maz001 Unfortunately, There isnt such functionality which allows the dropdown menu to go full-width within container width. Will forward your request to JA Dev. Team if the specific feature/functionality can be incorporated in future Template release coming from T3 framework although, the team is more focused currently on rolling out revolutionary T4 framework stable version 🙂
You may still try the following steps although please perceive this to be temporary workaround which may not derive the exact result as envisaged -
1) Following this documentation excerpt - Add full-width as extra class for parent menu item
2) Thereon, try add this to custom.css file and adjust width value accordingly :
.dropdown.full-width>.mega-dropdown-menu{min-width: 1200px !important;left:0 !important;
}