWidgetResizeHandler(q);
     if (resizer)
         resizer->setEnabled(active);
-#endif
 }
 
 bool QDockWidgetPrivate::isAnimating() const