> # ⚠️ PARTLY REINSTATED — read this note before the SUPERSEDED banner below
>
> A real offline mail replica **now exists**: `lib/offline-replica/**` +
> `app/api/offline/{sync,mail,status}` + `lib/offline-fallback-client.ts`. So the banner below
> ("that scope was dropped") is history, not current state. What was reinstated is the DATA MODEL
> and the SYNC PROTOCOL from this document; what was **not** reinstated is its process
> architecture — and that distinction is the whole reason the review's critical findings did not
> come back with it.
>
> | This document proposed | What was built |
> |---|---|
> | A persistent background worker holding credentials for the process lifetime | **No worker.** A cycle is request-scoped work in an API route using the request's own `jmap_stalwart_ctx` cookie, triggered by the renderer's live push connection — the same shape the search index already uses. |
> | An epoch-fenced multi-account `registry.json` | **No registry, no epochs.** Single-flight per account inside one process; all state in the SQLite file under a real transaction. |
> | Multi-account simultaneous sync | **One request, one account, one cycle**, with hard budgets. |
> | Engine reads a renderer setting to decide whether to sync | **The renderer decides when to sync.** The retention *policy* is durable inside the encrypted store (`/api/offline/status`), because the retention pass genuinely needs it server-side. |
> | An offline read layer with no coherence story for the webmail's local unread-count arithmetic | **The replica is a FALLBACK, never a cache in front of the server** — consulted only after a read has failed at the transport level, so an online session never sees a replica count. This is the answer to review finding H3, the one finding that genuinely returned. |
>
> Consequently: **C2, C3, C4, H1 and H4 remain moot** (they were all consequences of the worker,
> the registry, or a server-side engine reading renderer state), **C1 and H2 remain fixed** by what
> already shipped (`optionalDependencies` + guarded require; the key on an inherited fd), and
> **H3 is now in scope and answered** as above. See `lib/offline-replica/sync.ts`'s header for the
> finding-by-finding version of this table, kept next to the code it constrains.
>
> Two implementation bugs from the mobile client are regressed by name, because both fail silently
> and both cost user data: the **body-tier infinite redownload loop** (durable `gave_up` +
> `shed-by-cap` marks + inserted-not-attempted counting) and the **clock-jump guard that wiped the
> store** (persist the floor that was USED, never the one that was rejected, plus a separate
> `evictionAllowed` bit). Tests:
> `lib/offline-replica/__tests__/{store,retention}.test.ts`, and the real network-cut proof in
> `integration/tests/13-electron-offline-replica.spec.ts`.
>
> Still deliberately out of scope: attachment blobs, offline compose/outbox, delegated/shared
> accounts, and calendar/contacts/files replication.

> # ⚠️ SUPERSEDED — this is not what was built
>
> This document designs a **full offline mail replica**: a persistent background sync engine with
> JMAP `Foo/changes` cursors, three state machines, a retry ladder, reconcile/sweep logic and an
> epoch-fenced multi-account registry. **That scope was dropped.** After the adversarial review
> (`ELECTRON-OFFLINE-ENGINE-REVIEW.md`), the human narrowed the requirement to *"a SQLite index we
> can prompt against"* — retrieval to feed an LLM, refreshed on each delivery/change event.
>
> **What was actually built:** `lib/mail-index/**` + `app/api/offline/{reindex,search}` — an
> encrypted SQLite/FTS5 index over mail, calendar, contacts and file *metadata*, written by an
> ordinary request-scoped API route that the renderer's existing live JMAP push connection calls
> when something changes. No background worker, no cursors, no resident credentials. Staleness
> between refreshes is acceptable by design.
>
> Most of the review's CRITICAL and HIGH findings **stopped existing** rather than being fixed: C2,
> C3, C4, H1 and H2 were all consequences of a long-lived worker holding credentials, and there is
> no worker.
>
> **Still accurate and still worth reading here:**
> - §3 — the SQLite/SQLCipher binding investigation. `@signalapp/sqlcipher` is what shipped, for the
>   reasons given, and the `PRAGMA key` silent-no-op landmine is real (the shipped code asserts
>   `cipher_version` returns a non-empty *string*, per the review's correction).
> - §6 — `safeStorage`, including the Linux `basic_text` hazard. Shipped as described, with
>   `getSelectedStorageBackend()` correctly guarded to Linux only (a review finding).
> - §1 — the codebase survey (auth model, push pipeline, CSP, account model). All verified.
> - §2.4's hosted-deployment gate (`VNCMAIL_DESKTOP_STORE_DIR`) — shipped, and now covered by a test.
> - §14 — what was and was not empirically verified.
>
> **Wrong in hindsight, beyond the scope change:** §2.1's claim that Option A needs no new secret
> handling (the review's C2 is right — credentials are request-scoped, not resident); and §2.1's
> assumption that Next's output file tracing would carry the native module (it does not — the
> standalone build needs an explicit copy step, now in `scripts/assemble-standalone.mjs`).

# Electron Offline Engine — Design

Status: **superseded design, never implemented.** See the note above. Nothing outside this file was
changed by the pass that wrote it; `electron/main.ts`, `electron/preload.ts` and
`lib/jmap/client.ts` were untouched *at that time* (`main.ts` has since gained the index's store-dir
and key-channel wiring, which is a small fraction of what this document describes).

Repo: `brvncde-dotcom/vncmail-plus`, branch `claude/electron-offline-design`, worktree
`~/worktrees/vncmail-electron-sqlite`. Based on `claude/electron-desktop` (the working desktop
shell + RFC 8887 WebSocket push), HEAD `b15098a6`.

Companion documents:

- **`~/worktrees/vncmail-native-sync-impl/docs/DELTA-SYNC-ENGINE-DESIGN.md`** (revision 3, 2012
  lines) — the finalized, twice-adversarially-reviewed, implemented and real-device-verified JMAP
  delta-sync design for this program's React Native client. **This document is an adaptation of
  that one, not a replacement for it.** Read it first; it is the normative source for everything
  marked *[reused]* below. Cited as **M§n** throughout.
- `~/worktrees/vncmail-native-sync-impl/docs/DELTA-SYNC-DESIGN-REVIEW.md` — the adversarial review
  that produced M's revision 2 (findings S1–S16). Cited as **MR**.
- `docs/VNCMAIL-NATIVE-BUILD-MANUAL.md` — program narrative; §4 decision log, §5 (what the Electron
  shell already has), §8 known landmines.
- `~/.claude/skills/VNCprodbuild/SKILL.md` — the build plan this belongs to.

Normative references, cited by section: **RFC 8620** (JMAP core) and **RFC 8621** (JMAP Mail),
exactly as enumerated in M's preamble. This document does not re-derive any RFC reading; every
protocol-level claim is M's, verified there.

---

## 0. How to read this document

The JMAP delta-sync problem is platform-independent. M solved it, was attacked twice over it, and
shipped it. Re-deriving it here would produce a second, subtly different set of invariants for the
same protocol — which is how one client silently loses mail the other doesn't.

So every section is tagged:

| Tag | Meaning |
|---|---|
| **[reused]** | Adopted from M unchanged. The cited M section is normative; this document only records *that* it applies and any Electron-specific naming. Do not re-litigate. |
| **[adapted]** | M's decision holds but its mechanism doesn't, because Electron's runtime differs. The difference is stated explicitly. |
| **[new]** | No M counterpart, or M's answer is actively wrong here. Designed from scratch in this document. |

The genuinely new work is §2 (which process hosts the engine), §3 (SQLCipher from day one), §6 (key
storage), and the parts of §5/§7/§8 that follow from those. Everything else is M.

### 0.1 Scope

**In scope:** where the engine and its SQLite file live; which SQLite binding; whether encryption
ships on day one and how its key is stored; how the store keys against *this* repo's account model;
what the existing renderer-side push pipeline must and must not do once the engine exists; the
schema; triggering; the staged rollout with its verify-first gate.

**Out of scope, deliberately:**

- FTS5 index population (VNCprodbuild step 9). §7.5 reserves the hook. Note §3.4: FTS5 is
  empirically present in every candidate binding, so this is not a binding-selection input.
- Offline **compose/outbox**. Unlike the mobile app, **this repo has no outbox and no optimistic
  mutation layer at all** (§1.6) — so M§5.6's read-time overlay has nothing to overlay. v1 desktop
  offline is **read-only**. This is a scope decision, recorded in §5.4, not an oversight.
- Attachment blob storage (M§9.4's second bullet applies verbatim when it lands).
- Calendar / Contacts / Files delta sync.
- Shared/group ("delegated") mail. Account-scoped primary keys are in place from day one (M§9.3,
  MR S3) so adding it later is inserting rows.
- Code signing (VNCprodbuild step 9). It is *referenced* in §6.3 because it interacts with
  macOS key storage, but it is not resolved here.

**Non-goal:** compatibility with anything on disk today. There is nothing on disk today (§1.6).

---

## 1. What exists today, verified

File:line references are to this worktree at `b15098a6`. Everything in this section was read, and
every runtime claim in §3/§6 was executed against the Electron binary actually pinned by
`package.json` — see §3.1 for the transcript summary.

### 1.1 The desktop shell

`electron/main.ts` (225 lines) boots the **same** Next.js `output: "standalone"` artifact the
`Dockerfile` ships (`next.config.ts`'s `output: "standalone"`), as a **child process**:

- `getStandaloneServerEntry()` (`:26-31`) — `process.resourcesPath/standalone/server.js` when
  packaged, `.next/standalone/server.js` in dev.
- `startStandaloneServer()` (`:70-105`) — allocates a random free localhost port (`:33-48`), spawns
  `process.execPath` with `ELECTRON_RUN_AS_NODE: "1"` (`:85-94`) so no system Node is required, then
  polls until reachable (`:50-68`).
- `createMainWindow()` (`:114-143`) — `BrowserWindow` with `contextIsolation: true`,
  `nodeIntegration: false`, **`sandbox: true`**, preload at `dist-electron/preload.js`, loading
  `http://127.0.0.1:<port>`.
- The notification bridge: `ipcMain.handle("vnc:show-notification", …)` (`:155-176`), reached from
  `electron/preload.ts:16-27`'s `contextBridge.exposeInMainWorld("vnc", …)`, wrapped by
  `lib/electron-bridge.ts`'s `isElectronShell()` / `showElectronNotification()`, and called from
  `app/(main)/[locale]/page.tsx:1198-1222`. **This is the existing IPC pattern** — one
  `ipcMain.handle` + one `contextBridge` method, no channel registry, no streaming.

Packaging (`electron-builder.config.js`): the standalone server ships as `extraResources` copied
`from: ".next"` with a `standalone/**/*` filter — deliberately, to dodge app-builder-lib
unconditionally dropping a copy-root directory literally named `node_modules` (documented in that
file, and in the manual §5 as a bug found by actually launching a `--dir` build). Targets: macOS
dmg+zip **x64 and arm64**, Windows nsis x64, Linux AppImage+deb x64. Unsigned.
`scripts/build-electron.mjs` bundles `electron/*.ts` with esbuild, CJS, `external: ["electron",
"electron-updater"]`.

CI: `.github/workflows/electron-build.yml`, matrix macos/windows/ubuntu, **Node 22** on the runner
(note: not Electron's Node — see §3.3), `npm run test:electron` as a required gate before packaging.

### 1.2 The JMAP client, and where the credentials actually are

`lib/jmap/client.ts` (7413 lines) is a **renderer-side** class. `JMAPClient` (`:542`) holds
`serverUrl`, `username`, `password` and an `authHeader` built in the constructor as
`Basic ${btoa(username:password)}` (`:579-585`), or `Bearer …` via `static withBearer` (`:586-598`).
`authenticatedFetch` (`:672`) is plain browser `fetch()` straight to the mail server. There is no
JMAP proxy route in front of it for normal traffic.

