If you do Panels->Tear off copy... It creates a floating window without the "panels" menu.
If you create a modelPanel using MEL or Python's Maya.cmds it will create a viewport. Prior 2017, it will create a panel the same way normal viewports were, including that Panels menu.
But it's not there anymore with 2017
Doing it using modelPane commandl used to work... until now. How can I get it to work again?