Build the HistoryProviding seam and the per-board native undo stack

The provider seam 12 promised: HistoryProviding speaks 13's vocabulary
— register a HistoryStep (bare 06 phrase plus undo/redo closures
returning applied or skipped), canUndo/canRedo, action names, clear —
and no UndoManager type appears anywhere in it, proven by a fake that
satisfies the seam with counters. The base provider wraps a private
UndoManager with groupsByEvent off so coalescing stays the Writer call
site's decision; undo re-registers the reversed step from inside the
undo, which makes a stale-skipped step vanish for free and the
crossing loop fall through to the next. BoardUndoManager adapts the
protocol to the responder chain — a stackless UndoManager subclass
answering from the provider — so Pro's git provider inherits menu
enablement, dynamic titles, and the nil-target toolbar pair by binding
the protocol. One stack per board session, born in beginSession,
cleared in the close flush; every window over the board answers it
through windowWillReturnUndoManager. Headless probes shaped the
routing: a real NSTextView's own manager wins natively, but a field
editor's does not — BoardUndoRouting answers the per-window text
manager while any NSText is first responder, so a search-field typo
never crosses a board step.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/App/AppModel.swift

@@ -189,6 +189,25 @@ public final class AppModel {
     /// reads the pasteboard once and collects every staged tree it no longer names.
     public let clipboard = ClipboardStore()
 
+    // 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.
+    ///
+    /// Base binds the native stack — an `NSUndoManager` over inverse `WriteOperation`s
+    /// (13-native-undo.md) — and that is the default here because it is the *shared* code's
+    /// implementation: both targets compile it, and Pro runs it too until pro-m1 replaces this
+    /// closure with the git provider (06-history-undo.md). Nothing in this file is
+    /// edition-conditional; the edition difference is which closure the root installs.
+    ///
+    /// 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 is about to need the same store's
+    /// snapshots to compute inverses from. 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.
+    @ObservationIgnored
+    public var makeHistoryProvider: (BoardStore) -> any HistoryProviding = { _ in NativeHistoryProvider() }
+
     // MARK: Sessions
 
     /// One open board window and everything hanging off it.
@@ -201,6 +220,22 @@ public final class AppModel {
         /// open-now flag without matching by identity a second time.
         public let recordID: UUID
 
+        /// This board's undo/redo substrate — **one stack per board session, never per window**
+        /// (13-native-undo.md ▸ Rules). 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 edition's answer and nobody else's
+        /// (12-editions.md ▸ The provider seam) — see `AppModel.makeHistoryProvider`.
+        public let history: any HistoryProviding
+
+        /// 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> = []
@@ -507,7 +542,18 @@ public final class AppModel {
     /// 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?) {
-        sessions[ref] = BoardSession(store: store, recordID: recordID, cardRefs: [], access: access)
+        // 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".
+        let history = makeHistoryProvider(store)
+        sessions[ref] = BoardSession(
+            store: store,
+            recordID: recordID,
+            history: history,
+            undoManager: BoardUndoManager(history: history),
+            cardRefs: [],
+            access: access
+        )
         clearLaunchFailures(naming: [ref.path, store.rootURL.path])
         refreshRecents()
     }
@@ -705,6 +751,12 @@ public final class AppModel {
             },
             tearDown: { [weak self] in
                 guard let self, let session = sessions.removeValue(forKey: ref) else { return }
+                // 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()
             }

Kanban/App/BoardWindowHost.swift

@@ -264,6 +264,13 @@ struct BoardWindowHost: View {
             }
         }
 
+        // This window's answer to "what does ⌘Z act on" (13-native-undo.md ▸ Rules; 06 ▸ Undo
+        // routing) — the *session's* stack, read afresh on every ask so a torn-down board answers
+        // nothing rather than a stack with no board behind it. The Edit menu's Undo/Redo rows and
+        // the toolbar's pair are nil-target `undo:`/`redo:`, so this one line is what lights them
+        // up: `NSWindow` validates and crosses them against exactly this manager.
+        windowController.boardUndoManager = { appModel.session(for: ref)?.undoManager }
+
         // The window-title widget (03-board-ui.md § Board popover) — **board windows only**, which
         // is why it is installed here rather than in `WindowAccessor`: welcome, the bootstrap and
         // card windows share that machinery and have no board to describe. It goes in after the

Kanban/App/CardWindowHost.swift

@@ -523,6 +523,15 @@ struct CardWindowHost: View {
             CardToolbar.controller(body: bodyPresentation, rawSource: rawSource, attachments: attachments)
         )
 
+        // **The board's stack, not one of this window's own** (13-native-undo.md ▸ Rules: "not
+        // per-window: every window over a board (board window, its card windows) shares the store
+        // and shares the stack"). Same closure shape as the board window's, and deliberately the
+        // same object: ⌘Z with a card window in front crosses the board step the user last made,
+        // wherever they made it. The card's *text* surfaces are untouched by this — the body editor
+        // and the raw-source editor each vend their own manager to the responder chain, which is
+        // what keeps typing undo out of the board's stack (06-history-undo.md ▸ Undo routing).
+        windowController.boardUndoManager = { appModel.session(for: ref.board)?.undoManager }
+
         windowController.onAttach = { window in
             if let recordID,
                let saved = appModel.boardRegistry.cardWindowFrame(id: recordID, cardID: ref.cardIdentity) {

Kanban/App/WindowAccessor.swift

@@ -56,6 +56,26 @@ final class HostedWindowController: NSObject, NSWindowDelegate {
     /// window that has nothing to flush.
     var onCloseRequested: (() -> Void)?
 
+    /// This window's board undo stack, asked for afresh every time AppKit wants it — the board
+    /// window's and its card windows' shared answer (13-native-undo.md ▸ Rules: "one stack per
+    /// board, owned by the board session ... `window.undoManager` for board surfaces returns the
+    /// session's manager").
+    ///
+    /// A closure rather than a stored manager for two reasons: the session does not exist yet when
+    /// the window attaches, and it stops existing at teardown while the window is still closing —
+    /// answering `nil` then is what keeps a torn-down board's stack from being reachable through a
+    /// window that outlived it by a run-loop turn.
+    ///
+    /// `nil` on every window that is not showing a board (welcome, the bootstrap, the template
+    /// chooser), which `BoardUndoRouting` reads as "the platform default".
+    var boardUndoManager: (() -> UndoManager?)?
+
+    /// The text manager this window hands back while a field editor holds the keyboard, and the one
+    /// it hands back when there is no board — 06-history-undo.md ▸ Undo routing, via
+    /// `BoardUndoRouting`. Created on demand, per window, which is what AppKit itself would have
+    /// done for a window whose delegate answered nothing.
+    private lazy var textUndoManager = UndoManager()
+
     /// Set by `closeAfterFlush()` so the re-entrant `windowShouldClose` lets the close through
     /// instead of starting a second flush.
     private var isFlushed = false
@@ -189,6 +209,29 @@ final class HostedWindowController: NSObject, NSWindowDelegate {
         return false
     }
 
+    /// The window-level half of 06-history-undo.md ▸ Undo routing (see `BoardUndoRouting`, which
+    /// owns the rule and the reasoning): the board's stack when the keyboard is on the board, a
+    /// text manager of this window's own while a field editor has it.
+    ///
+    /// **Answered here rather than forwarded**, unlike the proxy's other selectors, on the one
+    /// condition that this window has a board: `responds(to:)` reports this method whatever the
+    /// previous delegate does, so a `nil` return would leave a window with *no* undo manager at all
+    /// rather than the one AppKit creates for a delegate that stays silent. A window with no board
+    /// still defers to SwiftUI's delegate if it has an opinion.
+    func windowWillReturnUndoManager(_ window: NSWindow) -> UndoManager? {
+        let board = boardUndoManager?()
+        if board == nil, let previousDelegate,
+           previousDelegate.responds(to: #selector(NSWindowDelegate.windowWillReturnUndoManager(_:))),
+           let inherited = previousDelegate.windowWillReturnUndoManager?(window) {
+            return inherited
+        }
+        return BoardUndoRouting.undoManager(
+            isTextEditing: BoardUndoRouting.isTextEditing(window.firstResponder),
+            board: board,
+            textFallback: textUndoManager
+        )
+    }
+
     func windowDidMove(_ notification: Notification) {
         reportFrame()
         previousDelegate?.windowDidMove?(notification)

Kanban/History/BoardUndoManager.swift

@@ -0,0 +1,133 @@
+import AppKit
+
+// MARK: - BoardUndoManager
+
+/// What AppKit is handed for a board's windows: an `UndoManager` that owns no stack and answers
+/// every question from the session's `HistoryProviding`.
+///
+/// ### Why an adapter exists at all
+///
+/// The responder chain speaks one currency. `NSWindow` implements `undo:`/`redo:` and validates
+/// those menu items itself, reading `canUndo`, `canRedo` and `undoMenuItemTitle` off whatever
+/// `UndoManager` the window's delegate hands back (`windowWillReturnUndoManager(_:)`) — which is
+/// exactly how the system's Edit ▸ Undo row and the toolbar's nil-target pair (`BoardToolbar`) light
+/// up, disable and retitle with no code of the app's own.
+///
+/// The seam, though, must not be an `NSUndoManager`: base's stack is one, Pro's is git
+/// (12-editions.md ▸ The provider seam), and a protocol that vended one could only ever have had a
+/// single implementation. So the substrate stays behind `HistoryProviding` and *this* object is the
+/// translation — one per board session, over whichever provider that session was composed with. Pro
+/// inherits the whole command surface (enablement, dynamic titles, ⌘Z, the toolbar pair) by binding
+/// its provider and changing nothing here, which is what "a user moving between editions relearns
+/// nothing" (12) has to mean in code.
+///
+/// ### It deliberately keeps its inherited stack empty
+///
+/// Every question is overridden, so anything that *did* register into this manager by accident (a
+/// stray AppKit client finding it through the window) would be invisible rather than half-live: it
+/// could never enable Undo, retitle it, or be crossed by ⌘Z. Nothing registers here today; the
+/// overrides are what make that safe rather than lucky.
+///
+/// `removeAllActions()` is deliberately **not** forwarded to `HistoryProviding.clear()`: AppKit
+/// clears a window's undo manager on paths this app does not control, and a card window closing must
+/// not empty the board's stack (13-native-undo.md ▸ Rules — the stack belongs to the *session*, and
+/// its one clearing point is that session's teardown).
+public final class BoardUndoManager: UndoManager {
+
+    /// The substrate this manager is a face for. Strong: the session owns both, and the manager is
+    /// only ever reachable while the session that made it is alive.
+    private let history: any HistoryProviding
+
+    public init(history: any HistoryProviding) {
+        self.history = history
+        super.init()
+    }
+
+    // MARK: Enablement
+
+    public override var canUndo: Bool { history.canUndo }
+
+    public override var canRedo: Bool { history.canRedo }
+
+    // MARK: Crossing
+
+    public override func undo() { history.undo() }
+
+    public override func redo() { history.redo() }
+
+    // MARK: Titles
+
+    /// `NSUndoManager`'s own vocabulary for "the phrase, without the verb" — `""` when there is
+    /// nothing to cross, which is what its menu-title composition expects.
+    public override var undoActionName: String { history.undoActionName ?? "" }
+
+    public override var redoActionName: String { history.redoActionName ?? "" }
+
+    /// "Undo Move 3 Cards" — composed and localized by the platform (`undoMenuTitle(forUndoActionName:)`
+    /// reads the `undo.strings` pattern), so the step vocabulary stays the bare phrase and this app
+    /// never spells the word "Undo" in a title.
+    ///
+    /// The trim is for the nameless case: the pattern is `Undo %@`, so an empty action name would
