lanework/Kanban/App/AppModel.swift

import AppKit
import Observation
import SwiftUI
import os

// MARK: - Scene ids

/// The scene identifiers, in one place because they are matched by string in three unrelated
/// spots — the scene declaration, `openWindow(id:)`, and `dismissWindow(id:)` — and a typo in any
/// one of them fails silently at runtime.
public enum WindowID {
    public static let welcome = "welcome"
    public static let restoreBootstrap = "restore-bootstrap"
    /// The template chooser (09-templates.md; File ▸ New Board… ⌥⌘N). Its own window rather than a
    /// sheet on welcome because ⌥⌘N is available *everywhere* (11-command-nexus.md) — including from
    /// a board window, and including when welcome is not open at all, which a sheet would have to
    /// conjure a host for.
    public static let templateChooser = "template-chooser"
    public static let board = "board"
    public static let card = "card"
}

// MARK: - App-wide preferences

/// The `UserDefaults` half of "App-wide state has the same home" (02-architecture.md § Per-board app
/// state): the app-scoped values that are scalars, kept out of the board registry because no board
/// owns them.
///
/// The keys are declared here rather than spelled at each `@AppStorage`, for the same reason
/// `WindowID` exists.
///
/// The domain is `UserDefaults.standard`, which the sandbox already scopes to this one app — the
/// same reason `AppStateHome` needs no bundle-id subfolder. A `@AppStorage` left to its own devices
/// reads exactly this domain, so nothing here has to be named at a binding site.
public enum AppPreferences {

    /// "Restore open boards at launch" (Settings, ⌘, — 11-command-nexus.md). **Default on.**
    public static let restoreOpenBoardsAtLaunchKey = "restoreOpenBoardsAtLaunch"

    /// Read outside a view, where `@AppStorage` is not available — the launch flow needs it before
    /// any scene exists. `object(forKey:)` rather than `bool(forKey:)` because the latter cannot
    /// tell "off" from "never set", and this preference defaults to *on*.
    public static var restoreOpenBoardsAtLaunch: Bool {
        UserDefaults.standard.object(forKey: restoreOpenBoardsAtLaunchKey) as? Bool ?? true
    }

    /// The last-used card-window size (05-card-window.md; 02 files it as app-wide, not per-board —
    /// "the last-used card-window size" is named there explicitly). Stored as a string because
    /// `NSSize` is not a property-list type and two more keys would be worse.
    public static let lastCardWindowSizeKey = "lastCardWindowSize"

    public static var lastCardWindowSize: CGSize? {
        guard let text = UserDefaults.standard.string(forKey: lastCardWindowSizeKey) else { return nil }
        let size = NSSizeFromString(text)
        guard size.width > 0, size.height > 0 else { return nil }
        return size
    }

    public static func setLastCardWindowSize(_ size: CGSize) {
        UserDefaults.standard.set(NSStringFromSize(size), forKey: lastCardWindowSizeKey)
    }

    // MARK: The comments pane's three bits

    /// **View ▸ Show Comments** — "a checkmark toggle à la Show Trash, and its choice is **app-wide
    /// and persisted across restarts**" (05-card-window.md ▸ The comments column, re-ruled
    /// 2026-07-29; 11-command-nexus.md).
    ///
    /// **One bit, and no content-derived auto-show**: checked, every card window carries the pane —
    /// a comment-less card shows the empty thread and the composer, because the invitation is the
    /// point; unchecked, threads and drafts are out of sight until the user says otherwise. The
    /// checkmark reads exactly this value, so the menu never lies, and deleting the last comment
    /// never closes the pane because nothing but this bit does.
    ///
    /// **Default on.** 05 does not spell a default, and the two candidate readings pull in opposite
    /// directions — the Show Trash bargain (a secondary surface, default off) against "the invitation
    /// is the point" (a pane whose empty state is its whole argument). The invitation wins: a
    /// comments feature nobody sees until they find a View-menu row is a feature that is not there,
    /// and the user who does not want it turns it off once, forever, which is what the persistence is
    /// for.
    public static let showCommentsKey = "showComments"

    public static var showComments: Bool {
        UserDefaults.standard.object(forKey: showCommentsKey) as? Bool ?? true
    }

    /// **View ▸ Comments Beside Body** — "checked = side-by-side (default), unchecked = body over
    /// comments; app-wide, persisted" (11-command-nexus.md; 05 ▸ Composition).
    ///
    /// Default **on**, which 05 does state: "side-by-side is the default".
    public static let commentsBesideBodyKey = "commentsBesideBody"

    public static var commentsBesideBody: Bool {
        UserDefaults.standard.object(forKey: commentsBesideBodyKey) as? Bool ?? true
    }

    /// The comments header's **sort-direction control** — "chronological ascending by default,
    /// flippable to newest-first (app-wide, persisted)" (05 ▸ The comments column; 11 files it under
    /// Configuration controls).
    ///
    /// Stored as "newest first" rather than as a direction so the default is `false` and the plain
    /// `bool(forKey:)` reading is the right one — the one preference here that does not need to tell
    /// "off" from "never set".
    public static let commentsNewestFirstKey = "commentsNewestFirst"

    public static var commentsNewestFirst: Bool {
        UserDefaults.standard.bool(forKey: commentsNewestFirstKey)
    }

    /// The quick-style row's recently-used backgrounds — an array of palette names / hex strings,
    /// most-recent-first (03-board-ui.md § Styling ▸ Controls: "Recents are app-wide and persist
    /// app-side (user preference, never board data)"; 11-command-nexus.md files it under the
    /// preferences that "need no UI"). Read and written by `StyleRecents`, which owns the list rule;
    /// the key is declared here with its neighbours for `WindowID`'s reason.
    public static let quickStyleBackgroundsKey = "quickStyleBackgrounds"

    /// The cached subscription facts behind the tier decision (12-editions.md ▸ The entitlement) —
    /// JSON-encoded `SubscriptionFacts`, read and written by `ProEntitlement`.
    ///
    /// A scalar default rather than a file in `AppStateHome` because it is two fields, which is the
    /// line that type's own note draws. **Not a secret and not a receipt**: the signed transaction
    /// store is StoreKit's and stays StoreKit's; this is a *cache of the last answer* whose worst
    /// case if edited by hand is one wrong tier until the next refresh corrects it, which is the
    /// same self-correction a fresh install already relies on.
    public static let subscriptionFactsKey = "subscriptionFacts"
}

// MARK: - Launch failures

/// A board that could not be restored or opened, as the welcome window renders it.
///
/// **A struct rather than the obvious tuple** only because SwiftUI needs identity to list these and
/// two failures can share a path (a board that failed, was retried, and failed again).
///
/// The join onto a recents row is `WelcomeRow.derive(recents:failures:)` — 02 § Launch and window
/// lifecycle wants the failure *on the board's row*, carrying fail-fast's specifics or the
/// unavailable state, and a failure naming no row (a first open of a folder that was never a board)
/// falls back to a list of its own. `path` is what the join matches on, which is why it is stored
/// rather than derived from the message.
public struct LaunchFailure: Identifiable, Sendable, Equatable {
    public let id = UUID()
    public let path: String
    public let message: String

    public init(path: String, message: String) {
        self.path = path
        self.message = message
    }

    /// What the row shows for a name: the folder, not the whole path. The path is the subtitle.
    public var displayName: String {
        URL(fileURLWithPath: path).deletingPathExtension().lastPathComponent
    }
}

// MARK: - Security-scoped access

/// One board's security-scoped access, held for the **whole session**.
///
/// `BoardRegistry.withScopedAccess(to:_:)` is the scoped-per-call form and is right for what it does
/// — resolving identities during a recents listing, where holding a scope open would be a leak. It is
/// exactly wrong for an open board: the store, the watcher, and every Writer call need access for
/// minutes or hours, and re-entering the scope per call would be both slower and racy against a
/// watcher thread that is already inside the folder.
///
/// So the pairing is explicit and its balance is the session's job: started when the board's window
/// opens, stopped in the close flush's teardown step. A class rather than a struct so the balance
/// cannot be duplicated by a copy.
///
/// **The URL matters, not the path.** A security-scoped URL is a token, not a string: a `URL`
/// rebuilt from `ref.path` grants nothing, which is why `AppModel.openBoard(at:)` stashes the
/// resolved URL for the host that is about to appear instead of letting it reconstruct one.
public final class ScopedAccess {

    public let url: URL
    private var started: Bool

    public init(_ url: URL) {
        self.url = url
        // `false` for a URL that is not security-scoped — a plain bookmark's, one the open panel
        // already blessed for the app's lifetime, anything inside the container. There is then
        // nothing to stop, and the pairing stays balanced either way.
        started = url.startAccessingSecurityScopedResource()
    }

    public func stop() {
        guard started else { return }
        started = false
        url.stopAccessingSecurityScopedResource()
    }
}

// MARK: - AppModel

/// The app's one piece of cross-window state: which boards are open, which card windows belong to
/// which board, and the two window actions AppKit-side code needs but cannot reach.
///
/// ### What lives here, and why it is not a singleton
///
/// The two registries (02-architecture.md § Layering ▸ Components and § Per-board app state) are
/// owned here because they are app-scoped and because "the app holds one instance, so a test can
/// hold its own without the two colliding" — `BoardStoreRegistry`'s own note. Everything else here is
/// window bookkeeping that has no other home: a `BoardStore` knows nothing about windows by design,
/// and a SwiftUI scene is a value that cannot hold state across a window's life.
///
/// ### Sessions are the join
///
/// A `BoardSession` is what makes the two halves of the app meet: the store the windows share, the
/// registry record they stamp, the card windows the close flush has to close first, and the
/// security-scoped access the whole thing runs inside. Its lifetime is exactly the board window's —
/// created when the host's load succeeds, removed by the close flush's last step. A card window with
/// no session is a card window with no board, which 02's ownership rule says cannot exist; the card
/// host reads that as "dismiss".
@MainActor
@Observable
public final class AppModel {

    // MARK: Registries

    public let storeRegistry = BoardStoreRegistry()
    public let boardRegistry: BoardRegistry

    /// The quick-style row's app-wide recents (03-board-ui.md § Styling ▸ Controls). Owned here for
    /// the registries' reason — app-scoped, and a test holds its own rather than colliding with the
    /// app's — and reached by the context menus through the environment, since a `BoardStore` is
    /// board-scoped and this list deliberately is not.
    public let styleRecents: StyleRecents

    /// The app's one drag session (DRAG-REORDER.md; 04-interactions.md ▸ Drag and drop).
    ///
    /// App-wide for the reason cross-board drags exist at all: **a drag crosses windows**, so the
    /// source board hides the dragged items while any other open board's drop delegates propose a
    /// landing spot for them. It lives here rather than as a global for `styleRecents`' reason — a
    /// test holds its own rather than colliding with the app's — and every board window reaches it
    /// through the environment.
    /// Internal rather than `public`, unlike its neighbours: the drag is entirely a UI-layer
    /// concern, and nothing outside this module has any business reaching into a gesture in flight.
    let dragSession = DragSession()

    /// The app's one clipboard (04-interactions.md ▸ Clipboard).
    ///
    /// App-wide for the drag session's reason turned up a level: a cut/copy **outlives the board it
    /// came from** — the pasteboard and the staged snapshot survive the source window closing, and
    /// survive the app quitting — so nothing board-scoped could own it. It lives here rather than as
    /// a singleton for `styleRecents`' reason (a test holds its own rather than colliding with the
    /// app's, which for this one also means staying off the machine's real pasteboard), and every
    /// board window reaches it through the environment.
    ///
    /// Building it here is also the **launch sweep** (04: "a sweep at launch"): the store's `init`
    /// reads the pasteboard once and collects every staged tree it no longer names.
    public let clipboard: ClipboardStore

    // MARK: The entitlement

    /// **Lanework Pro's entitlement** (12-editions.md ▸ The entitlement) — the cached, local answer
    /// to "is this a subscriber?", owned here for the registries' reason: it is app-scoped, and a
    /// test holds its own over its own defaults rather than colliding with the app's.
    ///
    /// Nothing on the board-open path awaits anything through this object. See `ProEntitlement` for
    /// why that is a property of its shape rather than a rule somebody has to remember.
    public let entitlement: ProEntitlement

    /// **The tier a board session composes under**, as an injectable seam.
    ///
    /// Defaulted to the real entitlement's local read and separated from it for `makeHistoryProvider`'s
    /// reason exactly: a test binds a tier without needing a StoreKit transaction, an App Store
    /// account, or a second `AppModel` initializer. `@MainActor` on the closure type because the
    /// entitlement it reads is main-actor state, and `@ObservationIgnored` because nothing renders
    /// from it — the tier reaches the UI, where it reaches it at all, through `entitlement`.
    ///
    /// **Read once per session, at composition, and never again** (12 ▸ The entitlement: "a lapse
    /// never interrupts an open session"). `beginSession` is the only caller.
    @ObservationIgnored
    public var currentTier: @MainActor () -> Tier = { .free }

    // MARK: The provider seam

    /// **The composition root for `HistoryProviding`** (12-editions.md ▸ The provider seam): what a
    /// board session's undo stack is built by, called once per board as its session begins.
    ///
    /// **The provider follows the board, not the tier alone** (re-ruled 2026-07-31 — 12 ▸ The
    /// provider seam; 13-native-undo.md's header; 06 ▸ Rules): a board's substrate is decided by what
    /// the board *is*, and the tier only decides whether git is on the table at all. The rule it
    /// replaced bound the native stack free-tier-wide and nothing at all on Pro's gitless boards,
    /// which made subscribing *remove* undo from a mode-none board — an upgrade that takes a feature
    /// away.
    ///
    /// It takes the store because that is what a provider is a history *of*: the git provider needs
    /// the board root it is a repository at, and the native one's steps are computed from the same
    /// store's snapshots. A property rather than an initializer argument so a test
    /// can bind a fake without a second `AppModel` initializer, `@ObservationIgnored` because
    /// nothing renders from it.
    ///
    /// ### The two answers, and the `nil` that is no longer one of them
    ///
    /// - **No `HistoryStore` at all** — the free tier, where `HistoryStore.compose` returns `nil`
    ///   without so much as a `stat`: the **native stack, on every board**. "The free tier binds it
    ///   everywhere (any `.git` inert)" (13), and 12 ▸ The free tier and `.git` names the boards that
    ///   covers by hand — "a formerly-subscribed user's board, a 1.x board, a repo-nested board …
    ///   native undo runs". The absent git state *is* the tier test; nothing here reads a flag.
    /// - **Mode `git`** (Pro only — no other tier composes a git state) — the git provider: undo as
    ///   forward restore commits over HEAD's first-parent ancestry (06).
    /// - **Mode `none` and mode `repoNested` alike** — the **native stack**, exactly as in the free
    ///   tier. "Boards without app-managed git — repo-nested included — bind 13-native-undo.md's
    ///   native stack in **every** tier" (03-board-ui.md ▸ Toolbar ▸ Catalog, re-ruled 2026-07-31
    ///   twice; 12 ▸ The provider seam; 13's header).
    ///
    /// **The repo-nested no-undo case is gone** (re-ruled 2026-07-31): 06's leave-strictly-alone
    /// stance "concerns *git*, and this stack never touches git — memory-only, journal-free,
    /// session-scoped — so what repo-nested denies is app-managed history, never ⌘Z" (13's header).
    /// It also made the Pro upgrade story exceptional, which was the other half of the same defect:
    /// the free tier could not tell such a board from a plain one and bound the native stack anyway,
    /// so subscribing *removed* undo from exactly the boards it left alone. Edit ▸ Undo/Redo and the
    /// toolbar pair now disable only under a lock and on an empty stack.
    ///
    /// This closure therefore never answers `nil`, and the seam stays optional for the seam's own
    /// reason: a test binds a substrate-less board through it (`BoardUndoManager.history`).
    ///
    /// The `HistoryStore` argument is what makes the git/gitless split decidable here, and it is why
    /// `beginSession` composes the git state *before* the provider: which substrate a board gets is a
    /// question about its repository, and a root that had to ask the disk itself would be a second
    /// detection.
    ///
    /// ### Consumers
    ///
    /// `beginSession` calls it once per board, and `bindHistoryProvider(for:)` calls it again on the
    /// one event that changes a board's answer under an open session — add-git's commanded flip,
    /// which swaps `none`'s native stack for `git`'s trail.
    @ObservationIgnored
    public var makeHistoryProvider: (BoardStore, Tier, HistoryStore?) -> (any HistoryProviding)? = { store, _, git in
        guard let git else { return NativeHistoryProvider() }
        switch git.mode {
        case .git: return GitHistoryProvider(boardRoot: store.rootURL)
        case .none, .repoNested: return NativeHistoryProvider()
        }
    }

    // MARK: Sessions

    /// One open board window and everything hanging off it.
    public struct BoardSession {

        /// The shared store — the same object every one of this board's windows renders.
        public let store: BoardStore

        /// Which registry record this board is, so the close flush can stamp counts and clear the
        /// open-now flag without matching by identity a second time.
        public let recordID: UUID

        /// This board's undo/redo substrate — **the board half of 13-native-undo.md ▸ Rules' two
        /// levels** (re-ruled 2026-07-31): one stack per board session, carrying board-surface
        /// gestures and the one coarse step each card window's close registers. A card window's own
        /// fine-grained stack is not here and never was the session's (`CardWindowUndo`, held by the
        /// window). It lives here for the store's reason exactly: the session is what every window
        /// over this board shares, and "undo is board-local".
        ///
        /// Which implementation it is, is the tier's answer and nobody else's
        /// (12-editions.md ▸ The provider seam) — see `AppModel.makeHistoryProvider`.
        ///
        /// **`nil` is a board with no undo at all, and no board the app composes is one any more**
        /// (re-ruled 2026-07-31 — see `AppModel.makeHistoryProvider`): boards without app-managed git,
        /// repo-nested included, bind the native stack in every tier, and git boards bind the git
        /// provider. What keeps the optionality is the seam rather than a board: a test binds a
        /// substrate-less session through `makeHistoryProvider`, and a store with no session at all
        /// registers nothing (`BoardStore.registerStep`). The command surface disables through
        /// `undoManager`, which answers the empty way over an absent substrate.
        ///
        /// A `var`, unlike `tier` beside it, and for one event only: **add-git**, the design's single
        /// sanctioned mid-session mode flip, *swaps* the substrate here on the board it flips —
        /// native out, git in, the in-session steps discarded with it
        /// (`bindHistoryProvider(for:)`). A tier lapse still cannot touch it — `tier` has no setter.
        public var history: (any HistoryProviding)?

        /// **The tier this board composed under** (12-editions.md ▸ The entitlement).
        ///
        /// A `let`, on a value type, set once by `beginSession` — which is the entire mechanism
        /// behind "a lapse never interrupts an open session: an open board finishes with the provider
        /// it composed; the next open composes the native stack over inert `.git`". There is no
        /// setter, no observation, and nothing anywhere that re-evaluates a live session's tier: a
        /// subscription ending mid-session is a fact about the *next* open and about nothing that is
        /// already on screen.
        ///
        /// It is recorded rather than merely used-and-discarded because the provider it selects is
        /// not the only thing that will ever ask. pro-m1's surfaces — the card window's History
        /// section, View ▸ History (12 ▸ Tier matrix) — are per-board questions asked long after
        /// composition, and they must get the answer this board actually opened with rather than
        /// whatever the entitlement happens to say when the sidebar renders.
        public let tier: Tier

        /// **This board's git state** (06-history-undo.md ▸ Rules; 02-architecture.md ▸ Components
        /// ▸ HistoryStore) — the detected mode, the repository behind it in git mode, and the
        /// add-git action the popover offers on a board that has none.
        ///
        /// `nil` under the free tier, and that is the inert posture made structural rather than
        /// remembered: with no object there is nothing to consult, nothing to detect with, and no
        /// path by which a free-tier session could touch `.git` (12-editions.md ▸ The free tier and
        /// `.git`). `HistoryStore.compose` is the one place the tier decides it.
        ///
        /// A `let` beside `tier`, for `tier`'s reason: which board this is a git story *of* is
        /// settled at composition and cannot change under an open session. What can change is the
        /// mode *inside* it, by add-git alone — the one commanded mid-session flip 06 allows.
        public let git: HistoryStore?

        /// The mode this board is being edited in, `none` when there is no git state at all — which
        /// is every free-tier session ("The free tier ships exactly one mode: `none`",
        /// 12-editions.md ▸ Tier matrix).
        ///
        /// **Its first consumer is the provider seam** — `makeHistoryProvider` reads exactly this to
        /// know whether the board has a repository to be an undo stack for, and it is the *mode*
        /// rather than the tier that decides (re-ruled 2026-07-31): `git` binds the git provider,
        /// `none` and `repoNested` alike the native stack. The popover's git section is the other
        /// reader — and the one place the two gitless modes still differ, since add-git is offered on
        /// one and explained away on the other.
        ///
        /// `@MainActor` because the state it reads is: a nested type does not inherit its enclosing
        /// type's isolation, and everything that asks a session what mode it is in is main-actor
        /// work anyway (a menu, a popover, a provider being composed).
        @MainActor
        public var gitMode: BoardGitMode { git?.mode ?? .none }

        /// The same stack, wearing the face AppKit needs (`BoardUndoManager`): what this board's
        /// windows hand back from `windowWillReturnUndoManager`, so the Edit menu's Undo/Redo rows
        /// and the toolbar's pair resolve to *this* board through the ordinary responder chain.
        ///
        /// Built once with the session rather than per window, because a second adapter would be a
        /// second answer to "what is this board's undo" — and card windows share this one.
        let undoManager: BoardUndoManager

        /// This board's open card windows. The close flush's step 1 reads it; the card hosts
        /// maintain it. Empty is the common case.
        public var cardRefs: Set<CardWindowRef> = []

        /// The scope the board is being read and written inside, released at teardown. `nil` when
        /// the board was opened from a URL that needed none.
        var access: ScopedAccess?
    }

    /// Keyed by board window, because that is the thing whose lifetime a session shares.
    ///
    /// Observed: a card window watches for its board's session disappearing and dismisses itself when
    /// it does — the safety net behind "card windows never outlive the board window".
    public private(set) var sessions: [BoardWindowRef: BoardSession] = [:]

    /// The end-session hooks, keyed the same way the card windows are.
    ///
    /// Beside `BoardSession.cardRefs` rather than inside it: the set is *membership* (what the close
    /// flush drains and what the safety net checks), this is the *seam table* (what it calls). They
    /// are only ever written together, by the two register/unregister methods below, which is what
    /// keeps them from becoming two answers to one question.
    @ObservationIgnored
    private var cardSessions: [CardWindowRef: any CardSessionFlushing] = [:]

    /// Boards whose close flush is already running — the re-entrancy guard.
    ///
    /// Needed because a board window can be told to close twice in quick succession: the
    /// `windowShouldClose` interception starts the flush, and the host's own disappear runs a second
    /// attempt as its safety net. The second must not re-enter a sequence that is mid-await.
    @ObservationIgnored
    private var closingBoards: Set<BoardWindowRef> = []

    // MARK: Window actions

    /// SwiftUI's window-opening action, captured from whatever scene view is alive.
    ///
    /// It exists because the two things that most need to open a window are not views:
    /// `AppDelegate.applicationShouldHandleReopen` (a Dock click with no windows must show welcome)
    /// and the close-flush coordinator (which dismisses card windows). Neither can read
    /// `@Environment`. The action stays valid after the view that supplied it is gone — it is a value
    /// addressed to the app, not to a window — which is precisely the windowless case it is for.
    ///
    /// `@ObservationIgnored` on both: nothing renders from them, and an assignment on every scene's
    /// appear would otherwise invalidate every observer for no reason.
    @ObservationIgnored
    public var windowOpener: OpenWindowAction?

    @ObservationIgnored
    public var windowDismisser: DismissWindowAction?

    /// URLs handed to `openBoard(at:)` before `windowOpener` existed to open them — a cold launch's
    /// Finder-open (`AppDelegate.application(_:open:)`) can arrive ahead of the first scene's
    /// `onAppear`. Held in order, replayed the moment `captureWindowActions` gives the app somewhere
    /// to open them, then discarded — the buffer is a doorway, not a second registry of intent.
    @ObservationIgnored
    private var pendingOpenURLs: [URL] = []

    /// What `CaptureOpenWindow` calls. A method rather than two assignments so the launch flow, which
    /// needs the actions before any `onAppear` has run, has one thing to call.
    ///
    /// Returns how many buffered Finder-open URLs it replayed — the restore bootstrap's input: a
    /// launch that already opened a document's board must not put welcome up beside it, and only this
    /// method knows the buffer wasn't empty.
    @discardableResult
    func captureWindowActions(open: OpenWindowAction, dismiss: DismissWindowAction) -> Int {
        windowOpener = open
        windowDismisser = dismiss

        guard !pendingOpenURLs.isEmpty else { return 0 }
        let urls = pendingOpenURLs
        pendingOpenURLs.removeAll()
        for url in urls {
            openBoard(at: url)
        }
        return urls.count
    }

    // MARK: Recents

    /// The recents list, cached: what the welcome window renders and what File ▸ Open Recent lists
    /// (02-architecture.md § Per-board app state — "The recents list *is* this registry sorted by
    /// last-opened").
    ///
    /// **Cached rather than read through on demand, and both halves of that are deliberate.**
    /// `BoardRegistry` is not `@Observable`, so a view reading it directly would never learn that a
    /// row was forgotten; and `recents()` resolves every record's bookmark, which is filesystem work
    /// no SwiftUI body should be doing on every evaluation — the File menu's command graph is
    /// rebuilt far more often than this list changes. So the list lives here as observable state and
    /// every path that can change the registry refreshes it explicitly (`refreshRecents()`).
    ///
    /// The honest residual: a registry mutated behind this object's back would show stale until the
    /// next refresh. There is no such path today — every writer goes through this type or through a
    /// session it owns — and welcome refreshes on appearance as the cheap belt-and-braces.
    public private(set) var recents: [RecentBoard] = []

    /// Re-reads the registry into `recents`. Called wherever the registry changes: a board opening,
    /// a board closing (the counts are stamped there), Forget, Clear Menu, and welcome appearing.
    public func refreshRecents() {
        recents = boardRegistry.recents()
    }

    /// The welcome row's Forget (11-command-nexus.md ▸ Welcome recent) — the record, plus any launch
    /// failure that row was carrying, plus the refresh, in one call so no caller can do one without
    /// the others.
    ///
    /// **Forgetting the board forgets the failure too.** The row *is* the failure's surface (02
    /// § Launch and window lifecycle); dropping the row while keeping the failure would relocate its
    /// message into the unmatched-failures list, which reads as the app declining to forget.
    public func forget(boardID: UUID) {
        clearLaunchFailures(naming: knownPaths(ofBoard: boardID))
        boardRegistry.forget(id: boardID)
        refreshRecents()
    }

    /// File ▸ Open Recent ▸ Clear Menu (11-command-nexus.md).
    ///
    /// **Finder clears the *menu*; here the registry is the menu**, so clearing removes every record
    /// — there is no second list to clear, and a "menu" that still knew about the boards it had
    /// stopped listing would be a distinction with no surface. What that costs is per-board settings
    /// (window frames, push-on-commit) for boards the user reopens later, which is exactly what
    /// Forget costs one row at a time and what 02's "its settings are conveniences" already accepts.
    ///
    /// It is Forget applied wholesale, so it clears failures the same way — the ones naming records,
    /// leaving a failure that named no row (and therefore no menu entry) standing in its own list.
    ///
    /// A board that is open right now keeps working: its session holds a record id that no longer
    /// resolves, and `BoardRegistry.update` treats an unknown id as a no-op for precisely this case.
    public func clearRecents() {
        clearLaunchFailures(naming: Set(recents.flatMap { recent in
            [recent.record.lastKnownPath, recent.url?.path].compactMap { $0 }
        }))
        boardRegistry.forgetAll()
        refreshRecents()
    }

    /// Every path a given record is known by — the one it was last seen at and, when its bookmark
    /// still resolves, where it lives now. The two can differ (a bookmark follows a move), and a
    /// failure recorded before the move names the older one.
    private func knownPaths(ofBoard id: UUID) -> Set<String> {
        var paths: Set<String> = []
        if let record = boardRegistry.record(id: id) {
            paths.insert(record.lastKnownPath)
        }
        if let url = recents.first(where: { $0.record.id == id })?.url {
            paths.insert(url.path)
        }
        return paths
    }

    // MARK: Launch failures

    /// Boards that failed to restore or open, newest last. Rendered on their own recents rows where
    /// one exists, and in a fallback list where none does — `WelcomeRow.derive(recents:failures:)`.
    public private(set) var launchFailures: [LaunchFailure] = []

    // MARK: Card-window placement

    /// Where the next card window cascades from (05-card-window.md, "New windows open at the
    /// last-used card-window size, cascaded").
    ///
    /// `NSWindow.cascadeTopLeft(from:)` is the whole mechanism: passing `.zero` places the window at
    /// its natural position and returns the point for the next one, so this is a running cursor
    /// rather than a computed grid. App-wide, not per-board: two boards' card windows cascade past
    /// each other rather than landing on top of one another.
    @ObservationIgnored
    var cardCascadePoint: NSPoint = .zero

    // MARK: Pending opens

    /// The security-scoped URL a board window is about to be built from, stashed between
    /// `openBoard(at:)` and the host's first appearance.
    ///
    /// The handoff exists because a window value has to be `Codable` and a scoped URL is not a
    /// string: by the time `BoardWindowHost` receives its `BoardWindowRef` the access token is gone
    /// unless something carried it across. The host claims it on appear; an unclaimed entry (a window
    /// that never opened) leaks one scope until quit, which is the cheapest failure available here.
    @ObservationIgnored
    private var pendingAccess: [BoardWindowRef: ScopedAccess] = [:]

    private static let logger = Logger(subsystem: "dev.rzen.indie.Kanban", category: "app-model")

    /// The app builds one of these with the real state home; a test passes its own for the reason
    /// `BoardRegistry` takes a storage URL at all — "injecting it is how a test stays out of the real
    /// Application Support directory" (`AppStateHome`). A suite that swept the real staging root
    /// would be sweeping the developer's own clipboard.
    ///
    /// `clipboardStagingRoot` is a separate parameter rather than derived from `registryStorageURL`'s
    /// folder because the two are injected for different reasons and by different callers: the UI-test
    /// fixture launch redirects both into one scratch root (`UITestLaunch`), a unit test usually wants
    /// only one of them, and deriving would silently move a test's staging directory the day it moved
    /// its registry file.
    public init(
        registryStorageURL: URL = BoardRegistry.defaultStorageURL,
        clipboardStagingRoot: URL = ClipboardStore.defaultStagingRoot,
        preferences: UserDefaults = .standard
    ) {
        boardRegistry = BoardRegistry(storageURL: registryStorageURL)
        styleRecents = StyleRecents(defaults: preferences)
        clipboard = ClipboardStore(stagingRoot: clipboardStagingRoot)
        // Reads the cached facts and nothing else — no StoreKit API is touched until
        // `ProEntitlement.start()`, which the app's launch calls and a test host never does.
        let entitlement = ProEntitlement(defaults: preferences)
        self.entitlement = entitlement
        // Bound after the stored properties are in place, so the closure captures the object rather
        // than a half-built `self`. This is the app's default wiring; a test that wants a tier
        // assigns over it.
        currentTier = { entitlement.tier }
        // Read once here rather than lazily, so File ▸ Open Recent is populated from the app's first
        // menu pass — a launch that restores boards never shows welcome, and a submenu that filled
        // in only after the first close would look broken. It costs one bookmark-resolution sweep at
        // launch, next to the one `restorables()` already runs.
        refreshRecents()
    }

    // MARK: - Launch restoration

    /// The launch-restoration gate, as a pure function (02-architecture.md § Launch and window
    /// lifecycle: "the preference gates only whether the flagged set is consulted; the flags are
    /// maintained regardless").
    ///
    /// `KanbanApp.init()` is where this actually runs — read once, before any scene exists, into a
    /// `let` rather than a computed property, because `restorables()` costs a bookmark resolution per
    /// known board and nothing should pay that on every scene-graph evaluation. An `App`'s `init` is
    /// not itself reachable from a test, so the decision is pulled out to here: two `Bool`s in, one
    /// out, provable without a real `UserDefaults` domain or a live registry.
    ///
    /// `nonisolated` because it is exactly as pure as that sentence claims — it touches no stored
    /// state, and `LaunchPlan.decide` (which is not main-actor-bound either, for the same reason)
    /// composes it into the three-way launch decision.
    public nonisolated static func shouldRestoreAtLaunch(preference: Bool, hasRestorables: Bool) -> Bool {
        preference && hasRestorables
    }

    // MARK: - Opening

    public var hasOpenBoards: Bool { !sessions.isEmpty }

    /// Opens a board window for `url`, or focuses the one this board already has.
    ///
    /// **The already-open check is by file identity, not by path** — `liveStore(for:)` resolves it —
    /// so a board reached through a resolved bookmark and the same board reached through the open
    /// panel land on one window even when the two URLs are spelled differently. Only when nothing is
    /// open for it does a ref get minted, and `openWindow(value:)` with an equal ref focuses rather
    /// than duplicates, which is the second half of "one board window per root".
    ///
    /// Security-scoped access starts here, *before* the window exists, because the host's very first
    /// act is a tree walk: a scope started after the load would be too late.
    ///
    /// **Called before any scene has appeared, and that's fine.** A cold launch's Finder-open can
    /// reach here before `windowOpener` is captured; the URL joins `pendingOpenURLs` and this same
    /// method runs again for it once `captureWindowActions` has something to open it with.
    public func openBoard(at url: URL) {
        guard let windowOpener else {
            pendingOpenURLs.append(url)
            return
        }

        if storeRegistry.liveStore(for: url) != nil, let existing = boardRef(forBoardAt: url) {
            windowOpener(id: WindowID.board, value: existing)
            return
        }

        let ref = BoardWindowRef(url: url)
        // Replacing a stash for the same ref would strand the old scope; there is no such case today
        // (an unopened window's ref is not reachable), but stopping the loser is free.
        pendingAccess.removeValue(forKey: ref)?.stop()
        pendingAccess[ref] = ScopedAccess(url)
        windowOpener(id: WindowID.board, value: ref)
    }

    /// Shows — or focuses — the welcome window. Its own scene id, so this works with no windows at
    /// all, which is the Dock-reactivation case (02: "Reactivation (Dock click) with no windows shows
    /// welcome").
    public func showWelcome() {
        windowOpener?(id: WindowID.welcome)
    }

    /// File ▸ New Board… (⌥⌘N) — shows, or focuses, the template chooser (09-templates.md).
    ///
    /// The command opens a *chooser*, never a board: the location is the save panel's question and
    /// the panel is the chooser's, so this method's whole job is the window.
    public func showTemplateChooser() {
        windowOpener?(id: WindowID.templateChooser)
    }

    /// The standard open panel behind File ▸ Open… ⌘O (11-command-nexus.md).
    ///
    /// **Validation is the open attempt itself** — there is no pre-flight check that a folder is a
    /// board. Fail-fast owns that verdict (01-storage-format.md § Malformed input) and it is the same
    /// verdict a restored board gets, so a folder that is not a board produces one error in one
    /// vocabulary rather than two near-identical rejections in two.
    ///
    /// `treatsFilePackagesAsDirectories` is what lets a `.kanban` package be *chosen* while
    /// `canChooseFiles` stays off: a package is a file to the panel otherwise, and boards are both
    /// packages and plain folders (01 § Board naming). The cost is that double-clicking a package
    /// navigates into it, which the welcome window's own open affordances will make moot.
    public func presentOpenPanel() {
        let panel = NSOpenPanel()
        panel.canChooseDirectories = true
        panel.canChooseFiles = false
        panel.treatsFilePackagesAsDirectories = true
        panel.allowsMultipleSelection = false
        panel.prompt = "Open"
        panel.message = "Choose a board folder."

        guard panel.runModal() == .OK, let url = panel.url else { return }
        openBoard(at: url)
    }

    /// The ref of the window already showing the board at `url`, if any — matched through the store,
    /// which is identity-keyed, rather than through the path.
    private func boardRef(forBoardAt url: URL) -> BoardWindowRef? {
        guard let store = storeRegistry.liveStore(for: url) else { return nil }
        return sessions.first { $0.value.store === store }?.key
    }

    // MARK: - Sessions

    public func session(for ref: BoardWindowRef) -> BoardSession? {
        sessions[ref]
    }

    /// Claims the scoped URL `openBoard(at:)` stashed for this window, or `nil` if it opened by some
    /// other route. Claiming removes it: the session owns the balance from here.
    func claimPendingAccess(for ref: BoardWindowRef) -> ScopedAccess? {
        pendingAccess.removeValue(forKey: ref)
    }

    /// Starts a board's session — the board window's host calls this once its load has succeeded.
    ///
    /// Two bookkeeping consequences of "this board is now open" ride along. The recents list is
    /// re-read, because `recordOpen` just moved this board to the top of it. And any launch failure
    /// naming this board is dropped: the board demonstrably opens, so a row still captioned with the
    /// old error would be reporting a condition that has stopped being true. That is not the silent
    /// drop 02 forbids — it forbids a failure that was never surfaced disappearing, not one the user
    /// has since fixed.
    func beginSession(ref: BoardWindowRef, store: BoardStore, recordID: UUID, access: ScopedAccess?) {
        // **The entitlement read** (12-editions.md ▸ The entitlement): "Pro state is read from
        // StoreKit's signed on-device transaction store at board-session composition — the open path
        // gains no network dependency." Synchronous, over facts already in memory, on the same line
        // as the provider it selects — which is the shape that makes "the open path never waits on
        // the App Store" checkable by reading four lines rather than by auditing a call graph. It is
        // also the *only* time this board asks: the answer becomes `BoardSession.tier` and nothing
        // re-derives it.
        let tier = currentTier()
        // **Mode detection** (06-history-undo.md ▸ Rules ▸ Detection: "checked at every board
        // open"), on the same line as the tier that gates it. Under `.free` this returns `nil`
        // without looking at the disk at all — the inert posture is unconditional there — and under
        // `.pro` it is one `stat` per open, freshly, so a board that gained or lost a `.git` since
        // its last open opens in the mode it now has.
        //
        // Deliberately *not* re-run anywhere: no reload path, no watcher event, nothing. "The
        // running session keeps its mode, and the watcher does not scan for `.git` appearing."
        //
        // **Before the provider**, which is new in pro-m1: which substrate a board's undo is depends
        // on the mode this line detects (`makeHistoryProvider`), and a root that had to look at the
        // disk itself would be a second detection able to disagree with this one.
        //
        // **The ledger is the store's own** (06 ▸ Interaction with external writers: "the Writer/echo
        // machinery — the EchoLedger — lets the auto-committer classify every observed change, per
        // file, as app-mediated or foreign"). `compose` defaults to a fresh one for the store-less
        // callers (the add-git surface, unit tests), and a session that took that default would hand
        // the committer a ledger nothing ever writes to: every commit this app made would classify
        // foreign and be authored `Lanework External`. The default is a fallback, never this path's.
        let git = HistoryStore.compose(boardRoot: store.rootURL, tier: tier, ledger: store.echoes)
        // The board's stack is born here, with the session that owns it, and dies in `tearDown`
        // below — the whole of 13-native-undo.md's session-only persistence: "the stack lives with
        // the board session and dies at close/quit ... standard macOS behavior". On Pro's git boards
        // it is instead the repository's own trail, which survives everything (06 ▸ Rules ▸ Undo
        // survives relaunch) — the seam's whole point.
        let history = makeHistoryProvider(store, tier, git)
        // **The loader's earlier-occurrence-wins history rung** (01-storage-format.md ▸ Fractal
        // layout ▸ Rules; `BoardLoader.IdentityHistoryRanker`): git-mode boards get a ranker,
        // everything else keeps injecting nothing. A *provider* rather than a ranker because each
        // load wants its own — see `BoardStore.makeIdentityHistoryRanker` — and because add-git
        // flips the mode mid-session, which this closure picks up for free by asking the git state
        // at the moment of each load rather than at composition.
        if let git {
            store.makeIdentityHistoryRanker = { [weak git] in git?.identityHistoryRanker }
            // **The auto-commit engine, wired into the session it commits for** (06-history-undo.md
            // ▸ Rules ▸ Auto-commit). Called on every Pro session and not only on git-mode ones,
            // because add-git can flip a board mid-session and the committer it builds then must land
            // in exactly this shape — `activateAutoCommit` remembers the wiring for that.
            git.activateAutoCommit { [weak store] committer in
                guard let store else { return }
                committer.currentSnapshot = { [weak store] in store?.snapshot }
                // 02-architecture.md ▸ Write-failure surfacing, through the strip the board window
                // already renders: a genuine commit failure means "your edits are saved, history has
                // stopped advancing", which is exactly what the standing suspension row says. Lock
                // contention and a held repository never reach here — neither is a failure.
                committer.reportFailure = { [weak store] failure in
                    store?.banners.suspendHistory(reason: failure.message)
                }
                committer.reportRecovery = { [weak store] in
                    store?.banners.clearHistorySuspension()
                }
                store.commitSeam = .binding(to: committer)
                // **The undo stack's ear on the committer** — every commit this engine lands, and
                // which of it was heal work (06 ▸ Rules ▸ The stack is HEAD's first-parent ancestry,
                // live; ▸ Heal commits are transparent to undo). Bound here rather than in
                // `wireGitUndo` because add-git builds a *new* committer, and this wiring is what
                // `activateAutoCommit` remembers on its behalf.
                committer.reportLanded = { [weak self, ref] window in
                    guard let provider = self?.sessions[ref]?.history as? GitHistoryProvider else { return }
                    provider.noteLanded(window)
                }
            }
            // **Add-git swaps the undo substrate too** (06 ▸ Rules ▸ Detection — the one commanded
            // mid-session mode flip; 13-native-undo.md's header — "discards the in-session native
            // stack and seeds the git trail from the root commit"). See `bindHistoryProvider(for:)`.
            git.didAddGit = { [weak self] in
                self?.bindHistoryProvider(for: ref)
            }
        }
        // **The binding 13-native-undo.md ▸ Rules' "registration at the Writer boundary" needs**: the
        // store is that boundary — every app-mediated mutation goes out through one of its write
        // methods — so it is the store that computes each inverse and registers it. What it cannot
        // know is *which* stack, because a stack belongs to a session and a store knows nothing about
        // windows; this line is where the session tells it. Weak on the store's side, so the loop
        // this closes (provider → step closures → store) is not a retain cycle.
        store.history = history
        sessions[ref] = BoardSession(
            store: store,
            recordID: recordID,
            history: history,
            tier: tier,
            git: git,
            // The lock's enablement half (13-native-undo.md ▸ Rules): Undo and Redo disable with the
            // other mutating commands while the board refuses writes, and the stack survives to
            // resume when it clears. Weak, so the adapter is never the reason a closed board's store
            // stays alive; a store that has gone answers "writable", which is moot — its stack went
            // with it.
            undoManager: BoardUndoManager(history: history, isReadOnly: { [weak store] in
                store?.isReadOnly ?? false
            }),
            cardRefs: [],
            access: access
        )
        wireGitUndo(history, store: store, git: git, ref: ref)
        clearLaunchFailures(naming: [ref.path, store.rootURL.path])
        refreshRecents()
    }

    // MARK: - The git provider's wiring

    /// Fills a `GitHistoryProvider`'s seams with the session it is the history of — and does nothing
    /// at all for any other substrate.
    ///
    /// Everything the git provider needs is a fact about *this* board that neither a repository nor a
    /// protocol could supply: which committer's debounce to settle first, whether the git surface is
    /// held, which card windows a restore's diff would disturb, and the bracket a wholesale tree
    /// change runs inside. Each arrives as a closure for `HistoryCommitSeam`'s reason — the provider
    /// stays a thing that knows about commits, and the model stays the only object that knows what a
    /// window is.
    private func wireGitUndo(
        _ history: (any HistoryProviding)?,
        store: BoardStore,
        git: HistoryStore?,
        ref: BoardWindowRef
    ) {
        guard let provider = history as? GitHistoryProvider, let git else { return }

        provider.flushPendingCommit = { [weak git] in
            await git?.committer?.flushNow()
        }
        provider.isHeld = { [weak git] in git?.committer?.pause != nil }
        provider.suspendCommitting = { [weak git] in git?.committer?.stop() }
        // The stage-around the settle released comes back with the committer: a card window still open
        // after the restore is still a session (`resumeCardSessionStaging(for:)`).
        provider.resumeCommitting = { [weak self, weak git] in
            git?.committer?.start()
            self?.resumeCardSessionStaging(for: ref)
        }
        // **A restore that failed cleanly** (06 ▸ Interaction with external writers: "surfaces as a
        // one-shot banner failure naming the operation and the error, the tree left as it was").
        //
        // Posted as a **loss row**, and the compromise is recorded rather than hidden: the true
        // failure class (`OneShotBanner`) carries a `BoardWriteError`, whose `operation` is the closed
        // `WriteOperation` vocabulary — and a git operation is deliberately not one of those
        // (`BoardStore.performWholesale`'s own note says so). The loss row is the nearest honest
        // class: warning tone, one-shot lifecycle, never auto-expires, and a free-form message that
        // can name both halves 06 asks for. A message-carrying failure class is the right fix and is
        // a banner-surface change, not this card's.
        provider.reportFailure = { [weak store] failure in
            store?.banners.postLoss(failure.description)
        }
        provider.runBracketed = { [weak store] subject, work in
            guard let store else { return await work() }
            // The completion phrase 10-accessibility.md gives a bracketed operation is the restore's
            // own subject — the sentence the trail now carries, spoken once when the reload lands.
            try? await store.performWholesale(announcing: subject) { await work() }
        }
        provider.settleSessions = { [weak self, weak provider] paths in
            guard let self, let provider else { return .proceed }
            let gate = self.settleGate(for: ref) { [weak provider] folder in
                // The card's uncommitted on-disk saves are reverted by the restore itself, which
                // compares this folder against the working tree rather than against HEAD — see
                // `GitRestoreOperation.plan`.
                provider?.noteDiscarded(cardFolderName: folder)
            }
            let outcome = await gate.settle(touching: paths)
            // **Only on `.proceed`** — a cancelled or failed settle leaves the board exactly as it
            // was, sessions and their staging included.
            if outcome == .proceed { self.releaseCardSessionStaging(for: ref) }
            return outcome
        }
        provider.seed()

        wireBranchSwitching(git: git, store: store, provider: provider, ref: ref)
    }

    /// **The branch controls' seams** (06-history-undo.md ▸ Branch switching) — the five things the
    /// switch's sequence needs that a repository cannot supply, plus the per-board stamp that makes an
    /// interrupted switch recognizable as this app's.
    ///
    /// Wired beside the undo provider's rather than in a place of its own, because the two are the
    /// same board's git session seen from two sides — and because both must be re-wired on exactly the
    /// same event, add-git's commanded mid-session flip (`bindHistoryProvider(for:)`).
    private func wireBranchSwitching(
        git: HistoryStore,
        store: BoardStore,
        provider: GitHistoryProvider,
        ref: BoardWindowRef
    ) {
        guard let switcher = git.switcher else { return }
        let recordID = sessions[ref]?.recordID

        switcher.flushPendingCommit = { [weak git] in await git?.committer?.flushNow() }
        switcher.isHeld = { [weak git] in git?.committer?.pause != nil }
        switcher.suspendCommitting = { [weak git] in git?.committer?.stop() }
        // As on the restore path: what the settle released is a session that has not ended, and the
        // window is still open on the other side of the checkout.
        switcher.resumeCommitting = { [weak self, weak git] in
            git?.committer?.start()
            self?.resumeCardSessionStaging(for: ref)
        }
        // **The undo/redo reseed** — the provider's own API, which is the relaunch reseed by
        // construction: "discarded and reseeded from the new HEAD's first-parent ancestry … redo
        // starts empty".
        switcher.reseedUndo = { [weak provider] in await provider?.reseed() }
        switcher.didSwitch = { [weak git] in await git?.refreshBranch() }
        switcher.runBracketed = { [weak store] announcement, work in
            guard let store else { return await work() }
            try? await store.performWholesale(announcing: announcement) { await work() }
        }
        switcher.beginProgress = { [weak store] label in
            store?.banners.beginOperation(label: label) ?? UUID()
        }
        switcher.updateProgress = { [weak store] id, label in
            store?.banners.updateOperation(id, label: label)
        }
        switcher.endProgress = { [weak store] id in store?.banners.endOperation(id) }
        // The loss row, on `GitHistoryProvider.reportFailure`'s recorded compromise — see it for why
        // a git failure cannot be a `OneShotBanner` today.
        switcher.reportFailure = { [weak store] failure in
            store?.banners.postLoss(failure.description)
        }
        switcher.reportRecovery = { [weak store] message in
            store?.banners.postLoss(message)
        }
        // **The per-board registry is the stamp's home** (`GitOperationStamp`). A session with no
        // record — a store-level test — simply carries no stamp, and recovery then has nothing to
        // recognize, which is the honest answer for a board the app has no state for.
        switcher.readStamp = { [weak self] in
            guard let self, let recordID else { return nil }
            return self.boardRegistry.gitOperationStamp(id: recordID)
        }
        switcher.writeStamp = { [weak self] stamp in
            guard let self, let recordID else { return }
            self.boardRegistry.setGitOperationStamp(id: recordID, stamp)
        }
        switcher.settleSessions = { [weak self, weak switcher] in
            guard let self, let switcher else { return .proceed }
            let gate = self.settleGate(
                for: ref,
                message: SessionSettleStep.branchSwitchMessage
            ) { [weak switcher] folder in
                switcher?.noteDiscarded(cardFolderName: folder)
            }
            // Every open session, not the ones a diff reaches — see `SessionSettleGate.settleAll`.
            let outcome = await gate.settleAll()
            // The switch's flush runs next and must find a tree it can settle whole — see
            // `releaseCardSessionStaging(for:)` for why the modal's own predicate is not enough.
            if outcome == .proceed { self.releaseCardSessionStaging(for: ref) }
            return outcome
        }

        // **The own-leftovers check, at open** (06 ▸ Rules ▸ Abnormal repo states). Beside the
        // committer's start, which is where a pause first becomes knowable, and before anything the
        // user does can land on top of a half-finished checkout.
        Task { await switcher.recoverInterruptedOperation() }
    }

    /// **The save-or-discard step for one board**, built from its open card windows
    /// (06-history-undo.md ▸ Rules ▸ Undo restore vs open Edit sessions; ▸ Branch switching).
    ///
    /// Built per ask rather than stored, because its whole content is "which card windows are open
    /// right now" — a set that changes under any operation slow enough to need the step at all.
    ///
    /// - Parameters:
    ///   - message: what the step says it is about. The two callers describe different consequences —
    ///     a restore changes the cards being edited, a switch replaces them — and 06 gives the step to
    ///     both without giving either the other's wording.
    ///   - didDiscard: told each card folder the Discard branch abandoned, so the operation behind the
    ///     gate can put that folder's uncommitted saves back to HEAD its own way (the restore folds it
    ///     into its plan; the switch reverts before it flushes).
    func settleGate(
        for ref: BoardWindowRef,
        message: String = SessionSettleStep.message,
        didDiscard: @escaping (String) -> Void = { _ in }
    ) -> SessionSettleGate {
        SessionSettleGate(
            sessions: { [weak self] in
                guard let self, let session = self.sessions[ref] else { return [] }
                return session.cardRefs.compactMap { cardRef in
                    guard let flushing = self.cardSessions[cardRef],
                          let settlement = flushing.settlement else { return nil }
                    return SettleableSession(
                        id: cardRef.cardID,
                        cardFolderName: cardRef.cardID,
                        needsSettling: settlement.needsSettling,
                        saveAll: settlement.saveAll,
                        discard: {
                            settlement.discard()
                            didDiscard(cardRef.cardID)
                        }
                    )
                }
            },
            ask: { await SessionSettleStep.ask(message: message) },
            focus: { [weak self] id in
                guard let self, let session = self.sessions[ref] else { return }
                guard let cardRef = session.cardRefs.first(where: { $0.cardID == id }) else { return }
                // Opening a window that is already open is how SwiftUI's value-addressed groups say
                // "bring that one forward" — the same call `BoardWindowHost` makes to open a card, and
                // the reason reopening a live card focuses its window rather than making a second one.
                self.windowOpener?(id: WindowID.card, value: cardRef)
            }
        )
    }

    /// **Swaps the board's undo substrate onto an already-open session** — add-git's one caller.
    ///
    /// 06 ▸ Rules ▸ Detection sanctions exactly one mid-session mode flip, the app's own add-git:
    /// "clicking it flips the open board into git mode immediately — the popover flows straight into
    /// the git controls, the first auto-commit follows". 13-native-undo.md's header spells out what
    /// that does to undo: "**Add-git swaps the substrate mid-session** — the commanded flip discards
    /// the in-session native stack and seeds the git trail from the root commit, the branch-switch
    /// discard-and-reseed precedent applied".
    ///
    /// ### The discard is the whole of it — there is no migration
    ///
    /// The board opened mode-none under any tier now carries a live `NativeHistoryProvider` with real
    /// steps on it (`makeHistoryProvider`), and those steps **die with the substrate**: they are
    /// in-memory inverse operations against a board that has just acquired a commit trail, and
    /// replaying one after the swap would walk the board back across a change the root commit already
    /// records as the baseline. The branch-switch precedent says the same thing about the same
    /// question — "the undo/redo stack does not survive a switch. It is discarded and reseeded from
    /// the new HEAD's first-parent ancestry … redo starts empty" (06 ▸ Branch switching) — so the old
    /// stack is cleared rather than merely dropped, and the git provider's `seed()` (in `wireGitUndo`)
    /// walks a trail whose only commit is the root, which is the stack's floor and not a step: ⌘Z is
    /// correctly empty the instant the flip lands.
    ///
    /// **Nothing is announced.** 06 gives the flip the popover's own flow ("straight into the git
    /// controls") and 13 gives the discard no surface at all, exactly as the branch switch's discard
    /// has none; what the user sees is the Edit rows and the toolbar pair revalidating through
    /// `BoardUndoManager` on AppKit's own cadence, which is the same machinery every other enablement
    /// change on this board rides. Nothing here posts a banner, and nothing should.
    ///
    /// ### Why live rather than at the next open
    ///
    /// A judgment call, recorded when the free-tier matrix still left this board with no provider at
    /// all: the mode flip already carries the *committer* through (`HistoryStore.activateAutoCommit`
    /// remembers its wiring for precisely this board); 12-editions.md's "an open board finishes with
    /// the provider it composed" is a rule about a **tier** lapsing, which cannot change a running
    /// session at all; and a board that visibly starts accumulating commits while ⌘Z answers from a
    /// stack the repository knows nothing about would read as a defect rather than as a policy.
    ///
    /// Called exactly once per board, structurally: `HistoryStore.addGit` refuses any mode but
    /// `none`, and flips to `.git` before it fires `didAddGit`.
    func bindHistoryProvider(for ref: BoardWindowRef) {
        guard var session = sessions[ref], let git = session.git, git.mode == .git else { return }
        guard let history = makeHistoryProvider(session.store, session.tier, git) else { return }
        guard history !== session.history else { return }
        // Before the reassignment, while `session.history` is still the substrate being replaced: the
        // in-flight native steps go with it, and any closure that outlives this line finds an empty
        // stack rather than inverses against a pre-repository board.
        session.history?.clear()
        session.history = history
        sessions[ref] = session
        session.store.history = history
        session.undoManager.history = history
        wireGitUndo(history, store: session.store, git: git, ref: ref)
    }

    /// Registers a card window with its board's session, so the close flush can find it.
    ///
    /// A card window whose board has no session is a card window with no board — the ownership rule
    /// says that cannot exist, and the host's own check dismisses it before reaching this. Recording
    /// the seam anyway would leave an entry nothing ever drains.
    func registerCardWindow(_ ref: CardWindowRef, session: any CardSessionFlushing) {
        guard sessions[ref.board] != nil else {
            Self.logger.debug("card window registered against a board with no session — ignored")
            return
        }
        sessions[ref.board]?.cardRefs.insert(ref)
        cardSessions[ref] = session
        // **The window *is* the commit unit** (06 ▸ Rules ▸ Auto-commit, widened 2026-07-31), so the
        // stage-around opens here — with the window — rather than at the body's first Edit→Preview
        // flip. From this line to `unregisterCardWindow` nothing this card's folder receives can land
        // in an interim commit.
        setCardSession(true, for: ref)
    }

    func unregisterCardWindow(_ ref: CardWindowRef) {
        sessions[ref.board]?.cardRefs.remove(ref)
        cardSessions[ref] = nil
        // **The close flush's release** — and, for a window that left without its session ending (a
        // crash-shaped teardown, a dismissal that raced the flush), the backstop that must not leave a
        // card folder excluded from staging forever. Both are the same line because both mean the same
        // thing: this window is no longer holding its folder back.
        setCardSession(false, for: ref)
    }

    /// Tokens the committer knows each card window's session by. Beside `cardSessions` for its
    /// reason: this is the seam table's third column, written only here.
    @ObservationIgnored
    private var cardSessionTokens: [CardWindowRef: UUID] = [:]

    /// **A card window's session opened or closed** (06-history-undo.md ▸ Rules ▸ Auto-commit: the
    /// committer "stages around the whole open card folder").
    ///
    /// This is the honest seam between the two halves of the rule: the host knows a window exists, the
    /// committer knows what staging is, and only the app model knows which board a card window belongs
    /// to and how to reach its committer. A board with no committer — the free tier, a Pro board with
    /// no repository — records nothing, which is the same `nil` every other git seam takes.
    ///
    /// The card's folder is handed over as a **closure**, not a URL: a card can change lane, or be
    /// moved into the trash, in the middle of a session, and what must be staged around is wherever
    /// it is at the moment of the commit. `BoardStore.cardBodyTarget` is the resolution that spans
    /// both containers, which is exactly why the body save uses it too.
    ///
    /// Idempotent both ways: re-opening reuses the token (a settle that released it, then a resume),
    /// and closing an already-closed session reaches a committer that has nothing to remove.
    func setCardSession(_ isOpen: Bool, for ref: CardWindowRef) {
        guard isOpen else {
            // **The token goes whether or not there is anyone left to tell.** A card window's own
            // teardown can land after its board's, and a token kept past the board it names would
            // outlive everything that could ever release it.
            guard let token = cardSessionTokens.removeValue(forKey: ref) else { return }
            sessions[ref.board]?.git?.committer?.endCardSession(token)
            return
        }
        guard let session = sessions[ref.board], let committer = session.git?.committer else { return }
        let token = cardSessionTokens[ref] ?? UUID()
        cardSessionTokens[ref] = token
        let cardID = ref.cardIdentity
        committer.beginCardSession(token) { [weak store = session.store] in
            guard let store,
                  let path = BoardStore.cardBodyTarget(cardID, in: store.snapshot) else { return nil }
            return path.folder(under: store.rootURL)
        }
    }

    /// **The stage-around releases at the settle step** (06 ▸ Branch switching; ▸ Rules ▸ Undo restore
    /// vs open Edit sessions) — every open card window on this board, unconditionally.
    ///
    /// ### Why unconditionally, rather than through the modal
    ///
    /// The save-or-discard step asks about *buffers* — "unsaved keystrokes, or on-disk ~700 ms saves
    /// the session hasn't committed" — and a window that is merely open, with a comment posted an hour
    /// ago and a clean editor, answers `needsSettling` with `false`. Under the widened stage-around
    /// that window is still holding its whole folder out of every commit, so leaving it held would
    /// walk a checkout onto a dirty tree and break the one guarantee the settle exists to buy: "with
    /// sessions settled the restore runs on a settled tree … it cannot fail dirty".
    ///
    /// Releasing is therefore structural and silent, and the modal keeps its own narrower predicate:
    /// Save All's flush then carries the session's commit, and Discard's reverted bytes are
    /// reconciled by the operation itself (`GitRestoreOperation.plan`, `GitBranchSwitcher`), which is
    /// why this runs *after* the gate has answered rather than before it.
    func releaseCardSessionStaging(for ref: BoardWindowRef) {
        for cardRef in sessions[ref]?.cardRefs ?? [] {
            setCardSession(false, for: cardRef)
        }
    }

    /// **The next session begins** — the other half of `releaseCardSessionStaging(for:)`, run when the
    /// operation behind the settle has finished with the tree.
    ///
    /// A window that is still open after a restore or a branch switch is still a session, and its
    /// folder must go back to being staged around. Idempotent, so the paths that resume without ever
    /// having released (a cancelled switch, the open-time leftover check) cost a dictionary lookup.
    func resumeCardSessionStaging(for ref: BoardWindowRef) {
        for cardRef in sessions[ref]?.cardRefs ?? [] {
            setCardSession(true, for: cardRef)
        }
    }

    // MARK: - Launch failures

    /// Records a board that could not be opened. Deliberately additive and never cleared on success:
    /// welcome is showing *because* something failed, and a list that emptied itself as other boards
    /// arrived would be the silent drop 02 rules out.
    public func recordLaunchFailure(path: String, message: String) {
        launchFailures.append(LaunchFailure(path: path, message: message))
    }

    /// Forgets the failures — the welcome window's dismissal of a list the user has read.
    public func clearLaunchFailures() {
        launchFailures.removeAll()
    }

    /// Forgets exactly the named failures — what the unmatched-failures list's Clear dismisses, so
    /// that pressing it never also erases a message still standing on a recents row the user has
    /// not looked at.
    public func clearLaunchFailures(ids: Set<UUID>) {
        launchFailures.removeAll { ids.contains($0.id) }
    }

    /// Drops every failure naming one of `paths` — the resolution path, used when a board opens
    /// successfully and when its record is forgotten. Paths are compared the way the welcome row's
    /// join compares them, so "this row's failure" means the same thing in both places.
    private func clearLaunchFailures(naming paths: Set<String>) {
        guard !paths.isEmpty else { return }
        let keys = Set(paths.map(WelcomeRow.pathKey))
        launchFailures.removeAll { keys.contains(WelcomeRow.pathKey($0.path)) }
    }

    // MARK: - Counts

    /// The lane and card counts stamped into the registry at close — **live items only** (02
    /// § Per-board app state, settled).
    ///
    /// > deleted lanes and cards don't count; the row advertises the board's working size, and the
    /// > trash is an errand, not inventory.
    ///
    /// **`.trash/` is excluded by construction** (02-architecture.md § Per-board app state,
    /// re-grounded 2026-07-28 for the materialized trash): this walks `snapshot.lanes`, and the
    /// trash is `snapshot.trash` — a sibling container, never a lane — so no filter is needed and
    /// none could be forgotten. The tombstone era's ancestor walk over `deleted:` flags is gone with
    /// the flag; a board an older version wrote counts its unmigrated cards until the migration moves
    /// them, which is the safe direction and lasts exactly one write.
    ///
    /// Static and pure: it is a fact about a snapshot, and the close flush is the wrong place to
    /// discover a counting bug.
    public static func liveCounts(of snapshot: BoardModel) -> (lanes: Int, cards: Int) {
        var lanes = 0
        var cards = 0
        for lane in snapshot.lanes {
            lanes += 1
            cards += lane.cards.count
        }
        return (lanes, cards)
    }

    /// The folder name, extension stripped (01-storage-format.md § Board naming) — `displayName`'s
    /// own fallback, and (02-architecture.md § Per-board app state) the registry record's
    /// *provisional* display name for a board recorded before its load has run: "fail-fast means the
    /// frontmatter can't be trusted, and the folder name is the Finder document name the user just
    /// picked". A first successful load replaces it with the cached title through the ordinary
    /// `displayName(of:)` path — there is no separate provisional vocabulary, just this one fallback
    /// used a moment earlier than usual.
    public static func folderDisplayName(of url: URL) -> String {
        url.deletingPathExtension().lastPathComponent
    }

    /// A board's display name: its `title`, falling back to the folder name sans extension
    /// (01-storage-format.md § Board naming).
    ///
    /// Read from `store.rootURL` rather than `snapshot.rootURL` so the fallback follows a rename the
    /// moment it is absorbed, instead of lagging by one reload (see `BoardStore.rootURL`).
    public static func displayName(of store: BoardStore) -> String {
        if let title = store.snapshot.title.value, !title.isEmpty {
            return title
        }
        return folderDisplayName(of: store.rootURL)
    }

    // MARK: - Closing

    /// Runs the close flush for one board and tears its session down.
    ///
    /// Idempotent by two guards: a board with no session has already closed, and a board already
    /// mid-flush is not started again. Both matter — the window's close interception and the host's
    /// disappear both call this, by design, because neither one alone fires on every path a window
    /// can leave by.
    public func closeBoard(ref: BoardWindowRef, cause: BoardCloseCause) async {
        guard sessions[ref] != nil, !closingBoards.contains(ref) else { return }
        closingBoards.insert(ref)
        defer { closingBoards.remove(ref) }

        await coordinator(for: ref).run(cause: cause)
        // The flush stamped this board's counts and (on a user close) cleared its open-now flag, so
        // the cached list is now one close out of date — and welcome is often the very next thing on
        // screen.
        refreshRecents()
    }

    /// The close flush's **pending-work step, without the teardown** — what File ▸ Duplicate runs
    /// before it copies (03-board-ui.md § Welcome screen & templates: "The copy is preceded by the
    /// close flush ... so neither the tree nor the copied history misses pending work").
    ///
    /// **Not `closeBoard`**, and the design says so itself: 09-templates.md ▸ Save as Template states
    /// the rule together with its exception — "with the pull-style mechanical exception committing an
    /// open Edit session's on-disk saves as-is, **sessions staying open**". A duplicate leaves the
    /// original on screen (03: "the original stays open too"), so what it needs is pending work
    /// *landed on disk*, not a session ended: no card window is dismissed, no record is stamped
    /// closed, nothing is torn down, and the board the user is looking at never blinks.
    ///
    /// It goes through `CloseFlushCoordinator` rather than calling the store directly so that the
    /// order of the three flushes — store pipeline, then editor saves, then the pending auto-commit
    /// (02's own order) — keeps having exactly one definition.
    public func flushPendingWork(for ref: BoardWindowRef) async {
        guard sessions[ref] != nil else { return }
        await coordinator(for: ref).flushPendingWork()
    }

    /// Whether any of this board's card windows is holding content the files do not have — a dirty
    /// Edit buffer or a typed-in raw-source outlet (`CardSessionFlushing.holdsUnsavedContent`).
    ///
    /// **One caller, one rule**: File ▸ Save as Template's carve-out from the read-only lock. Under
    /// the unwritable-location lock the item stays live — "reads the board, writes Application
    /// Support" — but only while no such session exists, because the lock has suspended exactly the
    /// saves that would flush one and 09-templates.md's never-misses-keystrokes guarantee outranks
    /// the item's availability (02-architecture.md ▸ Live-reload resilience, settled scoping).
    ///
    /// It asks the sessions rather than the store: the content in question is in *memory*, in the
    /// card windows, which is the whole reason the flush cannot reach it.
    public func hasUnsavedCardContent(for ref: BoardWindowRef) -> Bool {
        guard let session = sessions[ref] else { return false }
        return session.cardRefs.contains { cardSessions[$0]?.holdsUnsavedContent == true }
    }

    /// **The purchase flow's reopen offer, carried out** (12-editions.md ▸ The entitlement:
    /// "Subscribe takes effect at each board's next open ... The purchase flow offers to reopen open
    /// boards so the upgrade feels immediate").
    ///
    /// ### Close and open, through the ordinary paths
    ///
    /// There is no reopen-in-place mechanism here and deliberately so: the provider binding is a
    /// composition-time fact, so "apply the new tier to this board" *means* end its session and
    /// compose a new one. Doing that through `closeBoard` and `openBoard` — the same two calls ⌘W and
    /// welcome make — is what keeps every guarantee those paths carry: the close flush runs in its
    /// fixed order (card windows, pending work, registry stamp, teardown), the reopen resolves and
    /// re-scopes the board's URL exactly as a fresh open does, and the registry records both.
    ///
    /// ### Why the window is dismissed rather than reused
    ///
    /// A board window's identity is its root path (`BoardWindowRef`), so reopening the same board
    /// hands `openWindow(value:)` a ref it already has a window for — which *focuses* that window
    /// instead of building a new one, and the window it would focus is one whose host has already run
    /// its one-shot load. Dismissing first is what makes the reopen an open. The dismissals are all
    /// issued before any reopen, then given a run-loop turn to land: SwiftUI processes a window's
    /// teardown asynchronously, and asking for a value's window in the same turn it was dismissed is
    /// the one way this sequence can produce a focused corpse.
    ///
    /// ### Declining costs nothing, today least of all
    ///
    /// A board that says Not Now keeps the substrate it composed with, and since the provider follows
    /// the board (`makeHistoryProvider`), a gitless board's undo is the *same* native stack either
    /// way — declining costs undo nothing at all, and costs a git board only the trail it would have
    /// gained at its next open. The offer exists because "subscribe takes effect at each board's next
    /// open" (12 ▸ The entitlement) needs one, not because anything breaks without it.
    public func reopenOpenBoards() async {
        // Sorted for `flushAllBoardsForQuit`'s reason: a reproducible order rather than a `Set`'s.
        let refs = sessions.keys.sorted { $0.path < $1.path }
        guard !refs.isEmpty else { return }

        // The store's `rootURL` rather than the ref's path: a board renamed while open keeps the
        // path it was opened with, and reopening it there would open nothing (`BoardStore.rootURL`).
        var roots: [URL] = []
        for ref in refs {
            guard let session = sessions[ref] else { continue }
            roots.append(session.store.rootURL)
            await closeBoard(ref: ref, cause: .userClose)
            windowDismisser?(value: ref)
        }

        // One run-loop turn for the dismissals — see the note above. `Task.sleep` rather than
        // `Task.yield` because the main run loop, not the cooperative pool, is what has to advance.
        try? await Task.sleep(for: .milliseconds(150))

        for root in roots {
            openBoard(at: root)
        }
    }

    /// Quit: the same sequence, once per open board, **sequentially**.
    ///
    /// Sequential rather than concurrent so each board's ordering is the one 02 fixes rather than
    /// three interleavings of it, and in a stable board order so a quit is reproducible. Nothing here
    /// clears an open-now flag — that is what `.quit` means, and it is what makes the next launch
    /// restore this set (§ Launch and window lifecycle).
    public func flushAllBoardsForQuit() async {
        for ref in sessions.keys.sorted(by: { $0.path < $1.path }) {
            await closeBoard(ref: ref, cause: .quit)
        }
    }

    /// Wires a session into `CloseFlushCoordinator`'s seams. The ordering lives over there; this is
    /// only which real object each step touches.
    private func coordinator(for ref: BoardWindowRef) -> CloseFlushCoordinator {
        CloseFlushCoordinator(
            openCardRefs: { [weak self] in
                // Sorted so a board with several card windows commits and closes them in a stable
                // order rather than a `Set`'s.
                (self?.sessions[ref]?.cardRefs).map { $0.sorted { $0.cardID < $1.cardID } } ?? []
            },
            endCardSession: { [weak self] cardRef in
                await self?.cardSessions[cardRef]?.endSession()
                // **The release, here rather than only at the host's unregister** (06 ▸ Rules ▸
                // Auto-commit). The unregister does release it — that is what closes a window on its
                // own — but it arrives from the *window's* teardown, which this sequence waits for
                // only up to `cardDrainDeadline` and then proceeds anyway. A quit whose last window
                // was slow to disappear would then flush with the folder still staged around and leave
                // a settled session uncommitted, which is precisely what "nothing settled is ever left
                // ... uncommitted by closing" forbids. Ending the session is this step's own act, so
                // releasing what the session held is too. Idempotent with the unregister.
                self?.setCardSession(false, for: cardRef)
            },
            dismissCardWindow: { [weak self] cardRef in
                self?.windowDismisser?(value: cardRef)
            },
            storeFlush: { [weak self] in
                await self?.sessions[ref]?.store.awaitQuiescence()
            },
            // `editorFlush` stays nil, and now deliberately rather than for want of an editor: the
            // card windows' debounced body saves flush in **step 1**, inside each window's
            // `endSession()` (`CardWindowSession`), which is both earlier than this slot and where
            // 02-architecture.md puts them ("each open Edit session ends with its normal session
            // commit", then pending work). The slot stays for a board-level editor with no card
            // window of its own — the raw-source buffer is the candidate — so that the order
            // relative to `committerFlush` is already decided when one arrives.
            //
            // **`committerFlush` is filled now** (06-history-undo.md ▸ Rules ▸ Auto-commit: "Board
            // window close and app quit flush the pipeline — any pending editor save, then the
            // pending auto-commit — before teardown; nothing settled is ever left unsaved or
            // uncommitted by closing"). By the time it runs, step 1 has ended every card window's
            // session *and released its stage-around*, so each session — body, comments, purge and
            // all — lands in exactly one commit. `nil` on every board with no committer, which is the
            // whole free tier.
            committerFlush: { [weak self] in
                await self?.sessions[ref]?.git?.committer?.flushNow()
            },
            recordClose: { [weak self] in
                guard let self, let session = sessions[ref] else { return }
                let counts = Self.liveCounts(of: session.store.snapshot)
                boardRegistry.recordClose(
                    id: session.recordID,
                    displayName: Self.displayName(of: session.store),
                    laneCount: counts.lanes,
                    cardCount: counts.cards,
                    icon: session.store.snapshot.icon.value,
                    iconColor: session.store.snapshot.iconColor.value
                )
            },
            clearOpenNow: { [weak self] in
                guard let self, let session = sessions[ref] else { return }
                boardRegistry.clearOpenNow(id: session.recordID)
            },
            tearDown: { [weak self] in
                guard let self, let session = sessions.removeValue(forKey: ref) else { return }
                // The committer dies with the session it commits for, `history.clear()`'s reason
                // exactly: its debounce holds a closure over the store this line is about to release,
                // and a timer that outlived its board would fire against a repository nobody is
                // looking at. Its pending work has already been flushed by `committerFlush` above.
                session.git?.stopAutoCommit()
                // Session-only persistence, the other half of `beginSession` (13-native-undo.md
                // ▸ Rules): "the stack ... dies at close/quit", so reopening the board starts empty.
                // Cleared rather than merely dropped because the steps hold closures over the store
                // this line is about to release, and a stack that outlived its board would be a
                // retain cycle wearing an undo stack's clothes.
                session.history?.clear()
                storeRegistry.release(session.store)
                session.access?.stop()
            }
        )
    }
}