**But the credentials are also recoverable server-side, and that is the load-bearing fact for §2.**
`app/api/auth/session/route.ts`:

- `POST` stores `encryptSession(serverUrl, username, password)` — AES-256-GCM under
  `SESSION_SECRET` (`lib/auth/crypto.ts:23-33`) — in an **httpOnly** cookie
  `jmap_session[_<slot>]` (`lib/auth/session-cookie.ts`), one per account slot,
  `MAX_ACCOUNT_SLOTS = 50` (`lib/account-utils.ts`).
- `GET` returns only `{serverUrl, username}`; `PUT` returns the **full credentials** for session
  restoration, gated on `Sec-Fetch-*` headers proving a same-origin browser `fetch()`.
- OAuth/TOTP accounts instead park a **refresh token** in an httpOnly cookie
  (`app/api/auth/token/route.ts` POST), and `PUT` on that route mints a fresh access token from it —
  **rotating the stored refresh token whenever the server returns a new one** (`:104-106`). Remember
  that; §2.5 has to keep two independent refreshers from existing.

So: any code running in the standalone server process can, for any account slot, obtain either a
Basic auth header (decrypt the session cookie) or a bearer token (refresh-token grant) **without a
single new credential path, IPC message, or storage location.** This is not true of Electron's main
process, which sees none of those cookies.

### 1.3 The push pipeline as shipped, and the wall it hit

`setupPushNotifications()` (`:6130`) prefers RFC 8887 JMAP-over-WebSocket
(`getWebSocketUrl()`, `:3803-3809`, reading the `urn:ietf:params:jmap:websocket` capability off the
session — never hardcoded), falling back to SSE, then polling. Tight reconnect ladder (200 ms base /
5 s cap), 30 s heartbeat, and a 3-consecutive-handshake-failure circuit breaker
(`wsPermanentlyDisabled`, `:6060`ff).

The committed comment at `:6038-6059` records the empirical outcome, and it is the single most
important existing finding for this design:

> `stalwart.sandbox.vnc.de`'s `/jmap/ws` requires the same HTTP `Authorization` header as every
> other JMAP endpoint **on the WebSocket UPGRADE request itself**. The browser `WebSocket`
> constructor cannot attach custom headers (a WHATWG restriction; credentials-in-URL are rejected
> too). So from the renderer, every attempt fails the handshake and the circuit breaker correctly
> falls back to SSE.

And the alternative it explicitly declined (`:6055-6059`):

> opening it from Electron's main process via a header-capable client like the `ws` package "would
> mean piping raw credentials from the renderer to the main process over IPC, which is a materially
> bigger security-sensitive change than what was scoped here."

That objection is **correct for the main process and inapplicable to the standalone server** — which
already holds the credentials (§1.2) and would pipe nothing. §2.5 acts on this.

The transport-agnostic "genuine new mail" signal is `email-store.newEmailNotification`
(`stores/email-store.ts:3082-3086`, set by `refreshCurrentMailbox`), consumed once in
`app/(main)/[locale]/page.tsx:1198-1222`. It already fires identically over WS, SSE and polling.
**The engine must not add a second notification path.** §2.5 states the rule.

### 1.4 Multi-account model (differs from mobile — check, don't assume)

`stores/account-store.ts` — a Zustand `persist` store named `account-registry`, holding
`AccountEntry[]` with:

- `id`: `` `${username}@${new URL(serverUrl).hostname}` `` via
  `lib/account-utils.ts generateAccountId()`. **Same shape as mobile's `LocalAccountId`** — a
  genuine coincidence worth stating, because it means M§3.1's `LocalAccountId` type carries over
  verbatim.
- `cookieSlot: number` — **new relative to mobile.** The index into the per-slot cookie namespace of
  §1.2, assigned by `getNextCookieSlot()` (first free integer, reused after removal).
- `serverIdentifiers?: string[]` — server-confirmed account-id forms captured at login, used by the
  account-switch guard so a short login name canonicalized by the server is still recognized.
- `activeAccountId`, `defaultAccountId`; caps `MAX_ACCOUNTS_HTTP1 = 5` (HTTP/1.1 SSE-connection
  budget) lifting to `MAX_ACCOUNT_SLOTS = 50` once h2/h3 is observed.

Two consequences for the schema (§7):

1. The durable key is `accountId` (`username@host`), **never `cookieSlot`** — slots are recycled by
   `getNextCookieSlot()`, so a slot number is a transport detail with a shorter lifetime than the
   data. A `slot → accountId` confusion is a cross-account data-mixing bug of exactly M's D6 shape.
2. Any API surface addressed by slot (as §1.2's routes are) must **resolve slot → accountId and
   re-verify** against the session's confirmed username before touching the store. §5.3.

### 1.5 CSP — a hard constraint on option C

`proxy.ts:88-141` builds the CSP. In production:

```
script-src 'self' 'nonce-<n>'          # no 'unsafe-eval', no 'wasm-unsafe-eval'
connect-src 'self' https: wss:         # 'wss:' was added for §1.3's WS push
```

`'unsafe-eval'` exists **only** for `isDev` and the plugin-sandbox path. WebAssembly compilation
requires `'wasm-unsafe-eval'` or `'unsafe-eval'` under CSP3. §2.3.

### 1.6 What does *not* exist here (and does in the mobile repo)

This is the inverse of M§1.1/§1.2, and it is mostly good news:

| | mobile (`vncmail-native`) | here |
|---|---|---|
| Existing offline cache | `offline-sync.ts` + `offline-cache-store.ts`, carrying defects D1–D8 | **nothing.** No IndexedDB mail cache, no offline list, no offline read path. `lib/plugin-storage.ts` uses IndexedDB but only for plugin assets. |
| `Email/changes` / `Mailbox/changes` wrappers | already present, already driving an incremental list path | **none.** The only occurrence in the repo is a mock in `app/api/dev-jmap/[...path]/route.ts:1949`. Greenfield. |
| Outbox / optimistic mutations | `outbox-store.ts`, full-state idempotent queue | **none.** Mutations go straight to the server. |
| Push transport | SSE + FCM relay | WS (blocked, §1.3) → SSE → polling, all renderer-side |
| Stalwart integration fixture | in a *sibling* repo — MR S16 costed this as real cross-repo CI work | **in this repo**: `integration/docker-compose.yml` + 11 specs incl. `11-electron-notification.spec.ts`, which logs in against real Stalwart, injects mail over real SMTP, and asserts the native notification. Free to extend. |

**Therefore M§1.3's defect list D1–D8 does not apply here.** There is no legacy cache to inherit
bugs from, no `patchCache()` write-through to delete, no D4 cursor fast-forward in shipped code, and
M§14.1's "discard, don't migrate" is vacuous. What *does* carry over is the *class* of each defect
as a thing not to introduce — which is what M's invariants I1–I13 are for (§4.3).

One inherited defect *shape* is worth naming, because this repo has it too: `stores/file-store.ts`
and others use `try { localStorage.setItem(...) } catch { /* ignore */ }` in a dozen places — M's D2
pattern. **Banned in the sync path** (M I4). §7.2.

---

## 2. Decision 1 — which process hosts the engine **[new]**

M has no counterpart: React Native has one JS context. Electron has three candidate homes, and this
codebase makes the choice non-obvious in both directions.

### 2.1 Candidate A — engine + SQLite inside the standalone Next.js server process

The renderer reaches cached data through new `app/api/**` routes, exactly as it reaches everything
else server-side today.

**For:**

1. **The credentials are already there, encrypted, per account** (§1.2). No new credential path, no
   IPC carrying secrets, no second copy of the TOTP/refresh state machine. Every other candidate has
   to solve this, and B can only solve it by doing the thing `client.ts:6055-6059` explicitly
   declined.
2. **It unlocks real WS push, which the renderer structurally cannot have** (§1.3). A Node process
   can set `Authorization` on a WebSocket upgrade (`ws` package). This is not a side benefit: it
   converts a documented dead end into a working transport, on the server this program actually
   targets, with no Stalwart-side change. §2.5.
3. **No new IPC surface at all.** `window.vnc` stays a one-method bridge. Nothing about
   `contextIsolation: true` / `sandbox: true` has to be relaxed or extended.
4. **Native module packaging is already solved for this process.** The standalone server ships as
   `extraResources` **outside `app.asar`**, with its own traced `node_modules` — the exact copy path
   whose one footgun is already found, fixed and documented (`electron-builder.config.js`). A `.node`
   binary in an unpacked directory needs no `asarUnpack` reasoning at all.
5. **Blocking is cheapest here.** `better-sqlite3` and `@signalapp/sqlcipher` are synchronous
   (§3.2). Blocking this event loop delays local-cache HTTP responses; it does not block the
   renderer's paint (React runs in the renderer) and does not block window/menu/IPC handling (that's
   the main process). It is the *least* latency-critical of the three loops.
6. Reads are trivially observable — the existing integration suite drives the app over HTTP and can
   assert on new routes without any Electron-specific harness.

**Against:**

1. **This process is also what a hosted, multi-user Docker deployment runs.** Unconditional offline
   routes would have a shared server start caching *every user's* mail into a server-side SQLite
   file. This is the strongest argument against A and it must be closed by construction, not by
   convention — §2.4.
2. **Next.js output-file-tracing vs. a native module.** `serverExternalPackages` (already used for
   `esbuild`, `next.config.ts`) plus NFT must actually carry `prebuilds/**/*.node` into
   `.next/standalone/node_modules`. `node-gyp-build`'s resolution is directory-scan-based, which NFT
   handles specially but not infallibly. **Verify-first, §12 Stage A.**
3. The DB path must be handed in: `app.getPath('userData')` is a main-process API, so `main.ts` must
   pass it as an env var on spawn (`:85-94` already builds the env). One line, but it is a coupling.
4. Server-process lifetime is `window-all-closed` / `before-quit` (`main.ts:210-219`), so an
   in-flight cycle is killed by process death rather than by a cooperative abort. M's crash-recovery
   design (I1, M§6.3) already makes that safe — cost is one page — but a graceful-shutdown IPC is
   worth adding later.

### 2.2 Candidate B — engine + SQLite in `electron/main.ts`, over `contextBridge`/IPC

**For:**

1. **The standalone server's code stays byte-identical to a hosted deployment's.** A's §2.1-against-1
   simply does not arise: there is no Electron-only server code to accidentally ship in Docker.
2. Mirrors the existing notification bridge, so the pattern is familiar.
3. `electron-builder` already auto-unpacks `**/*.node` from the asar, and production `node_modules`
   are collected regardless of the narrow `files: ["dist-electron/**/*", "package.json"]` (that's why
   `electron-updater` is `external` in `build-electron.mjs` and still ships). Low packaging friction.
4. `safeStorage` (§6) lives in the main process natively — no bridging for the key.

**Against:**

1. **It requires exactly the thing `lib/jmap/client.ts:6055-6059` refused.** The engine needs
   credentials. The main process has none: the `jmap_session` / refresh-token cookies belong to the
   renderer's origin. So either the renderer ships the Basic header / bearer token over IPC (the
   declined "materially bigger security-sensitive change"), or the main process learns to read
   Electron's cookie jar (`session.defaultSession.cookies`) and re-implement `decryptSession` — which
   means shipping `SESSION_SECRET` into the main process too. Both are net-new secret handling to
   reach a place that currently, deliberately, holds no secrets.
2. **A large new IPC surface.** The offline read path is not one fire-and-forget notification; it is
   list queries, single-message reads, per-account status, settings changes and abort signals — each
   an `ipcMain.handle` returning structured data across `contextIsolation`. Every one is a new trust
   boundary in a window that is currently `sandbox: true` with a 12-line preload.
3. **Blocking hurts most here.** Synchronous SQLite on the main process's loop is jank in window
   dragging, menu response and IPC dispatch. Mitigable with `worker_threads`, but then B is A's
   complexity plus IPC.
4. The renderer's offline read path becomes Electron-only by construction, so the web/PWA deployment
   can never share it. That may be acceptable — but it is a fork, and A's routes would work in both.

### 2.3 Candidate C — engine in the renderer, WASM SQLite over OPFS

**Investigated, not assumed. Findings:**

- The official WASM build is `@sqlite.org/sqlite-wasm` (3.53.0-build1), which `sqlocal` (0.18.0)
  wraps for OPFS. **Neither has any encryption** — SQLCipher is a *fork* of SQLite's source, not a
  loadable extension, so an official build cannot have it.
- Encrypted WASM builds **do** exist on npm: `@7mind.io/sqlcipher-wasm` (1.2.0, "production-ready
  WebAssembly build of SQLCipher with real OpenSSL-based encryption") and `@aztec/sqlite3mc-wasm`
  (5.1.0, SQLite3MultipleCiphers 2.3.5 as WASM). So the honest answer to "does a WASM SQLCipher
  genuinely exist?" is **yes, but only from small third-party publishers** — not from the SQLite
  project, not from a vendor with a desktop-mail-scale user base. For a component whose failure mode
  is "the user's whole mailbox is readable on a stolen laptop", that provenance is the finding.
- **The CSP problem is decisive independently of encryption.** §1.5: production `script-src` is
  `'self' 'nonce-…'`. WASM compilation needs `'wasm-unsafe-eval'`. Adding it in `proxy.ts` widens the
  CSP for **every deployment of this product, including the hosted web one**, to buy a desktop-only
  feature. That is a security regression with the wrong blast radius.
- Even granted both, the encryption key would live in the renderer's JS heap — the same context that
  renders untrusted HTML mail bodies and hosts the plugin sandbox. A/B keep it in a Node process the
  renderer cannot address.
- Secondary, verify-first if C is ever revisited: the official `opfs` VFS uses `SharedArrayBuffer` +
  `Atomics.wait` and therefore needs COOP/COEP headers; `opfs-sahpool` does not. Neither is
  configured in `proxy.ts` today.

**Against, summarised:** requires a product-wide CSP widening, puts the key in the most exposed
context, and its only encrypted backends are unvetted third-party WASM builds. **For:** no IPC, no
native module, no packaging story, works identically in the browser PWA — a real benefit, and the
reason to keep C on record rather than dismiss it. If the offline store were *unencrypted* and
*browser-first*, C would be the right answer. It isn't either.

### 2.4 Decision: **A**, with the hosted-deployment gate as part of the design

The engine and the SQLite file live in the **standalone Next.js server process**. Rationale in
priority order: it is the only candidate where credentials are already present and correctly scoped
(§2.1-for-1); it is the only candidate that makes RFC 8887 push actually work (§2.1-for-2); it adds
no IPC and no preload surface; and its native-module packaging path is the one already exercised and
debugged in this repo.

A's one serious objection — the same process serves hosted multi-user deployments — is closed
structurally, not by convention. Three layers, all required:

1. **A desktop marker env var.** `main.ts`'s spawn env (`:85-94`) gains
   `VNCMAIL_DESKTOP_STORE_DIR=<app.getPath('userData')>/offline`. Absent or empty ⇒ the engine
   module is never constructed, and this also supplies §2.1-against-3's path. One variable does both
   jobs, so they cannot drift apart.
2. **Every new route refuses to run without it.** `app/api/offline/**` returns `404` (not 403 —
   nothing should learn the routes exist) when the marker is unset. This mirrors the existing
   "routes 503-on-misconfig" habit elsewhere in the program.
3. **A single-user assertion.** With the marker set, the engine asserts at open time that the store
   directory is per-OS-user (it is, being under `userData`) and records the resolved
   `serverUrl`+`username` of every account it materialises. A store whose recorded account set
   doesn't match the requesting session's is a purge trigger (§5.5), not a merge.

Additionally, and non-negotiably: **the engine runs on a `worker_threads` Worker inside the server
process, never on the request event loop.** Synchronous SQLite plus JMAP page application is exactly
the workload that turns a shared event loop into a latency problem, and M's own I11 (jobs strictly
sequential within an account, M§3.4) is naturally expressed as "one worker per account, one job at a
time" rather than as a hand-rolled mutex. API routes talk to the worker via `postMessage` and never
touch the database handle. This also localises §2.1-against-4: the worker gets an explicit
`terminate` path.

Rejected explicitly, for the record: B, because it can only be built by moving credentials into a
process that today holds none — the change `client.ts` already declined on its merits, and nothing
about an offline store makes that trade better. C, because it needs a product-wide CSP widening and
its only encrypted backends are unvetted.

### 2.5 Consequence for the already-working WS-push renderer code

This is the question that must not be answered by accident.

**The renderer's push pipeline stays exactly as it is. No line of `lib/jmap/client.ts` changes for
v1.** Its SSE/polling path is the renderer's own liveness for the *visible* list, it is working, and
it is what feeds `newEmailNotification` → `showElectronNotification` (§1.3). The engine does not
replace it and does not read from it.

Three rules, in order of how easy they are to get wrong:

1. **The engine gets its own push connection, and it is the header-capable one.** In the server
   process the engine opens `wss://…/jmap/ws` with an `Authorization` header (via `ws`), which is the
   connection the renderer cannot open (§1.3). It subscribes with `WebSocketPushEnable`, and treats
   the resulting `StateChange` exactly as M§10.4 specifies: **a wake signal, never a cursor.** M's
   two load-bearing rules (a pushed `newState` is never written as a cursor; state-equality against
   our cursor is a cheap safe dedupe) apply verbatim. *[reused: M§10.4]*
2. **The engine never fires a notification.** `newEmailNotification` remains the single source of
   the OS notification, in the renderer, via the existing bridge. An engine-side notification would
   double-notify on the common path (both connections see the same delivery) and diverge on the
   uncommon one. What the engine *may* do is expose "account X changed" on its status channel; the
   renderer decides whether to refresh, exactly as M§5.7 specifies the engine→email-store direction
   (and only that direction).
3. **Duplicate work is bounded and acceptable; duplicate *state* is not.** Yes, two connections to
   the same server per account, and both wake on the same delivery. That is deliberate, and it is
   M§5.7's argument transplanted: the renderer's list cursor and the engine's `/changes` cursors
   page differently, invalidate differently, and *one being wrong must not corrupt the other*. The
   engine must never read the renderer's `lastStates` (`client.ts:571`) and the renderer must never
   read the engine's cursors. The cost is one extra socket per account; the cap is
   `MAX_ACCOUNTS_HTTP1 = 5` today, and the engine's socket is server→server, so it does not consume
   the browser's per-origin HTTP/1.1 connection budget that cap exists to protect.

**Deferred, and worth stating so it isn't done silently:** once the engine's WS connection is proven,
the renderer's transport could be retired in favour of the engine pushing "account changed" down an
SSE/`EventSource` from the local server — one server-side socket per account instead of two, and the
renderer's circuit-breaker-into-SSE path becomes dead code. That is a *follow-up*, gated on the
engine's connection being verified against real Stalwart, not part of v1. Doing it in v1 would make
a working notification path depend on an unproven one.

---

## 3. Decision 2 — SQLite binding, and SQLCipher on day one **[new]**

M deferred `useSQLCipher` for one specific reason: Expo Go cannot load it, so a plaintext-first phase
was the only way to keep the day-to-day dev workflow (M§9.2, M§14.3 step 3.1, MR S4/V4). **Electron
has no Expo Go.** The deferral's entire justification is absent, so the question is genuinely open
here and has to be answered on the evidence.

### 3.1 What was measured, and how

All of the following was executed against the Electron binary this repo pins
(`electron@43.2.0`, resolved from `~/worktrees/vncmail-electron/node_modules` — this worktree has no
`node_modules` installed), both as the main process and under `ELECTRON_RUN_AS_NODE=1` (the mode the
standalone server actually runs in, `main.ts:88`):

| Fact | Result | How |
|---|---|---|
| Electron 43.2.0's bundled Node | **24.18.0**, ABI `modules=148`, `napi=10` | `process.versions` |
| Its bundled SQLite | **3.53.1, with `ENABLE_FTS5`** | `pragma compile_options` |
| `node:sqlite` present and working | yes; exports `DatabaseSync, StatementSync, Session, constants, backup`; no `ExperimentalWarning` observed | `require('node:sqlite')` |
| `node:sqlite` encryption | **none.** `compile_options` has no codec. `PRAGMA key='…'` is **silently accepted and does nothing** — the file was written with a `SQLite format 3` header and a plaintext canary string recoverable with `grep` | wrote a real file, read the bytes back |
| `node:sqlite` stability (Node 24) | **1.2 — Release Candidate** (RC since v24.15.0; no longer behind `--experimental-sqlite`), not stability-2 stable | Node 24 docs |
| `better-sqlite3@13.0.2` | installs with **zero build step**; ships in-tarball N-API prebuilds for `darwin-{arm64,x64}`, `linux-{arm64,x64}`, `linuxmusl-{arm64,x64}`, `win32-{arm64,x64}`; **loads in Electron 43 both as main process and under `ELECTRON_RUN_AS_NODE`**; FTS5 available; `PRAGMA key` silently a no-op | `npm install` + load in Electron |
| `better-sqlite3` 12.x vs 13.x | 12.x used `install: prebuild-install \|\| node-gyp rebuild` (per-ABI downloads). **13.0.0 dropped that** for `gypfile: false` + in-tarball prebuilds — i.e. moved to ABI-stable N-API. This is why no `electron-rebuild` is needed | npm metadata for 13.0.2 vs 12.11.1 |
| `better-sqlite3-multiple-ciphers` | latest is **12.11.1** (2026-06-18) — on the *old* 12.x prebuild-install model. Its GitHub release carries 98 Electron prebuilds, ABIs **121…146**. **Electron 43 needs ABI 148 — absent.** So it would fall through to `node-gyp rebuild`: a C++ toolchain + Python + Electron headers on every contributor machine and every CI runner | npm metadata + GitHub releases API |
| **`@signalapp/sqlcipher@4.0.3`** | **N-API** (`prebuildify --strip --napi`, `node-gyp-build`), in-tarball prebuilds for `darwin-{arm64,x64}`, `linux-{arm64,x64}`, `win32-{arm64,x64}`. **Loads in Electron 43 with no rebuild, both process modes.** Real **SQLCipher 4.10.0 community**; `PRAGMA cipher_version` reports it; **file header is ciphertext, canary absent from the bytes, wrong key rejected with `SQLITE_NOTADB`, right key reads the row back**; FTS5 available; `better-sqlite3`-shaped synchronous API (`db.exec`, `db.prepare().run()/all()`, `db.pragma()`); **AGPL-3.0-only**, matching this repo's own licence | `npm install` + full round-trip in Electron main process |

Two of those deserve to be called out as landmines rather than table rows:

- **`PRAGMA key` failing silently is the worst possible ergonomics.** On both `node:sqlite` and plain
  `better-sqlite3`, setting a key "works", the database works, and the mail is on disk in cleartext.
  There is no error to notice. Whatever binding ships, the store's open path must **assert
  encryption positively** — read `PRAGMA cipher_version` and refuse to proceed if it is empty — and
  a test must assert a canary string is *absent* from the raw file bytes. Both are in §11.
- **`better-sqlite3-multiple-ciphers` lags Electron by roughly one to two majors** (ABI 146 vs 148,
  and its 12.x base trails better-sqlite3's 13.x). That lag is structural, not a one-off: with
  per-ABI prebuilds, every Electron major bump re-opens the question. `@signalapp/sqlcipher`'s N-API
  prebuilds are immune to Electron majors by construction.

### 3.2 Recommendation: **ship SQLCipher on day one, via `@signalapp/sqlcipher`**

The friction the mobile design was avoiding **does not exist here**, and the evidence is unusually
clean: a package built and maintained specifically to run SQLCipher inside an Electron desktop
application, N-API so it needs no rebuild against Electron 43, prebuilds covering every platform this
repo actually packages (§1.1: darwin x64+arm64, win32 x64, linux x64 — all present), FTS5 already
compiled in, an API close enough to `better-sqlite3` that the backend is the same code either way,
and a licence identical to this repo's.

Verified working, in this environment, against this Electron version. Not inferred.

So: **no plaintext-first phase.** M§14.3's plain-then-encrypted staging exists to protect a dev
workflow that has no analogue here; importing it would mean deliberately shipping a plaintext
mailbox on disk for a phase, plus building and then discarding the store-format-migration machinery
of M§8.4.1 to get out of it. Both costs, no benefit.

Corollaries:

- The abstraction boundary of M§9.1 (`SyncStore` / `SyncTxn`) stays **exactly** as M specifies. It is
  what makes this reversible: if `@signalapp/sqlcipher` ever becomes untenable, `store-sqlite.ts` is
  the only file that changes. Do not skip it on the grounds that the binding question is now settled
  — the boundary is also what makes `store-memory.ts` possible, and M§13's contract-tests-against-two-backends
  is most of the test plan's value.
- M§8.4.1's **out-of-band store-format marker is still required**, for the reason V4 gives, minus one:
  `schemaVersion` still lives inside a file that a future format change could make unreadable, so it
  must be mirrored outside. What day-one encryption removes is only the *plain→cipher* transition
  that would otherwise be the marker's first customer. Keep the marker; it costs a JSON file.
- `node:sqlite` is **rejected**, on two independent grounds: no encryption at any price, and
  stability 1.2 (RC) for a component holding the user's mail. Its one advantage — zero dependencies —
  is worth nothing once encryption is a requirement. Worth re-evaluating only if a decision is ever
  taken to ship unencrypted.
- Plain `better-sqlite3@13.0.2` is the **fallback**, not the plan: adopt it only if Stage A (§12)
  finds `@signalapp/sqlcipher` cannot be packaged, and in that case the human decides between
  "unencrypted desktop store" and "no desktop store yet" (§13, open question 1).

### 3.3 Friction that does exist, stated plainly

Not zero, just small — and the human should see it rather than have it smoothed over:

1. **A ~6 MB native dependency with 6 platform prebuilds in the tarball**, entering
   `dependencies`. Install size and `npm ci` time grow for everyone, including web-only contributors
   who will never run Electron.
2. **NFT tracing is the real unknown** (§2.1-against-2). `serverExternalPackages` plus a verification
   that `prebuilds/**/*.node` reached `.next/standalone/node_modules` is Stage A's first task. If NFT
   won't carry it, the fallback is a copy step in `scripts/assemble-standalone.mjs` — which already
   exists precisely to patch up what standalone output omits, so this is a known-shaped fix.
3. **CI runs Node 22 while Electron bundles Node 24** (§1.1). Harmless for an N-API module (the
   prebuild is selected by platform+arch, not ABI) — but it *would* have been fatal for a
   per-ABI package, which is worth recording as another reason the N-API choice matters. Any
   `npm test` that loads the binding under the runner's own Node exercises a different Node than
   production; the binding load assertion must therefore run **inside Electron** (`npm run
   test:electron`), not only in vitest.
4. **Cross-arch macOS packaging.** CI builds both x64 and arm64 dmg/zip on one macOS runner.
   In-tarball prebuilds ship *all* platforms, so this works — whereas `prebuild-install` downloads
   only the host's, and would have broken the cross-arch target. Verify in Stage A that the arm64
   `.node` is what ends up in the arm64 build and vice versa.
5. **`linuxmusl` is not covered** by `@signalapp/sqlcipher` (`better-sqlite3` does cover it). Irrelevant
   for AppImage/deb (glibc); relevant if an Alpine-based container ever wants the engine — which,
   per §2.4, it must not.

### 3.4 What is *not* an input to this decision

FTS5 (VNCprodbuild step 9) is present in all three candidates — Electron's own bundled SQLite,
`better-sqlite3` 13, and `@signalapp/sqlcipher` (which additionally ships Signal's FTS5 segmenting
extension and an `initTokenizer()`). So the search step cannot be used to argue for a binding.
Recorded so a later session doesn't relitigate the choice on those grounds.

---

## 4. The sync engine itself — mostly M, verbatim

Everything in this section is **[reused]** unless marked otherwise. The M section cited is
normative; what follows is a map, not a restatement, so that a reader can tell reuse from
re-derivation at a glance.

### 4.1 Architecture: three state machines *[reused: M§2, M§3.4 I11]*

Per account: **A** delta (`Mailbox/changes` then `Email/changes`, one cursor each), **B** coverage
(the envelope-window enumeration; `/changes` structurally cannot deliver pre-existing mail, so
coverage owns history and is also the bootstrap), **C1** body-queue drain, **C2** body backfill
(MR S9 — without it, widening body retention silently does nothing for already-covered envelopes).

Logically independent state, **operationally serialised** (I11). Here that serialisation is
structural rather than disciplinary: one worker thread per account, one job at a time (§2.4). M's
warning stands regardless — "run bodies in parallel, it's separate state" is forbidden, and F48
(a body landing for an envelope destroyed in the same cycle) is what it costs.

Module layout: M§2.2 verbatim, relocated. `src/sync/**` in the mobile repo becomes **`lib/sync/**`**
here (this repo has no `src/`): `engine.ts`, `cursor.ts`, `apply.ts`, `coverage.ts`, `bodies.ts`,
`retention.ts`, `errors.ts`, `states.ts`, `store.ts`, `store-sqlite.ts`, `store-memory.ts`. M's hard
requirement that `apply.ts` be **pure** (no network, no storage, no store access) carries over
unchanged and is the single highest-leverage constraint in the document: it is what turns M§11's
failure-mode table into a vitest suite. `overlay.ts` is **not** ported — see §5.4.

### 4.2 Two record tiers, two retention windows *[reused: M§2.1]*

Envelope tier = `EMAIL_LIST_PROPERTIES` — which in *this* repo is
`lib/jmap/client.ts:139-154`: `id, threadId, mailboxIds, keywords, size, receivedAt, from, to, cc,
subject, preview, hasAttachment, blobId`. (Note the extra `blobId`, present so list rows can serve
drag-out to the filesystem as `.eml`; it belongs in the envelope tier here.) Body tier =
`bodyStructure, textBody, htmlBody, bodyValues, attachments, bcc, replyTo, sentAt`.

Independent retention: `offlineEnvelopeDays` ≫ `offlineBodyDays`, MB cap on **bodies only**. The
human decision M records — widen envelopes well beyond bodies, so a message never falls out of the
offline *list* over a body-size cap — is a program-level decision and applies here identically.
Concrete numbers remain open (§13).

### 4.3 Cursors, provenance and invariants *[reused: M§3]*

Adopted without modification:

- `SyncCursor` / `CoverageState` / `BodyQueueEntry` / `AccountSyncState` as M§3.1 defines them,
  including per-cursor failure counters (MR S6), `sweepFloor` + `deferredTargetFrom` (MR S2), and
  `gapMarkers`.
- **Branded state types** (M§3.2): `ChangesState` vs `SnapshotState`, `advanceCursor(key, next:
  ChangesState)` as the delta path's only cursor write, `seedCursor(key, commitment:
  EnumerationCommitment)` for bootstrap/reconcile, and `EnumerationCommitment` made genuinely
  unforgeable by an **unexported real `Symbol()`** tag — including M's implementation note that
  `declare const … : unique symbol` emits no runtime value and throws `ReferenceError` as a computed
  key. That note came out of M's actual build; it would have been re-discovered here otherwise.
- The ordering rule as I2, not the false "only a changes state, ever" of M's revision 1.
- **I1–I13 in full** (M§3.4). Cursor-last; provenance-as-ordering; monotonic-or-invalidated; no
  silent write loss; idempotent application; account containment; deletion provenance; no
  clock-dependent cursors; bounded work; no wedge; sequential execution; field-level state writes;
  corrupt-state-blob ⇒ resync.
- The "not a cursor" list (M§3.3): `Email/query`'s `queryState`, a pushed `StateChange.newState`,
  `sessionState`, Thread state, `EmailDelivery`.

Two Electron notes on I12/I4, both simplifications rather than changes: with a real SQLite
`BEGIN…COMMIT` in place from day one (§3.2), M's per-account mutex and read-merge-write discipline
become belt-and-braces rather than load-bearing (M§9.2 anticipated exactly this), and I4's "every
write either succeeds or raises" is the binding's default behaviour rather than something to enforce
against a fire-and-forget storage API.

Cursor keying: `(LocalAccountId, JmapAccountId, CursorType)`, all three required, for M§3.1's
reasons. `LocalAccountId` is this repo's `AccountEntry.id` (§1.4) — **never `cookieSlot`**.

### 4.4 Bootstrap *[reused: M§4]*

Replace-the-code-keep-the-shape does not apply (there is no `runOfflineSync` here), but M§4.1's
**mandatory order** does, and it is the one thing in this document most likely to be "optimised" into
a permanent data hole:

1. Capture both cursors **first**, in one JMAP request (`Mailbox/get {ids: []}` + `Email/get
   {ids: []}`), and `seedCursor` them inside one `EnumerationCommitment` that in the same transaction
   writes `coverage {phase:'scanning', targetFrom, sweepFloor: targetFrom}`.
2. Full `Mailbox/get` → upsert every mailbox row.
3. The seeded cursors are **live from here**: each cycle runs A1, A2, then B.
4. Scan reaches `targetFrom` ⇒ `coveredFrom = sweepFloor`, `phase = 'complete'`. Bootstrap has no
   delete sweep; only reconcile sweeps.

The cursor is deliberately *older* than the data, so the first delta cycle re-delivers some changes
we already have. That is I5 working. The cheaper opposite order silently loses mail.

### 4.5 Change application *[reused: M§5.1–§5.5]*

Order within a cycle (A1 → A2 → B → C1 → C2) and M's timeline argument for why delta-before-coverage
is safe *given I11* — the resurrection hazard, and the fact that the unsafe configuration is
concurrency, not ordering. `Mailbox/changes` `updatedProperties` count-only optimisation (RFC 8621
§2.2). `Email/changes`: `created` ⇒ envelope fetch + conditional body enqueue; `updated` **present**
⇒ a **3-property** `Email/get {id, keywords, mailboxIds}` and never a body (RFC 8621 §4.1 — the two
mutable properties); `updated` **absent** ⇒ unconditional no-op with the ids filtered out *before*
the fetch is issued (MR S16); `destroyed` ⇒ delete envelope + body + membership + queue row.
Create-then-update-then-destroy ordering within a page. `notFound` is normal, not an error.
Mailbox/Email transient inconsistency tolerated, never repaired (I7: no deletion by inference).

### 4.6 Pagination *[reused: M§6]*

Ascending keyset walk on `receivedAt` with `calculateTotal: false`; `after` is **spec-inclusive**
(RFC 8621 §4.4.1, MR S14) so boundary re-delivery is normal and deduped by id; forward progress needs
strictly-greater `max(receivedAt)`; the no-progress guard is `anchor`/`anchorOffset` first and only
then, on `anchorNotFound`, a +1 ms advance with a `WARN` and a durable gap marker. Position-based
paging is rejected for M§6.2's reason. Budgets per M§6.4 — but see §8.2 for the desktop numbers.

### 4.7 Errors, retry, reconcile, anti-wedge *[reused: M§7]*

The seven-class taxonomy (Transport / RateLimit / ServerTransient / RequestLimit / Auth / Fatal /
StateInvalid), with **exactly one class moving a cursor** and unrecognised method errors defaulting
to ServerTransient. Full-jitter backoff. "Offline is not an error." Partial-failure semantics inside
a page (records may commit partially; the cursor may not). The eight cursor-advance rules of M§7.5.
`cannotCalculateChanges` handled as RFC-mandated **without blanking the UI**, with the **pinned
`sweepFloor`** (MR S2) and with the freshly-seeded cursor **live immediately** so a wide-window
rebuild doesn't stall incoming mail (MR S9). `oldState` mismatch re-issued once before escalating,
plus the ≤4-reconciles-per-24 h ceiling (MR S10). The monotonically **shrinking** `maxChanges` ladder
with every rung clamped to rung 0 (MR S7 + V2), and per-cursor counters with "any job failed ⇒ cycle
failed for escalation purposes" (MR S6).

One Electron-specific input: this repo's client already has `RateLimitError` with `Retry-After`
parsing (`client.ts:54-62`, `authenticatedFetch`'s 429 branch) and a client-wide rate-limit gate.
The engine's own JMAP layer (§10.1) must reproduce that behaviour rather than inherit it, since it
will not be using `JMAPClient`.

---

## 5. Multi-account isolation, account identity, lifecycle **[adapted]**

Requirement confirmed at program level (manual §4: an offline cache must isolate per account,
including per-account keys). M§8 is the design; what changes is the identity plumbing, because this
repo's account model differs (§1.4).

### 5.1 Namespacing *[reused: M§8.1, adapted paths]*

```
<VNCMAIL_DESKTOP_STORE_DIR>/
  registry.json                       # ONLY: account ids present, purge tombstones,
                                      # monotonic epochs, store-format markers (M§8.4.1)
  accounts/<sha256(accountId)>.db     # one SQLCipher file per account: mailbox, envelope,
                                      # email_mailbox, body, body_queue, sync_state
```

Filenames are hashed, not `username@host`, so the directory listing is not a plaintext account
inventory on disk. **No cursor, coverage row, record or resync flag lives outside an account's own
file** — M§8.1's forward-compatibility requirement, which here is load-bearing on day one rather than
later, because §5.5's purge deletes the key and the file together and a cursor surviving that would
be advanced against a freshly-empty store.

`registry.json` is deliberately **plaintext** and M§8.1's accepted-limitation argument transfers with
one improvement: mobile's justification was that `account-store` already persists usernames to plain
AsyncStorage. Here, `account-store.ts`'s `persist` (`:219-227`, name `account-registry`) already puts
`username` and `email` for every account into renderer `localStorage`, so the registry adds no new
exposure — and hashing the filenames means the registry is the *only* place the account list appears
in the store directory. It must be readable before any key exists (that is the whole point of
M§8.4.1's format marker), so it cannot itself be encrypted.

`epoch` lives in the registry, outside the per-account namespace, because it must be monotonic
**across** a purge (M§8.3). Owner: `SyncStoreFactory`. Not writable from `SyncTxn`; a transaction
reads it to validate itself and rejects with `EpochMismatchError`.

### 5.2 JMAP-level accounts within one login *[reused: M§8.2, M§9.3]*

Cursors and **every SQL primary key** carry `jmap_account_id` (MR S3: JMAP ids are unique only within
an account). v1 syncs the **primary mail account only**; delegated/shared accounts stay online-only,
as they effectively are today.

Note this repo already carries the same evidence mobile did: `client.ts:388`'s
`namespaceMailboxIds()` prefixes ids when returning emails for a non-active account (five call sites:
`:632`, `:1271`, `:2138`, `:2185`, `:2323`). Same collision, same workaround, same conclusion —
account-scoped keys from day one.

### 5.3 Slot → account resolution **[new]**

The one piece of identity plumbing with no mobile counterpart, and the one most likely to produce a
cross-account write.

`app/api/offline/**` routes are addressed the way every other authenticated route here is: by
`?slot=N` (§1.2). The resolution rule, in order, all steps required:

1. Read `jmap_session[_slot]`; `decryptSession` ⇒ `{serverUrl, username}`.
2. `accountId = generateAccountId(username, serverUrl)` — the *server-confirmed* username from the
   cookie, not a client-supplied one.
3. Open the store for `accountId`. **Never** derive a path from `slot`. Slots are recycled by
   `getNextCookieSlot()`, so a stale slot number pointing at a re-added different account is an
   ordinary occurrence, not an edge case.
4. Cross-check against the JMAP session's own `username` (`client.ts:3822`'s `getSessionUsername()`,
   which exists precisely because a short login name may be canonicalized server-side — and which
   `AccountEntry.serverIdentifiers` was added to handle). A mismatch is a **hard error**, not a
   best-effort match.
5. Every commit re-validates `(accountId, epoch)` (I6), and every network call re-verifies that the
   engine's JMAP session still serves that account — **not only at cycle start**, because a cycle is
   long-lived. This is M§8.3's generalisation of `jmapClientServesActiveAccount`, and it is what
   makes M's D6 (persisted cross-account contamination) unreachable here rather than merely unlikely.

### 5.4 Local mutations: not applicable in v1, and why that is a decision **[new]**

M§5.6 makes the outbox the sole durable record of local intent and composes it into reads via a pure
`overlay.ts`; M§5.6.1 then requires fixing the outbox's fire-and-forget persistence, because that
promotion made its durability load-bearing (V1).

**None of that machinery exists here** (§1.6): no outbox, no optimistic mutation queue, no
`patchCache()`. Mutations go straight to the server and fail when offline.

**Decision: v1 desktop offline is read-only.** The durable store holds server-derived state only —
which is M§5.6's core property, reached by having no write path at all rather than by removing one.
Consequences, stated so they are chosen rather than discovered:

- Marking a message read while offline does not work at all (rather than working locally and
  syncing later). That is today's behaviour; the engine does not regress it.
- M§5.6.2's two explicit non-coverages (unread badge counts read a server-maintained
  `Mailbox.unreadEmails` scalar and cannot be overlaid; SQL/FTS predicates see server truth) are
  moot in v1 and become live the moment an outbox is added.
- **When offline mutations are added later, M§5.6 and §5.6.1 are the design** — including the
  durability requirement. Do not invent a write-through into `envelope`/`body`; that is the failure
  mode M removed rather than guarded (MR S11).

### 5.5 Logout, account removal, disable, purge *[reused: M§8.4]*

```
purgeAccount(accountId, reason: 'logout' | 'removed' | 'feature-disabled' | 'store-format-change'):
  1. registry: { accountId, purgePending: true }        # durable intent, crash-safe
  2. epoch++                                            # in-flight commits now rejected
  3. delete the SQLCipher key from safeStorage-protected key file   -- FIRST
  4. delete accounts/<hash>.db (+ -wal, -shm)
  5. registry: remove the entry, KEEP the epoch
```

Ordering 3-before-4 is the security property: an interrupted purge must leave **unreadable** data.
Crash between 1 and 5 ⇒ the next launch completes the purge **before any cycle starts**. Triggers:
`account-store.removeAccount`, logout (single or all), the offline-cache setting being turned off
(MR S13 — purge, with a confirming Settings copy, since re-enabling costs a full bootstrap), and a
store-format/schema marker mismatch (M§8.4.1). **`AuthenticationError` during a cycle is not a purge
signal** — a server hiccup returning 401 must never delete a user's offline mail.

**Lazy materialisation** (M§9.5, MR S13) is if anything more important here than on mobile: read
paths check the setting for that account **before** calling `open()`, and `open()` on a
non-materialised account returns an empty read-only store and creates **no file and no key**. A user
who never enables offline mail must not end up with an encrypted database and a keychain entry
materialised by a read path.

---

## 6. Where the encryption key lives **[new]**

Mobile used `expo-secure-store` (OS-keychain backed). Electron's equivalent is `safeStorage`.

### 6.1 What `safeStorage` actually is, verified

Measured in Electron 43.2.0 on macOS (main process, after `app.whenReady()`):
`isEncryptionAvailable() === true`, `encryptString`/`decryptString` round-trip correct, ciphertext
prefixed `v10` (Chromium's OSCrypt format). No Keychain prompt appeared.

Per Electron's documented behaviour (`docs/latest/api/safe-storage`):

- macOS: Keychain-backed. "Access to the system Keychain is required and these calls can block the
  current thread to collect user input."
- Windows: DPAPI; requires the `ready` event.
- **Linux: `isEncryptionAvailable()` returns true even when no secret store exists**, in which case
  items are "encrypted via hardcoded plaintext password" and `getSelectedStorageBackend()` returns
  **`basic_text`**. Real backends are `gnome_libsecret`, `kwallet` / `kwallet5` / `kwallet6`;
  `unknown` means it was called before `ready`. `setUsePlainTextEncryption()` forces an in-memory
  password on Linux and is a no-op elsewhere.

### 6.2 Decision: `safeStorage`, in the main process, with an explicit Linux gate

`safeStorage` (built in, no dependency) over `keytar` (unmaintained). Per-account key, generated
once as 32 random bytes, wrapped with `safeStorage.encryptString()` and written to
`<store dir>/keys/<sha256(accountId)>.bin`.

The awkward part, stated rather than hidden: **`safeStorage` is a main-process API, and §2.4 put the
engine in the server process.** Options, and the choice:

- ~~Give the server process its own key wrapping (e.g. a file with 0600 perms)~~ — rejected: that is
  a key protected by nothing but filesystem permissions, i.e. materially weaker than the OS keychain
  the rest of the desktop ecosystem uses, and it silently discards the one thing `safeStorage` buys.
- **Chosen: the key crosses the existing IPC bridge, in one direction, once per account per app
  launch.** `main.ts` gains a single `ipcMain.handle("vnc:offline-key", …)`-shaped path that unwraps
  the per-account key and hands it to the **server process** — *not* to the renderer. Mechanically
  this means main.ts fetches/creates+wraps the key and passes it to the standalone server over a
  small local channel established at spawn time (a `stdio` extra fd, or a one-shot loopback request
  authenticated by a nonce also passed in the spawn env). The renderer is never in the path and
  `window.vnc` gains nothing.

This is a real cost of choosing A over B — B would have had the key and the database in the same
process — and it is the one place where B is genuinely simpler. It is outweighed by §2.1-for-1/2:
moving the *engine* to main to co-locate the key would drag the *credentials* there too, which is a
much larger secret-handling change (§2.2-against-1). Moving 32 bytes once per launch is the smaller
of the two.

**Mechanism is deliberately left open** as an implementation choice between the extra-fd and
nonce-authenticated-loopback variants; both are small, and Stage A should pick whichever proves
cleaner against the packaged build. What is *not* open: the renderer must never see the key, and the
key must never be written unwrapped.

### 6.3 Caveats to design around, not discover

1. **Linux `basic_text` is the important one.** On a Linux desktop with no keyring daemon — an
   AppImage on a minimal WM, a container, a headless CI box — `isEncryptionAvailable()` returns
   **true** while the key is protected by a hardcoded password that is public knowledge. That is
   *worse than an honest failure*, because it looks like it worked. **Rule: at key-creation time,
   `getSelectedStorageBackend()` must be consulted, and `basic_text` must not silently proceed.**
   Recommended behaviour: refuse to materialise a store, surface "offline mail can't be stored
   securely on this system (no OS keyring available)", and offer an explicit opt-in that records the
   downgrade. The decision on whether that opt-in exists at all is a human one (§13, open question
   2).
2. **`ready` ordering.** `safeStorage` must not be touched before `app.whenReady()`, and
   `getSelectedStorageBackend()` returns `unknown` if it is. `main.ts:205-208` already does its work
   inside `whenReady().then(...)`, so the key path must sit there — and, since the server spawn
   happens inside `createMainWindow()`, the key must be resolved **before or as part of** the spawn.
3. **macOS Keychain vs. unsigned builds — flagged, not resolved.** Keychain ACLs are tied to app
   identity. Builds are currently **unsigned** (`electron-builder.config.js`, `hardenedRuntime:
   false`, VNCprodbuild step 9 open). Whether an ad-hoc-signed Electron app retains Keychain access
   across an `electron-updater` upgrade, or prompts, or silently loses the item — **could not be
   verified in this environment** and is not documented by Electron either way. The failure mode if
   it does lose access is not data loss but "offline mail must re-bootstrap after every update",
   which §5.5's purge-on-unreadable path handles gracefully. **Stage A must test this on a real
   packaged build across a simulated update.** It is also an argument for step 9 (signing) being a
   soft prerequisite for shipping the encrypted store to users, not merely a nice-to-have.
4. **A lost key is a purge, never a prompt.** If the wrapped key cannot be unwrapped, or the database
   opens but `PRAGMA cipher_version` is empty, or the key fails (`SQLITE_NOTADB`), the response is
   `purgeAccount(..., 'store-format-change')` and a fresh bootstrap. Never a "enter your password to
   recover" flow — the key was never derived from a user secret, so there is nothing to enter.

---

## 7. Storage interface and schema

### 7.1 Interface *[reused: M§9.1]*

`SyncStore` / `SyncTxn` / `SyncStoreFactory` exactly as M§9.1 defines them, including:
field-level state patches only and **no whole-struct `AccountSyncState` write** (I12, MR S1);
`advanceCursor(key, next: ChangesState)` and `seedCursor(key, commitment)`;
`putBodyIfEnvelopeExists` (F48); `enqueueBodies` insert-or-ignore that **never resets `attempts`**
(MR S12, F41); `listBodiesForEviction` reading `body.received_at` from the body table alone;
`listOrphanBodies`; `clearRecords()` clearing records **and the body queue** while *not* nulling
cursors; `loadAccountState()` throwing `CorruptStateError` so the caller applies I13; the
`StoreFormatMarker` read/write pair and `completePendingPurges()` running once at launch before any
cycle.

The engine imports `SyncStore` and nothing else about persistence — no SQL, no binding import, no
path strings outside `store*.ts`. Two backends: `store-sqlite.ts` (`@signalapp/sqlcipher`) and
`store-memory.ts` (unit tests, and the second implementation that proves the boundary).

**One Electron addition:** `SyncStoreFactory.open()` must assert encryption positively —
`PRAGMA cipher_version` non-empty — and throw otherwise. §3.1's silent-`PRAGMA key` landmine makes
this the difference between an encrypted store and a plaintext one.

### 7.2 Backend notes *[adapted: M§9.2]*

M§9.2's staging question (AsyncStorage vs `expo-sqlite`, plain vs cipher) is **closed here by §3.2**:
one backend, encrypted, from the first commit. M's contingency section does not apply — there is no
key-value fallback worth building in a process that has a filesystem.

Concrete choices for this binding:

- `PRAGMA journal_mode = WAL` and `synchronous = NORMAL`. WAL means the `-wal`/`-shm` siblings must
  be included in every delete path (§5.5 step 4) — a classic leak.
- `PRAGMA key` is set as the **first statement after open**, before any other statement, then
  `cipher_version` is asserted (§7.1).
- Synchronous API on a worker thread (§2.4), so a long `BEGIN…COMMIT` cannot stall an HTTP response.
- `transaction()` is a real `BEGIN…COMMIT`, so **cursor-last (I1) is enforced by the database** rather
  than by write ordering — the payoff M§9.2 predicted for shipping SQLite before the engine.
- `void setItem(...).catch(warn)` and `try { … } catch { /* ignore */ }` around a store write are
  **banned** in the sync path (I4; §1.6's note about `file-store.ts`).

### 7.3 Schema *[reused: M§9.3]*

M§9.3 verbatim: `mailbox`, `envelope`, `email_mailbox`, `body`, `body_queue`, `sync_state`, all
primary keys `(jmap_account_id, id)` per MR S3; `envelope_received` and `envelope_nobody` indexes
(the latter being job C2's driver); `email_mailbox_by_mailbox`; `body.received_at` present so
eviction is a single-table ordered scan (MR S12); **deliberately no foreign keys and no cascades**
(M§5.5's transient inconsistency is normal; a cascade on mailbox delete would delete mail, violating
I7).

One field to add for this repo: `envelope.blob_id`, since `blobId` is in this codebase's
`EMAIL_LIST_PROPERTIES` (§4.2).

`sync_state` living in the same file as the records is what makes §5.5's atomic wipe work.

### 7.4 What the renderer reads, and how

New routes under `app/api/offline/`, all gated per §2.4 and resolved per §5.3:

| Route | Backs |
|---|---|
| `GET /api/offline/emails?slot&mailboxId&limit&before` | the offline mailbox list (indexed `queryEnvelopes`) |
| `GET /api/offline/email/:id?slot` | a single cached message incl. body |
| `GET /api/offline/status?slot` | phase/progress/coverage/error for the UI |
| `POST /api/offline/sync?slot` | user-initiated "sync now" (coalesces, never aborts — M§10.3, D7) |
| `DELETE /api/offline/store?slot` | clear cache / purge (§5.5) |

The engine→UI direction only, per M§5.7: the engine notifies "account X changed"; the renderer
decides whether to refresh. The engine never reads renderer state.

### 7.5 Reserved hooks *[reused: M§9.4]*

FTS5 (step 9) hangs off `upsertEnvelopes` / `putBodyIfEnvelopeExists` as the only write paths for
indexable content — no engine change. §3.4: FTS5 is compiled in. Attachment blobs are out of scope;
when added, their deletion belongs in `deleteEmails` and `purge` so they cannot leak past an account
wipe.

---

## 8. Triggering **[adapted: M§10]**

The trigger *model* is M's; the trigger *set* is not, because a desktop app has different lifecycle
events than a mobile one (no `AppState` backgrounding, no OS-governed background budget, but real
window minimise/hide, system sleep/wake, and a process that can outlive its window on macOS).

### 8.1 Triggers

| # | Trigger | Jobs | Throttle | vs. M |
|---|---|---|---|---|
| T1 | Server process ready + an account's credentials resolvable | A, B, C | 2 s delay | M T1 |
| T2 | Window shown / focused (`BrowserWindow` `focus`, via a small IPC ping) | A, C | min 30 s since last cycle | M T2 (`AppState` → active) |
| T3 | User "sync now" (`POST /api/offline/sync`) | A, B, C | none; **coalesces into a running cycle, never aborts it** | M T3, closes M's D7 |
| T4 | Network regained | A, C | 3 s debounce + per-account jitter | M T4 |
| T5 | `StateChange` on the **engine's own** WS/SSE connection (§2.5) | A, C | 2 s debounce + M§10.4's state-equality check | M T5 |
| T6 | Retention setting changed | B (envelope widen), C2 (body widen), eviction only (narrow) | none | M T6 |
| T9 | **Unfinished work:** previous cycle `partial`, or any cursor `drainPending`, or `coverage.phase ∈ {scanning, reconciling}`, or a non-empty body queue | the unfinished job(s) | 5 s, subject to §8.3's chaining rule | M T9 (MR S8) |
| T10 | Offline caching disabled for an account | abort + purge (§5.5) | none | M T10 (MR S13) |
| **T11** | **System resume from sleep** (`powerMonitor` `resume`), and `unlock-screen` | A, C | 5 s debounce; treat as network-uncertain, so T4's logic applies | **[new]** — no mobile analogue; a laptop lid closed for a day is the single most common way a desktop cursor gets far behind |
| **T12** | **App quit requested** | none — *cooperative stop* | n/a | **[new]** — see §8.4 |

Explicitly **not** triggers: a periodic timer; opening a mailbox; opening a message; scrolling. The
engine must never be on the critical path of a UI interaction (M§10.2) — if it is, its budgets and
backoff become user-visible latency.

M's T8 (OS background refresh) has no counterpart: on desktop the process simply keeps running, so
`partial`+T9 covers it.

### 8.2 Budgets **[adapted: M§6.4]**

M's foreground/background split is replaced by a **window-visible / window-hidden** split. A hidden
window on a plugged-in laptop is not the constrained environment a backgrounded phone is, so the
hidden column is *lower for politeness to the server and the user's battery*, not because an OS will
kill us:

| Bound | Window visible | Window hidden / minimised |
|---|---|---|
| Pages per cycle, per cursor | 40 | 20 |
| Wall clock per cycle | 90 s soft deadline, checked between pages | 60 s |
| Body queue items per cycle (C1+C2) | 200 | 100 |
| Coverage pages per cycle | 25 | 15 |

Exceeding a budget is a **normal** outcome (`partial`), not an error (M§6.4): the cursor stands at
the last committed page, `drainPending` stays true, T9 resumes. This is also the answer to a server
whose `hasMoreChanges` never goes false (F14).

### 8.3 Single-flight, coalescing, chaining *[reused: M§10.3]*

Per `LocalAccountId`: a second trigger during a cycle sets `wakePending` and awaits the same promise
— it never aborts (M's D7). Chained cycles continue **only while `madeProgress` is true**, so fixing
M's stall (MR S8) does not create a hot loop.

Abort triggers here: logout/purge, offline caching disabled (T10), the account being removed, network
loss, a budget deadline, **and app quit (T12)**. All leave a committed cursor and resumable state.

**Cross-account:** M is limited to the active account because `jmapClient` is a renderer singleton.
That constraint does **not** exist here — the engine constructs its own per-account JMAP layer from
per-slot credentials (§5.3), so it can sync **all logged-in accounts**, active or not, with a worker
per account. This is a genuine capability gain from choosing A, and one of the few places this design
is *more* capable than M. It is also a new load consideration: up to 5 accounts × (1 WS + delta
traffic) against one Stalwart. M§7.2's jitter is what keeps T4/T11 from producing a synchronised
stampede, and it becomes more important here than there.

### 8.4 Process lifetime **[new]**

M§10.5's headless-callability constraint holds trivially — the engine has no React, no store, no
component dependency by construction (§2.4). Two Electron-specific rules:

- **`main.ts` currently kills the server on `window-all-closed` and `before-quit` (`:210-219`) with
  `serverProcess.kill()`** — SIGTERM, no coordination. A cycle dies mid-page. That is *safe* (I1: the
  cursor is the last fully-applied page; cost is one page's refetch) but wasteful, and it is worth
  T12: a `before-quit` that asks the engine to stop at the next page boundary, with a short timeout
  before falling through to the existing kill. Small change, and it must not be allowed to delay quit
  perceptibly.
- **macOS keeps the app alive with no windows.** `window-all-closed` does not `app.quit()` on darwin
  (`:210-215`) yet *does* stop the server. So on macOS today, closing the window stops sync and
  reopening restarts it. Acceptable for v1; worth revisiting if "sync while closed" is ever wanted,
  because that is the only configuration where a desktop mail client can usefully sync with no UI.

---

## 9. Failure modes **[reused + new rows]**

**M§11's table (F1–F49) applies in full and is not reproduced here.** Every row is a JMAP-protocol or
engine-state scenario, and none of them changes because the host process changed. The ones most worth
re-reading before implementing: F1 (kill mid-drain), F3 (kill mid-bootstrap), F4 (kill mid-purge),
F9 (`cannotCalculateChanges`), F26 (`updated` for an id we don't hold), F37 (concurrent write vs.
commit), F38 (retention widened during reconcile — M's worst potential data-loss bug), F44
(clock jump), F47 (one cursor healthy, one wedged), F48 (body for a destroyed envelope).

