#if os(macOS)
  import AppKit
  import SwiftUI
  // Native windows must never silently discard a focused or failed edit.
  struct SheetCloseGuard: NSViewRepresentable {
    let model: SheetModel
    func makeNSView(context: Context) -> CloseGuardView {
      let view = CloseGuardView()
      view.model = model
      return view
    }
    func updateNSView(_ view: CloseGuardView, context: Context) { view.model = model }
  }
  @MainActor final class CloseGuardView: NSView, NSWindowDelegate {
    weak var model: SheetModel?
    // AppKit delegate messaging and mutation occur on the window’s main thread.
    nonisolated(unsafe) weak var previousDelegate: (any NSWindowDelegate)?
    override func viewDidMoveToWindow() {
      super.viewDidMoveToWindow()
      guard let window, window.delegate !== self else { return }
      SheetCloseRegistry.views.removeAll { $0.value == nil }
      if !SheetCloseRegistry.views.contains(where: { $0.value === self }) {
        SheetCloseRegistry.views.append(WeakCloseGuard(value: self))
      }
      previousDelegate = window.delegate
      window.delegate = self
    }
    override func responds(to selector: Selector!) -> Bool {
      super.responds(to: selector) || previousDelegate?.responds(to: selector) == true
    }
    override func forwardingTarget(for selector: Selector!) -> Any? { previousDelegate }
    func windowShouldClose(_ sender: NSWindow) -> Bool {
      guard let model else { return previousDelegate?.windowShouldClose?(sender) ?? true }
      if model.busy {
        model.error = "Wait for the current save to finish before closing this window."
        return false
      }
      guard model.hasUnconfirmedChanges else {
        return previousDelegate?.windowShouldClose?(sender) ?? true
      }
      let alert = NSAlert()
      alert.messageText = "Keep your cell edit?"
      alert.informativeText = "This cell has changes that have not been saved."
      alert.addButton(withTitle: "Save")
      alert.addButton(withTitle: "Discard")
      alert.addButton(withTitle: "Cancel")
      alert.beginSheetModal(for: sender) { response in
        Task { @MainActor in
          if response == .alertFirstButtonReturn {
            await model.saveAll()
            if !model.hasUnconfirmedChanges { sender.performClose(nil) }
          } else if response == .alertSecondButtonReturn {
            model.discardAll()
            sender.performClose(nil)
          }
        }
      }
      return false
    }
  }
  @MainActor struct WeakCloseGuard { weak var value: CloseGuardView? }
  @MainActor enum SheetCloseRegistry { static var views: [WeakCloseGuard] = [] }
  @MainActor public final class SheetsApplicationDelegate: NSObject, NSApplicationDelegate {
    public func applicationShouldTerminate(_ sender: NSApplication) -> NSApplication.TerminateReply
    {
      let models = SheetCloseRegistry.views.compactMap { $0.value?.model }
      if models.contains(where: { $0.busy }) { return .terminateCancel }
      let pending = models.filter { $0.hasUnconfirmedChanges }
      if pending.isEmpty { return .terminateNow }
      let alert = NSAlert()
      alert.messageText = "Save your spreadsheet edits before quitting?"
      alert.addButton(withTitle: "Save")
      alert.addButton(withTitle: "Discard")
      alert.addButton(withTitle: "Cancel")
      switch alert.runModal() {
      case .alertFirstButtonReturn:
        Task { @MainActor in
          for model in pending { await model.saveAll() }
          sender.reply(toApplicationShouldTerminate: !pending.contains { $0.hasUnconfirmedChanges })
        }
        return .terminateLater
      case .alertSecondButtonReturn:
        pending.forEach { $0.discardAll() }
        return .terminateNow
      default: return .terminateCancel
      }
    }
  }
#endif