+    /// otherwise leave a trailing space where `NSUndoManager` itself answers a bare "Undo".
+    public override var undoMenuItemTitle: String {
+        undoMenuTitle(forUndoActionName: undoActionName).trimmingCharacters(in: .whitespaces)
+    }
+
+    public override var redoMenuItemTitle: String {
+        redoMenuTitle(forUndoActionName: redoActionName).trimmingCharacters(in: .whitespaces)
+    }
+}
+
+// MARK: - BoardUndoRouting
+
+/// Which undo a hosted window answers with, given what holds the keyboard — 06-history-undo.md
+/// ▸ Undo routing, which 12 records as **edition-independent**: "focus decides text-undo vs
+/// board-undo; only the substrate behind board-undo differs".
+///
+/// ### Most of the rule is AppKit's, and is not here
+///
+/// "While a text-editing surface is focused ... ⌘Z/⇧⌘Z are that editor's own text undo" is the
+/// platform's own behaviour for a text view that vends a manager: `NSWindow.undo:` crosses the
+/// *first responder's* `undoManager`, and both of this app's real editors vend their own through the
+/// text-view delegate (`CardBodySurface`, `CardRawSourceView` — each with an `UndoManager` created
+/// per editor, emptied when the session ends). Those never reach this file.
+///
+/// ### What is here is the case AppKit gets wrong
+///
+/// A **field editor** — the shared `NSTextView` that appears inside every focused `NSTextField` and
+/// `NSSearchField` — vends no manager of its own to the window, so `undo:` falls through to the
+/// window's, and a reflexive ⌘Z over a typo in the search field or the board-rename field would
+/// cross a *board* step. 06 rules that out by name: "Control-class text fields route the same way
+/// (settled): the search field ... and the popover's text fields ... own ⌘Z/⇧⌘Z as field-local text
+/// undo while focused — a reflexive undo over a typo must never become a tree checkout." So the
+/// window-level answer is the board's stack **only when no text-editing surface holds the
+/// keyboard**, and a per-window text manager otherwise — which is also exactly what AppKit would
+/// have created for such a window on its own, so nothing about typing in a field changes.
+///
+/// Pure and free of windows on purpose: the decision is three lines that are invisible until they
+/// are wrong, and the responder it reads is the only part a test cannot conjure.
+public enum BoardUndoRouting {
+
+    /// Whether `responder` is a text-editing surface — every `NSTextView`, field editors included,
+    /// which `NSText` is precisely the ancestor of. An `NSTextField` that has focus without editing
+    /// is *not* one: AppKit installs the field editor the moment editing begins, and until then the
+    /// board is what the keyboard is acting on.
+    public static func isTextEditing(_ responder: NSResponder?) -> Bool {
+        responder is NSText
+    }
+
+    /// The manager a window's delegate should answer with.
+    ///
+    /// `board` is `nil` for every window that is not showing a board — welcome, the restore
+    /// bootstrap, the template chooser, and a board or card window whose session has already been
+    /// torn down. Those get the text manager too, which is the platform default they had before this
+    /// seam existed.
+    public static func undoManager(
+        isTextEditing: Bool,
+        board: UndoManager?,
+        textFallback: UndoManager
+    ) -> UndoManager {
+        guard !isTextEditing, let board else { return textFallback }
+        return board
+    }
+}

Kanban/History/HistoryProviding.swift

@@ -0,0 +1,143 @@
+import Foundation
+
+// MARK: - HistoryStepOutcome
+
+/// What happened when a step was asked to walk its write back (or forward again).
+///
+/// The second case is 13-native-undo.md ▸ Rules ▸ **staleness validation**, in the vocabulary the
+/// provider needs it in: "target folder gone, or the field no longer holding the step's after-value
+/// → the step is **skipped, not applied**: popped from the stack ... and ⌘Z falls through to the
+/// next step". The provider reads this answer and nothing else — the *predicate* (field-level,
+/// settled) and the info-tone banner that explains a skip both belong to the step, which is the only
+/// side that knows what it wrote and which board to say it on.
+public enum HistoryStepOutcome: Equatable, Sendable {
+
+    /// The step ran. Its mirror image joins the opposite stack.
+    case applied
+
+    /// The step declined: the board no longer holds the value this step's write left, so applying
+    /// its inverse would clobber somebody else's newer edit. Nothing ran, the step is dropped, and
+    /// the crossing continues with the next one down.
+    case skipped
+}
+
+// MARK: - HistoryStep
+
+/// One undoable step: a name, and the pair of actions that walk the board backwards and forwards
+/// across it.
+///
+/// ### An operation pair, not an `NSUndoManager` registration
+///
+/// 13-native-undo.md ▸ Rules puts every undoable change at the Writer boundary — "each Writer call
+/// site registers the inverse operation, computed from the pre-write snapshot the store already
+/// holds: move → move back ...; rename → restore title" — and both halves of that write are already
+/// in the caller's hands: the before-value *is* the inverse, and the after-value is what the write
+/// set (which is also what the staleness predicate compares). A step is therefore that pair, in the
+/// design's own terms, and deliberately says nothing about how a stack stores it: base's stack is
+/// `NSUndoManager`-backed and Pro's is git (12-editions.md ▸ The provider seam), and neither
+/// substrate appears here.
+///
+/// ### `name` is the 06 vocabulary, unprefixed
+///
+/// "The 06 vocabulary supplies menu titles ('Undo Move 3 Cards'), via NSUndoManager's dynamic
+/// retitling — the same naming machinery both editions use" (13). What the step carries is the bare
+/// phrase — `"Move Card"`, `"Move 3 Cards"`, `"Rename Lane"` — in the vocabulary of
+/// 06-history-undo.md ▸ Commit messages, plural-folded by the same rule ("one gesture, one undo
+/// step — a multi-card move is one step with a plural title"). The **"Undo "/"Redo " prefix is
+/// never part of it**: the platform composes and localizes that (`BoardUndoManager`), and a step
+/// that spelled it would read "Undo Undo Move Card" in the Edit menu.
+///
+/// ### Both closures are `@MainActor`
+///
+/// Everything they touch — the store, the snapshot, the banners — is, and a step exists to be run
+/// from a menu command. Marking them says so at the seam instead of leaving each provider to
+/// rediscover it.
+public struct HistoryStep {
+
+    /// The menu phrase, unprefixed — see the type's note.
+    public let name: String
+
+    /// Walks the board back across this step. Registered at the Writer boundary as the *inverse* of
+    /// the write that just landed.
+    public let undo: @MainActor () -> HistoryStepOutcome
+
+    /// Walks it forward again — the original write, replayed. Reached only after `undo` applied,
+    /// because that is the only way a step reaches the redo stack.
+    public let redo: @MainActor () -> HistoryStepOutcome
+
+    public init(
+        name: String,
