QQuickWidget renders offset/black bar when embedding via createWindowContainer

We are using a QApplication that uses a QtWebEngineView. If you try to embed this application into another application via winId some bugs appear: 

// create a QWindow* of the QtWebEngine-Application
QWindow* webEngineWindow = QWindow::fromWinId( winid );
// embed QWindow into a QWidget
QWidget* widget = QWidget::createWindowContainer( webEngineWindow );  

Without the createWindowContainer() function the extern application works fine but is placed in a seperate window. We want to have it inside a QWidget in our main application. After calling the createWindowContainer() function a black bar appears on top and on side of the widget:

Furthermore there is a weird offset of about 10-20 pixels between the mouse and the underlying buttons etc as you can see in the following gif: