fix(mobile): skip move indexes for empty and single-thread sections

[juliusmarminge]

Home and the tablet sidebar rebuild move availability as thread shells stream. Even an empty pinned section indexed every thread, including settled history, although no row could move.

Return early for sections with fewer than two rows. Empty sections still return an empty map; singleton rows still deny both directions. The existing pending-reorder lock remains first. Five production lines, no new cache, timer, dependency, or native-menu behavior.

Release simulator evidence

Same optimized iOS Release app and Hermes -O bundle, with only this source file changed. Home visible with 1,210 shells and the settled shelf expanded. Both runs replayed the same pre-run database snapshot and emitted 300 foreground stream events plus 120 background events over 15 seconds. Captures include startup/finalization, about 20.8 seconds each. One simulator, one capture at a time.

Sampled JS work	Before	After
Home move-availability memo, inclusive stacks	395	173
Memo share of active JS samples	19.2%	11.3%
computeThreadMoveAvailability, inclusive stacks	313	102
Active JS samples / total samples	2,056 / 13,512	1,531 / 11,330

Measurement details · Before Hermes profile · After Hermes profile

This is one controlled pair, not an overall app-speed or battery claim. Sampling cadence, host scheduling, GC, and generated background text vary. An earlier manual-capture pair was excluded because its baseline had sampling gaps up to 370 ms and unequal capture windows. The fixture server uses the earlier audit checkout. This changes CPU work only; screenshots would show identical menus.

Verification

• Nine focused tests passed, including 4,000-seed randomized parity with the reference planner and empty/singleton writable/read-only cases with hidden rows.
• Targeted lint passed on both changed files; formatting and diff checks passed.
• Bundle source-map comparison confirmed only threadOrder.ts changed. Unrelated local widget edits were excluded and restored byte-for-byte.
• Both mobile list callers use this helper; web, desktop, contracts, providers, and connection protocols are unchanged.

Implementation: Claude Opus 5.5 through T3 Code. Integration, review, and release profiling: GPT-6 Astra through the Codex harness.

[macroscopeapp]

Approvability

Verdict: Approved at 82d061a

Macroscope's review found this PR approvable — This is a localized, behavior-preserving optimization that skips unnecessary indexing for empty and single-row thread sections. Focused tests verify identical move availability, including hidden-row and writable/read-only cases.

No code changes detected at 3b5e737. Prior analysis still applies.

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📥 Commits

Reviewing files that changed from the base of the PR and between f623cad and 82d061a.

📒 Files selected for processing (2)

• apps/mobile/src/features/threads/threadOrder.ts
• apps/mobile/src/features/threads/threadOrderAvailability.test.ts

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📝 Walkthrough

Walkthrough

computeThreadMoveAvailability now handles empty and single-row lists before building indexes from allThreads. Tests cover both cases with hidden rows in writable and non-writable environments.

Changes

Short-list move availability

Layer / File(s)	Summary
Handle short lists apps/mobile/src/features/threads/threadOrder.ts, apps/mobile/src/features/threads/threadOrderAvailability.test.ts	The function returns an empty map for an empty list and marks a single row as unable to move in either direction. Tests cover both cases with hidden rows and compare single-row results with the reference planner.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~7 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 82d06

No actionable merge-blocking risk remains in the reviewed changes.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly describes the fix for move indexing in empty and single-thread sections.
Description check	✅ Passed	The description explains the problem, change, and focused verification. It also limits the performance claim. The scope rationale is implied by the small, focused fix, but the template asks the author…

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	5.0 KiB	—	6.8 KiB	✅
Codex	Thread snapshot wire	—	3.8 KiB	—	4.9 KiB	✅
Codex	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Codex	Live turn WebSocket decoded	—	20.9 KiB	—	29.3 KiB	✅
Codex	Live turn messages	—	2	—	8	✅
Claude	Total thread wire	—	5.0 KiB	—	6.8 KiB	✅
Claude	Thread snapshot wire	—	3.8 KiB	—	4.9 KiB	✅
Claude	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Claude	Live turn WebSocket decoded	—	21.2 KiB	—	29.3 KiB	✅
Claude	Live turn messages	—	1	—	8	✅

Baseline: unavailable · PR result: 3b5e737 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 108.5 KiB
• Claude decoded thread snapshot: 108.8 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.