ListSelectionBoxWidthReader.swift

import SwiftUI

/// Reads the width of the selection box next to a List row when editing mode is .active
///
/// Note: On iOS 15 the listRowInsets do not respond to updates so if you want to offset
/// the cell you should use negative leading padding.
public struct ListSelectionBoxWidthReader<Content: View>: View {
  @State private var boxWidth: CGFloat = 0
  private var content: (CGFloat) -> Content

  public init(@ViewBuilder content: @escaping (CGFloat) -> Content) {
    self.content = content
  }

  public var body: some View {
    content(boxWidth).background(
      GeometryReader { listGeometry in
        List {
          ForEach([0], id: \.self) { _ in
            GeometryReader { cellGeometry in
              HStack {
                let listX = listGeometry.frame(in: .global).origin.x
                let cellX = cellGeometry.frame(in: .global).origin.x
                Rectangle().onAppear {
                  self.boxWidth = cellX - listX
                }.onChange(of: cellX - listX) { newWidth in
                  self.boxWidth = newWidth
                }
                Spacer()
              }
            }
          }
          .onMove(perform: { _, _ in })
          .listRowInsets(EdgeInsets(top: 0, leading: 0, bottom: 0, trailing: 0))
        }
        .opacity(0)
        .disabled(true)
        .frame(height: 1) // iOS 15 fix: 0 height does not work so we hide and disable
      }
    )
  }
}

Usage.swift

/// View with editMode active and the selection box hidden
struct ExampleView: View {
  var items = ["a", "b", "c"]
  
  var body: some View {
    ListSelectionBoxWidthReader { selectionBoxWidth in
      List {
        ForEach(items, id: \.self) { item in
          Text(item)
            .padding(.leading, -selectionBoxWidth)
        }
        .onMove(perform: { _, _ in })
        .listRowInsets(EdgeInsets(top: 0, leading: 0, bottom: 0, trailing: 0))
      }
    }
    .environment(\.editMode, .constant(.active))
    .listStyle(PlainListStyle())
  }
}

Make sure to change the editMode on the ListSelectionBoxWidthReader and not just on the inner List.