Take window frame into account when dragging dock widget

Dock widgets with custom titlebar are frameless by default, so this
isn't usually a problem. However, the user can override the default and
make it non-frameless (by just removing the native title bar, not the frame).

Task-number: QTBUG-58843
Change-Id: Iff7a2a719cb2f326f2f81bbfb1b53ba01a63f3e1
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/widgets/widgets/qdockwidget.cpp

@@ -930,7 +930,9 @@ bool QDockWidgetPrivate::mouseMoveEvent(QMouseEvent *event)
     }
 
     if (state->dragging && !state->nca) {
-        QPoint pos = event->globalPos() - state->pressPos;
+        QMargins windowMargins = q->window()->windowHandle()->frameMargins();
+        QPoint windowMarginOffset = QPoint(windowMargins.left(), windowMargins.top());
+        QPoint pos = event->globalPos() - state->pressPos - windowMarginOffset;
 
         QDockWidgetGroupWindow *floatingTab = qobject_cast<QDockWidgetGroupWindow*>(parent);
         if (floatingTab && !q->isFloating())