When adding a Map using the MapKit this changes the appearance of the NavigationView and the TabBar.

I have tried adding the following modifier to the Map

.toolbarBackground(Color("White"), for: .navigationBar)

But the navigation toolbar still has a shadow, and the TabBar has the default translucent background colour with shadow.

Root

init() {
    // this is not the same as manipulating the proxy directly
    let appearance = UINavigationBarAppearance()
    
    // this overrides everything you have set up earlier.
    appearance.configureWithTransparentBackground()
    
    appearance.shadowColor = .clear
    
    //In the following two lines you make sure that you apply the style for good
    UINavigationBar.appearance().scrollEdgeAppearance = appearance
    UINavigationBar.appearance().standardAppearance = appearance
    
    
    UITabBar.appearance().barTintColor = UIColor.white)
    UITabBar.appearance().backgroundColor = UIColor.white)
    UITabBar.appearance().shadowImage = UIImage()
    UITabBar.appearance().backgroundImage = UIImage()
    
    UINavigationBar.appearance().isTranslucent = false
    UIToolbar.appearance().backgroundColor = UIColor.white)
    UIToolbar.appearance().isTranslucent = false
    UIToolbar.appearance().setShadowImage(UIImage(), forToolbarPosition: .any)
}

struct MainView: View {
    var body: some View {
        TabView {
            ContentView()
                .tabItem {
                    Label("Menu", systemImage: "list.dash")
                }
        }
    }
}

The Tabbar has a solid white colour with no shadow, as well as navigation bars.

Content View

struct ContentView: View {
    var body: some View {
        NavigationView {
            NavigationLink(destination: MapView()) {
              Text("Hello, World!")
            }
        }
        .navigationTitle("Content")
    }
}

MapView

struct MapView: View {
    @State private var region = MKCoordinateRegion(center: CLLocationCoordinate2D(latitude: 51.507222, longitude: -0.1275), span: MKCoordinateSpan(latitudeDelta: 0.5, longitudeDelta: 0.5))

    var body: some View {
        Map(coordinateRegion: $region)
            .frame(width: 400, height: 300)
    }
}