30 lines
13 KiB
Markdown
30 lines
13 KiB
Markdown
# Native Undo
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The undo/redo substrate for **every board without app-managed git, in every tier** (re-ruled 2026-07-31 — the provider follows the board, not the tier alone; formerly free-tier-only, which made a Pro upgrade *remove* undo from mode-none boards): the free tier binds it everywhere (any `.git` inert — 12-editions.md), and Pro binds it on mode-none **and repo-nested** boards alike (the repo-nested no-undo case retired 2026-07-31: leave-strictly-alone 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; the upgrade story is thereby exceptionless), switching to the git provider (06-history-undo.md) where the board's own git exists. **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; a subscription lapse still never interrupts (12). The design problem is not NSUndoManager itself — it is native undo over **files-are-truth**: the disk can change underneath the stack, because the app is not the only writer.
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## Rules
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- **Two levels: one stack per board, one per open card window** (re-ruled 2026-07-31 — the session-coarsening model, superseding the pure one-stack rule): the **board stack** is owned by the board session and shared by board surfaces; a **card window owns its own stack** for the session it represents — every gesture issued in that window (comment post/delete/edit, body Edit sessions, style/details changes, attachment ops where undoable) registers there at fine grain, and `window.undoManager` answers with it (standard per-window AppKit scoping). Disk stays live throughout — files-first untouched; this is history granularity only. **Window close coarsens**: the session's net effect registers on the board stack as **one coarse step named "Changes to '⟨card⟩'"** (ruled 2026-07-31 — the board row reads "Undo Changes to 'Fix login'": plural and scope-flavored, distinct from every fine verb, honest about folding many kinds; the fine body-edit wording never leaks onto the board menu), values-based, whose undo restores the card subtree to its session-start state — deleted comments included — and whose redo reapplies the net effect; a session with no net change registers nothing. The coarse step is transactional at apply time: staleness validation runs per component (the field-level predicate below), and any stale component skips the whole step — never a partial session revert. **Session steps anchor by card identity, never by path** (ruled 2026-07-31): the coarse step — and the window's fine steps it folds — stores the card's UUID plus expected values, and apply-time validation resolves the card's *current* folder exactly the way the window itself always resolves its card (the per-snapshot UUID walk; `writeCardBody` already resolves trash locations on purpose). A tracked relocation — a lane move mid-session or after close, a trash move — therefore never stales the step; only genuine content changes do, which is what the validation exists to catch. A card that resolves nowhere (purged, or moved out of the board) is the honest skip. 06 ▸ Undo routing applies unchanged — text-editing surfaces get their session-scoped text undo above either stack. **Two stacks over one open card are the blessed shape** (2026-07-31): a board-issued gesture on a card whose window is open registers on the board stack while the window's own gestures register on the window stack — no ordering relation between the two, interleaving decided by ⌘Z focus (06 ▸ Undo routing); routing board gestures into the open window's stack was considered and rejected, since board ⌘Z must never see card-session steps.
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- **Registration at the Writer boundary.** Every app-mediated mutation already passes through the Writer as a `WriteOperation` (02-architecture.md) — that closed enum is the exact inventory of undoable operations. Each Writer call site registers the inverse operation, computed from the pre-write snapshot the store already holds: move → move back (original lane, original `order`); reorder → restore original `order`; rename → restore title; restyle → restore prior style; resize → restore prior width; Edit-session body save → restore prior body bytes; card or lane delete (⌫) → move back out of `.trash/` (lanes rejoined the trash 2026-07-29 — the recreate-from-capture inverse retires with the last destructive delete); restore-by-move → move back in; create → remove the created folder.
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- **What is not undoable** (settled): **Permanently delete** (the trash's Delete, Empty Trash) — `purgeIsUnrecoverable` stays true in base, and the existing confirmation rule (03-board-ui.md) already fires on all base boards, since none have git history: the confirm *is* the safety. **The duplicate-id remint** (01-storage-format.md — a silent scheduled heal since 2026-07-29, formerly the user-gated Repair) — heals aren't user gestures, so nothing enters the stack, and undoing one would recreate the duplicate id it exists to remove. Permanently delete matches its existing "destructive, confirmed, final" posture; the remint sits outside undo as all heals do. **Raw Source Apply** (blessed 2026-07-31): the hatch writes byte-for-byte outside every contract — no `modified` stamp, no attribution clear, and no history step at either level; an Apply-only session folds to no coarse step, and an Apply mixed into a session is invisible to the fold. The hatch's story is "you edited the file," and files-are-truth covers it — on git boards the write commits like any disk change (05-card-window.md's carve-outs are the same statement from the stamping side).
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- **Coalescing follows commit granularity** (settled; window scoping added 2026-07-31): one gesture, one undo step — a multi-card move is one step with a plural title; an Edit session is one step, registered at the Edit→Preview flip (the effective Save — 05-card-window.md) **on the card window's stack**, like every window gesture; the window close registers the one coarse session step on the board stack (Rules above); a styling batch is one step (03's one-gesture-one-commit rule, substrate swapped). The 06 vocabulary supplies menu titles ("Undo Move 3 Cards"), via NSUndoManager's dynamic retitling — the same naming machinery both editions use.
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- **Session-only persistence** (settled): the stack lives with the board session and dies at close/quit — standard macOS behavior. Git undo's survive-relaunch property is a Pro difference, stated honestly (12's matrix).
