When I want to show a simple widget, it works in win32 but not under uwp. I read somewhere long time ago (not user if qt docs or something else), that the pop ups are not allowed in uwp, but this has apparently changed.
Look for the app "Eyes Guard" in MS store, their reminder is tool window outside the main window. So it is possible and it would be nice if Qt would support it.
The other problem of the uwp build, that the widget is sometimes not even shown properly because of some rendering issue..