+        undo: @escaping @MainActor () -> HistoryStepOutcome,
+        redo: @escaping @MainActor () -> HistoryStepOutcome
+    ) {
+        self.name = name
+        self.undo = undo
+        self.redo = redo
+    }
+
+    /// The same step read backwards — what a provider puts on the opposite stack once this one has
+    /// applied. The name does not change, which is the whole of "Undo Move Card" becoming "Redo Move
+    /// Card": the phrase names the *gesture*, not the direction.
+    public var reversed: HistoryStep {
+        HistoryStep(name: name, undo: redo, redo: undo)
+    }
+}
+
+// MARK: - HistoryProviding
+
+/// The undo/redo substrate, behind one protocol boundary (12-editions.md ▸ The provider seam).
+///
+/// ### One per board session
+///
+/// "One stack per board, owned by the board session. Not per-window: every window over a board
+/// (board window, its card windows) shares the store and shares the stack" (13-native-undo.md
+/// ▸ Rules). `AppModel.BoardSession` is where that ownership lives, and the composition root binds
+/// which implementation it gets: base binds `NativeHistoryProvider` (an `NSUndoManager` stack over
+/// inverse `WriteOperation`s), Pro binds the git provider in pro-m1 (undo as forward restore commits
+/// over HEAD's first-parent ancestry — 06-history-undo.md), Teams inherits Pro's.
+///
+/// ### What this protocol deliberately does not say
+///
+/// - **No `NSUndoManager`, anywhere in the signature.** It is base's implementation detail, and a
+///   seam that vended one would be a seam only one provider could ever satisfy — the opposite of
+///   the reason the split exists at all ("base's native undo is the first proof the seam is real",
+///   12). AppKit still needs an `UndoManager` to hand the responder chain; that adapter is
+///   `BoardUndoManager`, which sits *over* this protocol rather than inside it.
+/// - **No persistence promise.** Base's stack dies with the session (13); Pro's survives relaunch
+///   because git does (06). Both are honest implementations of these seven members.
+/// - **No routing.** Which surface ⌘Z reaches is focus's answer, not the substrate's
+///   (06 ▸ Undo routing, edition-independent) — `BoardUndoRouting`.
+@MainActor
+public protocol HistoryProviding: AnyObject {
+
+    /// Records one undoable step, on top of the undo stack, clearing the redo stack — the classic
+    /// rule, and the one every substrate shares.
+    ///
+    /// Called once per *gesture*, never once per write: "coalescing follows commit granularity ...
+    /// a multi-card move is one step with a plural title; an Edit session is one step, registered at
+    /// the Edit→Preview flip" (13 ▸ Rules).
+    func register(_ step: HistoryStep)
+
+    /// Whether there is a step to cross. What the Edit menu's Undo row and the toolbar's Undo item
+    /// enable on, through the same responder-chain answer.
+    var canUndo: Bool { get }
+
+    var canRedo: Bool { get }
+
+    /// The name of the step ⌘Z would cross, or `nil` when there is none — the phrase the menu title
+    /// is composed from ("Move 3 Cards" → "Undo Move 3 Cards").
+    var undoActionName: String? { get }
+
+    var redoActionName: String? { get }
+
+    /// Crosses one step backwards. A stale step is skipped rather than applied, and the crossing
+    /// falls through to the next one (13 ▸ Rules ▸ staleness validation); an empty stack does
+    /// nothing.
+    func undo()
+
+    func redo()
+
+    /// Drops every step in both directions — session-only persistence (13 ▸ Rules), run at the board
+    /// session's teardown. Also what a substrate that must re-seed (a branch switch, 06) calls first.
+    func clear()
+}

Kanban/History/NativeHistoryProvider.swift

@@ -0,0 +1,127 @@
+import Foundation
+
+// MARK: - NativeHistoryProvider
+
+/// The base edition's undo substrate: one `NSUndoManager`-backed stack per board session
+/// (13-native-undo.md).
+///
+/// ### Why `NSUndoManager` at all, when the steps are ours
+///
+/// Because the *command surface* is the platform's. Edit ▸ Undo and Edit ▸ Redo are the system's own
+/// nil-target `undo:`/`redo:` rows (11-command-nexus.md), the toolbar pair carries the same actions
+/// (`BoardToolbar`), and both light up, disable and **retitle** from whatever `UndoManager` the
+/// focused window hands back — "Undo Move 3 Cards" is `NSUndoManager`'s dynamic retitling, which 13
+/// names as the mechanism and 12 records as the one both editions share. Reimplementing the stack
+/// over two arrays would mean reimplementing that, badly.
+///
+/// It stays *behind* `HistoryProviding` regardless: nothing outside this file learns that base's
+/// stack is an `NSUndoManager`, which is what lets pro-m1 bind git to the same seam
+/// (12 ▸ The provider seam).
+///
+/// ### One `register` call is exactly one step
+///
+/// `groupsByEvent` is turned **off** and every registration is wrapped in its own group. The default
+/// (on) closes a group at the end of the run-loop turn, which would silently fold two gestures that
+/// happened to land in one event into a single ⌘Z — the opposite of 13's "one gesture, one undo
+/// step", and the coalescing decision belongs to the Writer call site (a multi-card move registers
+/// *one* step with a plural title), never to the run loop's timing.
+///
+/// ### Undo flips to redo by re-registering
+///
+/// A step that applies pushes its own mirror image back onto the manager from inside the undo — and
+/// `NSUndoManager` routes a registration made while it is undoing onto the **redo** stack, and vice
+/// versa. That single rule gives the whole classic dance (undo → redo → undo …) with no second stack
+/// of our own, and it is why a *skipped* step leaves nothing behind: it registers nothing, so the
+/// empty group is discarded and the step is simply gone (13: "popped from the stack ... and ⌘Z falls
+/// through to the next step").
+@MainActor
+public final class NativeHistoryProvider: HistoryProviding {
+
+    /// The stack. `private` and never vended: see the type's note.
+    private let manager = UndoManager()
+
+    /// Set by the step just crossed when it declined to apply, read by the crossing loop below.
+    /// A flag rather than a return value because the manager, not this object, calls the step.
+    private var lastCrossingSkipped = false
+
+    public init() {
+        // See "One `register` call is exactly one step", above.
+        manager.groupsByEvent = false
+    }
+
+    // MARK: - HistoryProviding
+
+    public var canUndo: Bool { manager.canUndo }
+
+    public var canRedo: Bool { manager.canRedo }
+
+    /// `NSUndoManager` answers `""` — not `nil` — for a stack with nothing on it *and* for a step
+    /// registered without a name, so the emptiness check is the honest one.
+    public var undoActionName: String? {
+        guard canUndo, !manager.undoActionName.isEmpty else { return nil }
+        return manager.undoActionName
+    }
+
+    public var redoActionName: String? {
+        guard canRedo, !manager.redoActionName.isEmpty else { return nil }
+        return manager.redoActionName
+    }
+
+    public func register(_ step: HistoryStep) {
+        push(step)
+    }
+
+    public func undo() {
+        cross(manager.undo, while: { self.manager.canUndo })
+    }
+
+    public func redo() {
+        cross(manager.redo, while: { self.manager.canRedo })
+    }
+
+    public func clear() {
+        manager.removeAllActions()
+    }
+
+    // MARK: - The stack
+
+    /// Registers `step` as one group of its own, named for the menu.
+    ///
+    /// The target is `self` and the payload rides in the closure, which is `NSUndoManager`'s
+    /// block-based form: the manager references its target **unowned** (its own documented rule), so
+    /// the provider owning the manager that references the provider is not a cycle.
+    private func push(_ step: HistoryStep) {
+        manager.beginUndoGrouping()
+        manager.registerUndo(withTarget: self) { provider in
+            provider.apply(step)
+        }
+        // Inside the group, deliberately: the name belongs to the group being closed, and on the
+        // way back it is what retitles Redo.
+        manager.setActionName(step.name)
+        manager.endUndoGrouping()
+    }
+
+    /// Runs one step's action and records what happened.
+    private func apply(_ step: HistoryStep) {
+        switch step.undo() {
+        case .applied:
+            // Lands on the opposite stack — see "Undo flips to redo by re-registering".
+            push(step.reversed)
+        case .skipped:
+            lastCrossingSkipped = true
+        }
+    }
+
+    /// Crosses one step, and keeps going while the steps it crosses decline to apply — 13's
+    /// fall-through: "the step is skipped, not applied ... and ⌘Z falls through to the next step".
+    ///
+    /// The `while` guard is the manager's own emptiness, so a stack of nothing but stale steps
+    /// empties itself and stops rather than spinning.
+    private func cross(_ step: () -> Void, while more: () -> Bool) {
+        repeat {
+            guard more() else { return }
+            lastCrossingSkipped = false
+            step()
+        } while lastCrossingSkipped
+    }
+}