There are many pieces to dragging widgets between displays; this is just one.

On classic ash, NativeWidgetAura::SetBounds() uses aura::client::ScreenPositionClient::SetBounds() which delegates back into ash to support migrating windows between displays.

aura::Window::SetBoundsInScreen() uses the same ScreenPositionClient, so you get the same behavior whether you set bounds on a widget or on the window (either directly or via WmWindowAura).

Under mus, we have a ScreenPositionClient per ui::Window, but that's just for handling aura children of the mus ui::Window. It's not used to position widgets.

Right now NativeWidgetMus::SetBounds() and WmWindowMus::SetBoundsInScreen() don't share code. This caused  issue 644895 , the shelf being positioned incorrectly on the second display.

We probably need some kind of delegate out of ui/views/mus back to whatever part of the system knows about both views::Widget and display management. In particular, it needs to be able to handle moving widgets between displays. I'm not sure if that will live in //ash or //ui or elsewhere.