perf(client-runtime): merge shell deltas once per received batch

[saphid]

Problem

Bulk sidebar changes, such as settling or snoozing many threads, or the replay after a reconnect, arrive as many thread.updated shell deltas. The client already publishes once per received batch. But makeEnvironmentShellState still applied each delta with the single-event reducer, which scans the thread array twice and copies the whole array for every changed row. With a few thousand threads, a burst costs hundreds of milliseconds of main-thread work.

Change

Add applyShellStreamEvents, which applies a contiguous run of deltas with one lazy row index per collection and builds each changed collection's array once. makeEnvironmentShellState collects deltas between snapshots and synchronized markers and flushes them in order. A single delta still takes the existing single-event path. Stale-sequence rejection, row identity for unchanged rows, ordering (including remove-then-reinsert), repository-identity retention, and active/archive exclusivity all match the single-event reducer.

Scope and approval

Small, focused performance fix in the shell sync hot path, so it uses the no-prior-issue exception. Client runtime only, and the contract is unchanged.

Verification

• Equivalence: new shellReducer.test.ts cases check the batched result against a sequential fold of applyShellStreamEvent. They cover mixed project and thread deltas, remove-then-reinsert order, stale and unknown events, and unchanged rows keeping object identity.

• Boundary cost: new shell-sync.test.ts cases drive the real makeEnvironmentShellState subscription with bounded queues and stream milestones. On main, 50 updates to existing rows read 5,200 ids, over the 550/700 bounds (log). With the fix they stay within bounds (log).

• CPU, single-event reducer vs. this PR, 16-event batches, median of 5 samples, identical final snapshots (log):

  Changed rows	main	This PR
  1,000	31 ms	7 ms
  2,000	112 ms	21 ms
  8,000	733 ms	376 ms

  Thread-array copies at 8,000 rows fell from 8,000 to 500.

• vp test run packages/client-runtime/src/state/shellReducer.test.ts packages/client-runtime/src/state/shell-sync.test.ts: 41 passed, rerun after rebasing on current main.

• vp lint on the touched files and typecheck in packages/client-runtime: clean.

• Independent review by Claude Opus 5.5 (high) through a T3 delegated task: no correctness defects. It noted that indexing is still O(N) per batch, which is the remaining cost at 8,000 rows.

• Not measured: rendering time in a browser. The numbers cover the reducer and subscription only.

Found by a GPT-6.1 Sol (high) audit and implemented by GPT-6.1 Sol (high) in T3 Code. Reviewed by Claude Opus 5.5. Coordinated by Claude Opus 5.5 in Claude Code via T3 Code. Agent-authored, sponsored by Alex Southwell.

🤖 Generated with Claude Code

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📝 Walkthrough

Walkthrough

Shell stream processing now applies delta events in batches. The reducer preserves sequence handling and collection identities, while synchronization tests cover existing rows and snapshot sequence resets.

Changes

Shell stream batching

Layer / File(s)	Summary
Batch delta reduction packages/client-runtime/src/state/shellReducer.ts, packages/client-runtime/src/state/shellReducer.test.ts	applyShellStreamEvents applies ordered project and thread deltas. Tests compare batched and sequential reductions and cover stale events, collection identity, and repository identity.
Stream batch integration packages/client-runtime/src/state/shell.ts, packages/client-runtime/src/state/shell-sync.test.ts	applyItems flushes accumulated deltas before snapshots and synchronization markers, then flushes remaining events. Tests cover batching with existing rows and a snapshot sequence reset.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Refactor

Sequence Diagram(s)

sequenceDiagram
  participant StreamItems
  participant applyItems
  participant applyShellStreamEvents
  StreamItems->>applyItems: provide delta events
  applyItems->>applyShellStreamEvents: flush deltas before snapshot or synchronization marker
  applyShellStreamEvents-->>applyItems: return updated shell snapshot

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Suggested reviewers: juliusmarminge

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Merge Risk | 🔵 Low · up to cb105

Merge Risk: 🔵 Low · up to cb105

Stale stream events can cause unnecessary state updates. This is a bounded issue that can be addressed before merge or accepted for follow-up.

Security Architecture Review

Security architecture risk: 🔵 Low · up to cb105

Batching remains within the existing client synchronization path, and no introduced security weakness was established. The principal transitions appear preserved, but complete equivalence depends on row-identity assumptions that were not fully verified across all snapshot sources.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The directly affected production outcome is one environment's client shell state and cached snapshot, including project and thread collections. The inspected change does not add server-side mutation authority or expand credential access; downstream external library consumers remain outside verified coverage.

Trust Boundaries and Controls

• observed — The HTTP snapshot loader continues using the authenticated environment request path and schema decoding. The changed reducer operates after snapshot loading or stream receipt, retains stale-sequence rejection, and does not establish a replacement authentication or authorization boundary.

Resilience and Maintainability Implications

• observed — Reduction remains synchronous within the existing serialized snapshot-write path. Persistence still avoids snapshots with pending deferred-link rows, logs save failures, and flushes through the existing lifecycle finalizer. Stale deltas can clear the surrounding state's error and queue persistence in both the base and head; that behavior predates this PR.

Hardening Proposals

• proposed — If duplicate-ID snapshots must be supported, explicitly define and enforce their handling at the snapshot boundary or reducer API. This would make the Map-backed reducer's identity precondition explicit; the review did not establish an attacker-reachable duplicate-ID production path.

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Pre-merge checks | 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check		The description includes all required sections. It clearly explains the performance problem, the batching change, scope and approval rationale, focused verification, measured results, limitations, and…
Title check		The title clearly and concisely identifies the main change: merging client-runtime shell deltas once per received batch for performance.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

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Actionable comments posted: 1

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Inline comments:
Review comments at @packages/client-runtime/src/state/shell.ts:
- Around line 198-203: Update flushDeltas to compare the result of
applyShellStreamEvents with the current snapshot before replacing next. When
they are identical, preserve the current state so a stale-only batch does not
update status or clear an existing error; retain the existing state update
behavior when the snapshot changes.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: Path: .coderabbit.config.ts
• Review profile: CHILL
• Plan: Advanced
• Run ID: 395c0da0-39ea-4699-919f-f1d071852e1d

📥 Commits

Reviewing files that changed from the base of the PR and between 6ba6a0d and cb10577.

📒 Files selected for processing (4)

• packages/client-runtime/src/state/shell-sync.test.ts
• packages/client-runtime/src/state/shell.ts
• packages/client-runtime/src/state/shellReducer.test.ts
• packages/client-runtime/src/state/shellReducer.ts

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[macroscopeapp]

Approvability

Verdict: Approved at cb10577

Macroscope's review found this PR approvable — This is a focused client-runtime optimization that reduces repeated shell-array scans and copies while preserving existing event ordering, cursor handling, and snapshot behavior. The production changes are isolated and backed by targeted equivalence and synchronization tests, with no schema, default, deployment, or sensitive-package impact.

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