fix(ui): silent save-loss — snapshot cursor advanced before persist resolved (CWD-9)

[eliotlim]

Problem

Any transient page-save failure was reported as "Saved" while silently dropping the edit: the save path advanced its snapshot cursor before awaiting persistence, so the post-failure retry recomputed an identical snapshot, hit the no-op skip, and resolved as success. Board: CWD-9 (first ship of the Concurrency & Write Durability epic OB-76).

Solution

Advance lastSnapshot.current only after onSave resolves, with a monotonic sequence guard so out-of-order resolves of overlapping saves can't regress the cursor (worst case is one redundant re-send, never loss). Deliberately minimal: no saver redesign — backoff/queueing is CWD-6/7. Two invariant comments document the unbounded-retry contract and the pageId-remount assumption.
Key file: packages/ui/src/screens/BlockPageDocument.tsx (+ new test file). New deps: none.

Before / After

No visual change — existing save-pill states only; what changed is when they're truthful.

Behavior	Before	After
Save fails once (transient network error)	Retry no-op-skips, pill shows Saved, edit lost on reload	Retry re-sends the same payload; Saved appears only after a 2xx (tested: BlockPageDocument.test.tsx "re-sends a rejected snapshot")
Save failing repeatedly	Pill lies Saved after first retry	Pill alternates saving/save-failed and keeps retrying (documented as unbounded until CWD-6/7)
Genuine no-change save	No-op skip, no request	Unchanged — regression-tested
Two saves resolve out of order	n/a (cursor already advanced)	Seq guard ignores the stale resolve; next save re-sends newest content

Test procedure

pnpm install && pnpm --filter @book.dev/ui test -- src/screens/__tests__/BlockPageDocument.test.tsx
pnpm verify

Review verification (Quinn): mutation test — reverting the one-line fix makes the new test fail on exactly the silent-loss assertion; interleaving probes confirmed no lost update with a mid-flight second edit.

Operational notes

None — no API/schema/config change, no deployment ordering. Deliberate deferral: retry backoff + durable queue land in CWD-6/7; conflict UX in CWD-10.

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Verify: green (full pnpm verify + full pre-push suite run). Reviews cleared: code (Quinn — APPROVE; concurrency verdict CORRECT; 3 pre-endorsed findings applied in 83125ac).

🤖 Generated with Claude Code

https://claude.ai/code/session_018RUicirLRuuZvjJN13N6yN

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Project	Deployment	Actions	Updated (UTC)
app.book.pub	Ready	Preview	Aug 14, 2026 12:56pm