Electron-specific additions:

| # | Scenario | Rule |
|---|---|---|
| **E1** | Server child process SIGTERM'd on window close / quit (`main.ts:210-219`) | Same class as M's F1: the cursor is the last fully-applied page (I1), `drainPending` survives, T1+T9 resume on next launch. Cost ≤1 page. T12 (§8.4) reduces it to ~0 but is not required for correctness. |
| **E2** | Native module fails to load in the packaged build (NFT dropped `prebuilds/`, asar, wrong arch) | Engine never constructs; `/api/offline/**` returns 404 exactly as in a hosted deployment; the app is fully functional online-only. **Must never be a launch failure.** This is also why Stage A verifies packaging before any engine code exists. |
| **E3** | `safeStorage` reports `basic_text` (Linux, no keyring) | Do **not** materialise a store. Surface "offline mail can't be stored securely here". Optional recorded opt-in (§6.3.1, §13 q2). Never silently encrypt with the public hardcoded password. |
| **E4** | Wrapped key unwraps but the DB rejects it (`SQLITE_NOTADB`), or `cipher_version` is empty | `purgeAccount(..., 'store-format-change')` + fresh bootstrap. Never a user-facing recovery prompt (§6.3.4). |
| **E5** | Keychain item lost across an `electron-updater` upgrade of an unsigned build | Same as E4 — re-bootstrap, one full sync. Cost is bandwidth, not data. Verify empirically (§12 Stage A); it is an argument for code signing. |
| **E6** | The desktop marker env var is absent (hosted Docker deployment, or a dev `next dev` run) | Engine module never constructed; routes 404. **No SQLite file is created anywhere.** The single most important non-failure in the document (§2.4). |
| **E7** | Two app instances launched against the same `userData` | Second instance's SQLite open fails or blocks on the WAL lock. Handle by requesting Electron's single-instance lock (`app.requestSingleInstanceLock()`) in `main.ts` — **not currently requested**, and worth doing on its own merits regardless of this engine. |
| **E8** | Slot reused: account A removed, account B added into A's freed `cookieSlot` | §5.3's resolve-by-cookie-then-verify-against-session makes this a no-op: B's cookie yields B's `accountId`, so B's store opens. A's store is already gone via §5.5's `removeAccount` purge. This row exists because resolving *by slot* would have been the natural shortcut and would have merged two accounts' mail. |
| **E9** | Engine's WS connection succeeds while the renderer's fails (the expected steady state, §1.3) | Correct and intended. Renderer keeps SSE for its list; engine uses WS for its cursors; **neither reads the other's state** (§2.5 rule 3). No notification is fired by the engine (§2.5 rule 2). |
| **E10** | Both connections wake on the same delivery | Both do their own work; the renderer refreshes the visible list, the engine advances its cursors. Duplicate *fetches*, never duplicate *writes* — they own disjoint state (M§5.7). |
| **E11** | Engine and renderer both refresh an OAuth access token, and the server rotates refresh tokens (`app/api/auth/token/route.ts:104-106`) | **Real hazard.** Two independent refreshers can invalidate each other's grant and log the user out. Rule: **the engine never refreshes independently.** It obtains tokens only through the existing `PUT /api/auth/token` route, in-process, so there is exactly one refresher and one rotation writer — the route. If that proves insufficient under concurrency, serialise it with a per-slot lock in the route itself. |
| **E12** | Worker thread crashes (OOM on a huge body, native fault) | Cycle counts as `failed`, cursor unchanged (M§7.1 — a crash is not StateInvalid), worker respawned with backoff, escalation ladder applies via the cursor's counters. Never a purge. |

---

## 10. Required changes outside the engine

### 10.1 A server-side JMAP layer **[new]**

The engine cannot use `lib/jmap/client.ts`: it is a browser-`fetch` renderer class holding
credentials in memory (§1.2), and importing it server-side would drag the whole 7413-line surface
into the server bundle. It needs a small, focused JMAP client of its own under `lib/sync/jmap/`,
with **only** what M§12.1/§12.2 specify:

- Typed results, not `null`-collapsing: `JmapResult<T>`, `JmapMethodError` with M§12.1's
  `JmapMethodErrorType` union including `'unknown'` defaulting to ServerTransient. **M's D5 is the
  bug that caused its D4; building the taxonomy in from the first commit is how it never exists
  here.**
- `getEmailChangesResult` / `getMailboxChangesResult` returning **branded** `ChangesState`, plus
  `updatedProperties: string[] | null` on the Mailbox result (RFC 8621 §2.2).
- `getEmailProperties(ids, properties, accountId)` returning its `state` as a **`SnapshotState`**, so
  M's D4 shape is a compile error rather than a code review question.
- `getMailboxProperties` for the `updatedProperties` patch path.
- `queryEmailWindow({after, before, limit, sort, anchor, anchorOffset})` for §4.6's keyset scan,
  surfacing `anchorNotFound` distinctly.
- `captureStates(accountId)` — the one-request `Mailbox/get{ids:[]}` + `Email/get{ids:[]}` pair of
  §4.4, returning branded `SnapshotState`s.
- Request-level error parsing (RFC 8620 §3.6.1 `application/problem+json`, `urn:…:error:limit` with
  `limit: maxSizeRequest | maxCallsInRequest | maxConcurrentRequests | rateLimit`), an `AbortSignal`,
  and a per-request timeout. Note `client.ts`'s plain-`fetch` path has no timeout today, so a hung
  socket hangs a cycle — do not reproduce that.
- A header-capable WebSocket (`ws`) for §2.5 rule 1, with `WebSocketPushEnable`.
- `RateLimitError` + `Retry-After` handling equivalent to `client.ts:54-62`.

**No `as ChangesState` / `as SnapshotState` cast may exist outside this layer's response parsers**
(M§6.3). Worth an eslint `no-restricted-syntax` rule.

### 10.2 Settings

`stores/settings-store.ts` gains, per account: `offlineCacheEnabled` (default **off**, per the
program decision M records), `offlineEnvelopeDays`, `offlineBodyDays`, `offlineMaxMB`. Enabling and
disabling both need confirming copy — disabling **purges** (§5.5).

### 10.3 Electron shell

`electron/main.ts`: pass `VNCMAIL_DESKTOP_STORE_DIR` on spawn (§2.4); resolve/create the per-account
wrapped key after `whenReady()` and hand it to the server process (§6.2); add T11's `powerMonitor`
hooks and T12's cooperative `before-quit`; request the single-instance lock (E7). `electron/preload.ts`
gains **nothing** — the renderer talks to the engine over HTTP, not IPC.

### 10.4 UI

New: an offline-status surface (phase, coverage, last error, storage used, "sync now",
"clear offline mail") reading `GET /api/offline/status`; and an offline read path in the mail list /
message view that falls back to `/api/offline/emails` and `/api/offline/email/:id` when the JMAP
request fails and the account has a materialised store. `stores/email-store.ts` is the natural place
for the fallback, mirroring where the mobile app does it — and, per M§9.5, it must check
`offlineCacheEnabled` **before** calling anything that could materialise a store.

### 10.5 Packaging

`serverExternalPackages: ['@signalapp/sqlcipher']` in `next.config.ts`; verification (and if needed a
copy step in `scripts/assemble-standalone.mjs`) that `prebuilds/**/*.node` reaches
`.next/standalone/node_modules`; a CI assertion that the packaged app can open an encrypted store on
every matrix OS.

---

## 11. Test plan

The `[QA]` gate. `apply.ts` being pure is what makes most of it cheap — that is why it is a hard
requirement (§4.1).

**Unit, no network (vitest, already the repo's runner):** M§13's unit list in full — every M§11 row
expressible as `apply(localState, page, fetched) → mutations`; the cursor state machine's eight
rules; `classify()` over the whole taxonomy including the unknown-type default; the escalation ladder
asserted **monotonically non-increasing** across a range of `maxObjectsInGet` including values below
250 and below 25 (MR V2); backoff monotonic/jittered/capped with `Retry-After` override; retention
F23/F23B/F24/F24B/F25 and the F44 clock-jump guard; reconcile floor pinning (F38) and the reconcile
ceiling (F39). Minus the outbox-durability tests, which have no subject here (§5.4).

**Type-level, compiled by `npm run typecheck`** (M§13's insistence that a type test only earns its
keep if a regression fails the build): `advanceCursor` rejects a `SnapshotState`; a plain object
literal is rejected where `EnumerationCommitment` is expected; no `as ChangesState`/`as SnapshotState`
cast exists outside the JMAP layer.

**`SyncStore` contract tests against both backends** (`store-memory`, `store-sqlite`), including
M's S1 lost-update sequence (F37) and `clearRecords` clearing the body queue (F35).

**Encryption, new and non-negotiable (§3.1's landmine):**

- After a write-and-close, the raw `.db` bytes contain **no** canary string and the header is **not**
  `SQLite format 3`.
- Opening with a wrong key fails; with the right key succeeds.
- `open()` throws if `PRAGMA cipher_version` is empty — i.e. the assertion of §7.1 actually fires if
  someone swaps in a non-cipher binding.
- The format marker: mismatched/stale/absent ⇒ `purgeAccount('store-format-change')` at launch
  **before** any cycle; a crash between materialising a store and writing its marker leaves a
  mismatch (safe), not a false match (M§8.4.1).

**Integration against real Stalwart — cheap here, unlike mobile.** `integration/docker-compose.yml`
+ `integration/tests/` already exist in this repo with a real Stalwart, real SMTP injection and an
Electron spec (§1.6). Extend with M§13's integration list, all of which apply:

- Bootstrap → deliver mail *during* the coverage scan → assert the first delta cycle picks it up.
  M calls this the highest-value test in the list and it is the §4.4 ordering test.
- Multi-page drain with `maxChanges` forced to 2; kill the server child between pages; relaunch;
  assert convergence with no duplicates or omissions (F1/E1).
- Flag toggle from a second client → assert the envelope's `keywords` update and **no body refetch**
  (a network assertion, not just a state assertion — this is the §4.5 3-property rule).
- Mailbox delete with `onDestroyRemoveEmails` both true and false (F7).
- Force `cannotCalculateChanges` → assert reconcile runs, records stay readable throughout, delta
  keeps flowing during the enumeration (F49), and the sweep deletes exactly the server-absent ids.
- **Widen retention mid-reconcile** → assert nothing in the gap is deleted (F38). M calls this the
  test for its worst potential data-loss bug.
- Two-account isolation, plus an explicit regression for M's D6: interleave account switching with
  in-flight fetches, assert no row lands under the wrong account. Add E8: remove an account, add a
  different one that lands in the freed `cookieSlot`, assert no bleed.
- Purge: kill mid-purge, relaunch, assert no records and no surviving cursor (F4/F22).
- **E6, the hosted-deployment gate:** boot the standalone server *without* the marker env var, hit
  every `/api/offline/**` route, assert 404 and assert **no file was created** anywhere.
- **E9/E10:** with the engine's WS connection live, assert exactly one OS notification per delivery
  and that the renderer's path is the one that fired it.

**Electron-level (`npm run test:electron`, the existing required CI gate):** the packaged app opens
an encrypted store on each matrix OS (E2 negative case: a build with the binding deliberately
removed still launches and works online-only); the Linux runner asserts the `basic_text` refusal path
(E3) since a GitHub Linux runner has no keyring — a free, realistic test of the exact configuration
§6.3.1 is about.

**Property/fuzz (M§13, cheap and high yield):** generate random legal change pages with M§5.4's
permitted overlaps and random kill points; assert the store converges to the same state as a
from-scratch bootstrap.

