Single line list row separator (edge to edge).

Content.swift

import SwiftUI

struct Row: View {
    let text: String
    let showDivider: Bool

    var body: some View {
        VStack(alignment: .leading) {
            Text(text)
                .padding(showDivider ? [.leading, .top] : [.leading, .top, .bottom])
            if showDivider {
                Divider().background(Color(UIColor.separator))
            }
        }
    }
}

struct ContentView: View {
    
    enum Flavor: String, CaseIterable, Identifiable {
        var id: String { self.rawValue }
        case vanilla, chocolate, strawberry
    }
        
    var body: some View {
        List {
            ForEach(Flavor.allCases, id: \.self) { flavor in
                Row(text: "\(flavor.rawValue)", showDivider:  flavor == Flavor.allCases.last ? false : true)
            }
            .listRowSeparator(.hidden)
            .listRowInsets(EdgeInsets(top: 0, leading: 0, bottom: 0, trailing: 0))
            .listRowBackground(Color(UIColor.systemYellow).opacity(0.2))
        }
        .listStyle(InsetGroupedListStyle())
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

This will create the following list with single line style separators: