For more context about this algorithm, check out this video: https://youtu.be/M3eJIo9h0fg

TodoList+FixedSorting.swift

import SwiftUI

struct Todo: Identifiable {
    let id: UUID
    var title = "Untitled"
    var isComplete = false
    
    static var sample: [Todo] {
        [
            Todo(
                id: UUID(),
                title: "Milk",
                isComplete: false
            ),
            Todo(
                id: UUID(),
                title: "Buy eggs",
                isComplete: false
            ),
            Todo(
                id: UUID(),
                title: "Clean room",
                isComplete: true
            )
        ]
    }
}

struct ContentView: View {
    @State private var todos = Todo.sample
    
    var body: some View {
        NavigationView {
            List($todos) { $todo in
                HStack {
                    Button {
                        withAnimation {
                            todo.isComplete.toggle()
                            sort()
                        }
                    } label: {
                        Image(systemName: todo.isComplete ? "checkmark.square" : "square")
                    }

                    Text(todo.title)
                }
                .foregroundColor(todo.isComplete ? .gray : nil)
            }
            .toolbar {
                ToolbarItem(placement: .navigationBarTrailing) {
                    Button {
                        withAnimation { todos.append(Todo(id: UUID())) }
                    } label: {
                        Text("Add")
                    }

                }
            }
            .navigationTitle("Todo List")
        }
    }
    
    // This sorting algorithm is not stable!
    //func sort() {
    //    todos.sort { !$0.isComplete && $1.isComplete }
    //}
    
    //This is the fixed version!
    func sort() {
        todos = todos
            .enumerated()
            .sorted {
                (!$0.element.isComplete && $1.element.isComplete)
                || ($0.offset < $1.offset)
            }
            .map(\.element)
    }
}

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