Customize the separator insets on lists in SwiftUI. Only tested on iOS 15. Does not work on SidebarListStyles or interface idioms that aren't pad / phone.

Example.swift

struct ContentView: View {
  
  var body: some View {
    List {
      ForEach(0..<100) { index in
        Text("Index: \(index)")
          .listRowSeparatorInsets(
            .init(
              top: 0, 
              left: CGFloat(index) * 5, 
              bottom: 0, 
              right: 0
            )
          )
      }
    }
    .listStyle(.insetGrouped)
  }
}

View+ListRowSeparatorInsets.swift

extension View {
  /// The top and bottom insets are ignored
  public func listRowSeparatorInsets(_ insets: UIEdgeInsets) -> some View {
    self.modifier(ListRowSeparatorInsetsModifier(insets: insets))
  }
}

struct ListRowSeparatorInsetsModifier: ViewModifier {

  let insets: UIEdgeInsets

  func body(content: Content) -> some View {
    content.background {
      Representable(insets: insets)
        .hidden()
    }
  }

  struct Representable: UIViewRepresentable {
    // Forces layoutMargins to take effect.
    private static let paddingInset = UIEdgeInsets(top: 0, left: 1, bottom: 0, right: 0)

    let insets: UIEdgeInsets

    func makeCoordinator() -> Coordinator {
      Coordinator()
    }

    func makeUIView(context: Context) -> some UIView { .init() }

    func updateUIView(_ uiView: UIViewType, context: Context) {
      guard let cell = context.coordinator.cellCache else {
        guard let superview = uiView.superview,
              // TODO: a better reflection solution, would be nice to grab the host directly
              let host = Mirror(reflecting: superview).descendant("host", "some") as? UIView,
              let cell = host.superview?.superview as? UITableViewCell else {
          return
        }
        updateInsets(cell)
        context.coordinator.cellCache = cell
        return
      }
      updateInsets(cell)
      context.coordinator.cellCache = cell
    }

    func updateInsets(_ cell: UITableViewCell) {
      guard insets != cell.layoutMargins else { return }
      if cell.separatorInset != Self.paddingInset {
        cell.separatorInset = Self.paddingInset
      }
      cell.layoutMargins = insets
    }

    class Coordinator {
      var cellCache: UITableViewCell?
    }
  }
}

Oh, I bet the reason the insets need to be some non-zero amount is because they’re being applied to the table view. So when we set it to something non-zero we are taking control from the table view.