---

## 12. Rollout, and the verify-first gate

M§14's shape, with M's own lesson applied: its V4 finding was that a whole staging decision rested on
an untested premise (`expo-sqlite` works in Expo Go). The premises here have been tested (§3.1) —
**except the packaging ones**, which cannot be tested without installing into this repo and building.
So Stage A exists for exactly those.

**Stage A — packaging and key storage, before a line of engine code.** *All of it is verification;
none of it is engine logic. If any item fails, the design changes before it is built, not after.*

1. Add `@signalapp/sqlcipher` + `serverExternalPackages`. Run `npm run build:standalone` and assert
   `prebuilds/<platform>-<arch>/@signalapp+sqlcipher.node` is present under
   `.next/standalone/node_modules`. If NFT dropped it, add the copy step to
   `assemble-standalone.mjs`.
2. Open an encrypted database from an `app/api/**` route in a **packaged** (`--dir`) build on macOS,
   and confirm the canary/header assertions of §11 against the real file. Repeat on Windows and Linux
   in CI.
3. Confirm cross-arch macOS packaging ships the right `.node` in each of the x64 and arm64 outputs
   (§3.3.4).
4. Resolve a `safeStorage`-wrapped key in `main.ts` and get it into the server process (§6.2); pick
   between the extra-fd and nonce-loopback variants on what actually proves cleaner.
5. On Linux, assert `getSelectedStorageBackend()` and that `basic_text` takes the refusal path (E3).
6. **Simulate an `electron-updater` upgrade of an unsigned build and check Keychain access survives**
   (E5, §6.3.3). This is the one item that could plausibly change the shipping plan — if an unsigned
   build loses its key on every update, the encrypted store should probably wait for step 9
   (signing), and the human should be told so rather than shipping a store that re-bootstraps
   monthly.

**Stage B — pure logic, no engine.** `states.ts` (with M's `Symbol()` note), `errors.ts`, `apply.ts`,
`retention.ts`, fully unit-tested. Type-level tests wired into `npm run typecheck`.

**Stage C — store.** `SyncStore` + `store-memory.ts` + `store-sqlite.ts` + the format marker + the
contract tests against both backends + the §11 encryption tests.

**Stage D — JMAP layer** (§10.1), with the taxonomy and branded returns. Includes the header-capable
WebSocket, tested against the real fixture — this is where §2.5 rule 1 gets proven or disproven, and
if the fixture's `/jmap/ws` behaves like the sandbox's (§1.3) this is where we find out that
server-side WS works.

**Stage E — cursors + delta drain** (A1/A2). **Stage F — coverage + bootstrap** (B). **Stage G —
bodies** (C1, C2). **Stage H — triggers, routes, UI.**

Feature flag: `offlineCacheEnabled`, default off, per account (§10.2). It gates route registration
and trigger registration, not just the engine body.

The Electron smoke gate (`npm run test:electron`) and the integration suite must stay green at every
stage.

---

## 13. Summary of key decisions

1. **The engine and the SQLite file live in the standalone Next.js server process (option A)**,
   on a `worker_threads` Worker, not on the request loop. Decisive reasons: the per-account
   credentials are *already there* in httpOnly encrypted cookies (§1.2), so nothing secret crosses a
   process boundary; and a Node process can put an `Authorization` header on a WebSocket upgrade,
   which is the exact thing that makes RFC 8887 push unreachable from the renderer today
   (`client.ts:6038-6059`). Option B was rejected because it can only be built by moving credentials
   into a process that currently holds none — the change `client.ts` explicitly declined. Option C
   was rejected because WASM SQLite needs `'wasm-unsafe-eval'` added to the **product-wide** CSP
   (`proxy.ts`), and its only encrypted backends are small third-party WASM builds.
2. **The renderer's push pipeline does not change.** The engine gets its own header-capable
   connection; `StateChange` is a wake signal and never a cursor (M§10.4); the engine **never fires a
   notification** — `newEmailNotification` stays the single source; and the two cursors never read
   each other's state. Two sockets per account is the deliberate price of that isolation. Collapsing
   to one (renderer listening to the local server instead of Stalwart) is a *follow-up*, gated on the
   engine's connection being proven.
3. **SQLCipher ships on day one, via `@signalapp/sqlcipher@4.0.3`.** Verified in this environment
   against Electron 43.2.0: N-API prebuilds load with **no rebuild** in both the main process and
   `ELECTRON_RUN_AS_NODE`, real SQLCipher 4.10.0, encrypted file header, wrong key rejected, FTS5
   present, AGPL-3.0-only matching this repo. The mobile design's plaintext-first phase existed
   solely because Expo Go cannot load SQLCipher; **that constraint has no Electron analogue**, and
   importing the staging anyway would mean shipping a plaintext mailbox for a phase plus building and
   discarding a migration path to leave it.
4. **`node:sqlite` rejected** (no encryption at any price — `PRAGMA key` is a *silent no-op* that
   leaves the mailbox in cleartext; and stability 1.2/RC in Node 24, which is what Electron 43
   bundles). **`better-sqlite3-multiple-ciphers` rejected**: newest release has Electron prebuilds
   up to ABI 146, Electron 43 needs 148, so it means a C++ toolchain on every machine — and that lag
   recurs at every Electron major by construction. Plain `better-sqlite3@13.0.2` is the fallback
   only.
5. **Every store open asserts `PRAGMA cipher_version` is non-empty, and a test asserts a canary is
   absent from the raw file bytes.** Silent-plaintext is the sharpest landmine found in this
   investigation and it has no error to notice.
6. **The hosted-deployment gate is part of the design, not a convention.** The same server process
   runs in Docker for many users. One env var (`VNCMAIL_DESKTOP_STORE_DIR`, set only by `main.ts`)
   both enables the engine and supplies its path; the routes 404 without it; a test asserts no file
   is created without it (E6).
7. **Keys: `safeStorage`** (built in, no `keytar`), per account, wrapped and written under the store
   directory; the unwrapped key goes main → **server** process only, never to the renderer.
   Consulting `getSelectedStorageBackend()` is mandatory: Linux returns
   `isEncryptionAvailable() === true` while using a *public hardcoded password* (`basic_text`), which
   is worse than an honest failure.
8. **Account identity is `AccountEntry.id` (`username@host`), never `cookieSlot`.** Slots are
   recycled; resolution is cookie → `decryptSession` → `generateAccountId` → cross-check against the
   session's confirmed username, and every commit re-validates `(accountId, epoch)` (E8).
9. **v1 desktop offline is read-only.** This repo has no outbox and no optimistic mutation layer, so
   M§5.6's read-time overlay has nothing to overlay. When offline mutations land, M§5.6/§5.6.1 is the
   design — including its durability requirement — and a write-through into `envelope`/`body` remains
   forbidden.
10. **The engine syncs all logged-in accounts, not just the active one** — a capability the mobile
    engine lacks because its JMAP client is a renderer singleton. Consequence: M§7.2's jitter matters
    more here, since T4/T11 fire for every account at once.
11. **Everything else is the mobile design, deliberately unchanged**: the three state machines with
    sequential execution (I11), independent envelope/body retention tiers, cursor provenance as an
    ordering rule with branded types and an unforgeable `EnumerationCommitment`, capture-cursors-
    before-enumerate, the 3-property `updated` fetch, cursor-last, the seven-class error taxonomy
    with exactly one class moving a cursor, the pinned reconcile sweep floor, the monotonically
    shrinking `maxChanges` ladder with per-cursor counters, no-deletion-by-inference, account-scoped
    primary keys, the purge ordering (key before file), lazy materialisation, and the F1–F49 failure
    table.

### Open questions for the human — resolved 2026-08-04

1. **If Stage A item 6 shows an unsigned build loses its Keychain item across updates** — left open
   deliberately; this is an empirical Stage A outcome, not something to decide blind. Resolve once
   Stage A actually runs and reports what happens.
2. **Linux with no keyring (`basic_text`, §6.3.1 / E3): refuse outright.** Decided — matches this
   design's own "cheapest wrong answer" philosophy (§ discussion of `safeStorage`'s silent-downgrade
   landmine): an honest failure beats a store that looks encrypted but isn't. No opt-in path in v1.
3. **Concrete retention defaults: same as the mobile design's defaults.** Decided —
   `offlineEnvelopeDays: 365`, `offlineBodyDays: 30`, MB cap on bodies matching mobile's default
   (§ mobile M§2.1's shipped numbers). Consistent behavior across platforms for v1; revisit only if
   desktop usage patterns actually show it's too narrow — don't widen speculatively.
4. **§2.5 single-socket follow-up: not scheduled for v1.** Stays a gated follow-up as designed —
   don't collapse to one connection until the engine's own connection is proven in production.
5. **Stage A as its own PR ahead of the offline engine: not decided separately** — proceed with the
   implementation sequence as designed (Stage A is the first step regardless of PR boundaries); the
   human can split the PR at merge time if desired, this doesn't block starting work.

Also decided: **this design goes through an independent adversarial review next**, matching the
mobile engine's process (design → review → implementation), before any implementation code is
written — same reasoning as before: this is the same class of highest-stakes, hardest-to-undo
component (a persistent, encrypted, multi-account local store), and the review process caught real
critical bugs in the mobile design that this document's own author-side verification did not.

None of these blocks starting the review.

---

## 14. What was verified, and what was not

Stated explicitly, in M's spirit — its V4 finding was precisely that an untested premise had been
presented as settled.

**Verified by execution in this environment, against `electron@43.2.0`:** Electron's bundled Node
(24.18.0) and ABI (148/napi 10); its bundled SQLite (3.53.1, FTS5 on); `node:sqlite`'s presence,
absence of encryption, and the *silent* no-op of `PRAGMA key` including the plaintext canary in the
file bytes; `better-sqlite3@13.0.2`'s in-tarball N-API prebuilds and successful load in both process
modes; `@signalapp/sqlcipher@4.0.3`'s load in Electron, SQLCipher 4.10.0, encrypted header, absent
canary, wrong-key rejection, right-key read-back, and FTS5; `safeStorage.isEncryptionAvailable()`,
round-trip and `v10` ciphertext prefix on macOS.

**Verified by reading published metadata:** `better-sqlite3` 13's move off `prebuild-install`;
`better-sqlite3-multiple-ciphers`' Electron ABI coverage (121–146, no 148) and release cadence;
the existence and provenance of `@7mind.io/sqlcipher-wasm` and `@aztec/sqlite3mc-wasm`; `node:sqlite`'s
stability index (1.2, RC) in Node 24; `safeStorage`'s Linux `basic_text` fallback and
`getSelectedStorageBackend()` values.

**NOT verified — flagged for Stage A, in descending order of how much they could change the design:**

1. Whether an **unsigned** Electron app retains its macOS Keychain item across an `electron-updater`
   upgrade (§6.3.3, E5). Not documented by Electron either way. Could change *when* the encrypted
   store ships.
2. Whether Next.js **output file tracing** carries `@signalapp/sqlcipher`'s `prebuilds/` into
   `.next/standalone/node_modules` (§2.1-against-2, §3.3.2). Fallback is a copy step in a script
   that already exists for exactly this class of omission.
3. Whether the cross-arch macOS build ships the correct per-arch `.node` (§3.3.4).
4. Whether the integration fixture's Stalwart `/jmap/ws` accepts a header-authenticated upgrade — the
   sandbox's does *require* the header (§1.3), which is what makes the server-side connection work in
   principle, but it has not been driven from Node here. Stage D.
5. The choice of mechanism for getting the unwrapped key from `main.ts` into the server process
   (extra fd vs. nonce-authenticated loopback) — deliberately left to whichever proves cleaner
   against a packaged build (§6.2).