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- **Foreign writes never join the stack** (settled): NSUndoManager can only undo what the app mediated. An agent's or hand edit is not a step — the honest capability gap vs Pro (12's matrix). Foreign changes also do not clear the stack wholesale; collisions are handled lazily, per step, by validation:
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- **Staleness validation before every apply** (settled): an inverse operation re-checks its target against the disk — a fresh read of the target at ⌘Z time (blessed 2026-07-29: not the store snapshot, which is by construction one reload behind the app's own writes; a rapid ⌘Z run validated against the snapshot would compare pre-state and false-skip every step). **The predicate is field-level** (settled — ruled 2026-07-27): each step registers both sides of its write anyway (the before-value is the inverse; the after-value is what its write set), so validation compares the targeted field's current value against the expected after-value — nearly free, and truer to never-surprise-the-file than an existence-only check (an inverse rename must not clobber a foreign rename on a still-existing card; body steps compare bytes). Target folder gone, or the field no longer holding the step's after-value → the step is **skipped, not applied**: popped from the stack with an info-tone banner ("Undo skipped — 'Fix login' changed outside Lanework"), and ⌘Z falls through to the next step. Never apply a stale inverse on top of someone else's newer write. **Delete steps validate their undo by existence only** (blessed 2026-07-31): a delete's forward write sets nothing but the `modified` stamp (the arrival rank mint retired 2026-07-31), and a clock reading is not a choosable after-value — pinning it would false-skip the restore whenever an agent touched the trashed card; the undo therefore expects only that the trashed folder still exists, while the redo stays field-level via the restore's `order` write. **Invalidation is lazy** (settled — ruled 2026-07-27): staleness is discovered at ⌘Z time, never by background pruning — the EchoLedger's foreign diffs do not eagerly drop colliding steps. The stack always looks full; with the field-level predicate a skip fires only on a genuine per-field collision, and a skipped step's banner explains itself, where eager pruning would shrink the stack invisibly mid-session.
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- **Locks disable the stack** (settled): every read-only lock (vanished root, failed reload after wholesale ops, unwritable location — 02-architecture.md) disables Undo/Redo with the other mutating commands; the stack itself survives the lock and resumes when it clears. Steps landed before a lock validate like any other at apply time.
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## Interaction with the trash
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⌫'s undo is the move back — a delete is a move into `.trash/` (cards resettled 2026-07-28; lanes rejoined 2026-07-29), so its undo is the ordinary inverse move, returning a card to its source lane and rank, a lane to its strip position (subtree intact — it never left the folder); a restore-by-move undoes the same way in reverse. The stack and the trash never conflict — they are the same folder moves addressed by recency instead of by selection. The old lane-delete recreate-from-capture inverse is **retired** — no destructive delete remains outside a trash, so nothing needs byte capture. A **permanent delete registers no step** — the trash's Delete and Empty Trash are not undoable (Rules above), lanes and their freight included; the confirm is the safety.
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**Comments keep the no-capture rule true — on the window stack** (re-ruled 2026-07-31, superseding the board-stack routing): a comment delete is a move into the card's `comments/.trash/` (01-storage-format.md ▸ Enhanced schema — the materialized-trash pattern one level down), its inverse the ordinary move back, and the step lives on the **card window's own stack** (Rules above) — the board stack never carries a granular comment step, so the old stale-after-close skip scenario cannot arise. **The purge of `comments/.trash/` defers with the coarse step** (re-ruled 2026-07-31, superseding purge-at-close): the coarse close step's undo restores deleted comments, so their backing lives as long as the step does — the purge runs when the coarse step leaves the board stack **cleanly** — undone-and-superseded, or dropped off the end — or when the board session ends; **a stale-skipped step's backing instead survives to board-session end** (ruled 2026-07-31, decoupling skip from purge): the skip banner says nothing was applied, and an irreversible purge riding that gesture would be surprise loss — the skip is exactly when the user may want to inspect what the collision left; crash residue still sweeps at the next card-window open — **and residue is defined by the purge-deferral condition itself** (ruled 2026-07-31): `comments/.trash/` content referenced by a live coarse step on the board stack is a step's backing, not residue — the open-time sweep consults the stack and skips owned content, re-arming when the owning step leaves the stack (which is exactly when the deferred purge wanted to run; one condition, two consumers). Reopening a window can therefore never destroy its prior session's undo backing. Unowned content sweeps as before, armed-then-cleared like every heal memo. On Pro the substrate is history: the close commit nets delete-plus-purge to a removal, revert restores it, so purge rides the close flush there as before — purge timing follows the undo substrate's need.
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## Out of scope
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- **Attachment operations, v1** (deferred — ratified 2026-07-27): attach → remove is a clean inverse, but remove-attachment → re-add requires the removed file to survive somewhere (a staging area with a lifecycle — App Support, bounded, its own cleanup rules; possibly shared with 04 ▸ Clipboard's staging). The deferral is the ruling: attachment add/remove registers **no undo step** in v1 (the operations remain, as today, confirmed-or-benign); the staging design pass reopens post-2.0.
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- **EchoLedger-synthesized foreign undo** (deferred, wishlist — WISHLIST.md item 6): the ledger already classifies foreign diffs for announcements; it could synthesize inverse operations and push foreign steps onto the stack, narrowing the gap to Pro. Real design needed (ordering vs app steps, attribution, user expectations) — not assumed by this doc.
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## Open questions
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None currently — the staleness predicate (field-level) and invalidation timing (lazy) were ruled 2026-07-27 and are settled in Rules above.
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