perf(web): defer composer draft serialization

[t3dotgg]

Typing serializes every saved web draft, including image bytes, before the storage debounce runs.

Capture the immutable state and defer the draft walk and JSON serialization until the write flushes. Keep hydration, migrations, attachment verification, and final flushes. Clear canceled state references.

This continues the web portion of #9049 by @StiensWout. His Git authorship and the existing co-author trailer are preserved. The mobile storage migration remains separate. The original PR is unchanged.

The real store test verifies that 20 prompt updates cause no saved-attachment reads or JSON serialization before one flush. The final prompt, image data, and assistant citations restore correctly. All 107 focused tests, web typecheck, and targeted lint pass. No browser or device was used. The original PR's browser recording is not new verification.

Part of #9661.

Created with GPT-6 Astra (preview) in Codex.

---

Note

Medium Risk
Changes when and how composer drafts are written to localStorage; behavior depends on flush/rehydrate paths staying correct, but migrations and unload flush are preserved.

Overview
Composer draft persistence no longer walks drafts or runs JSON.stringify on every keystroke. Zustand partialize now only captures the live store snapshot; the custom persist storage runs partializeComposerDraftStoreState and stringifies inside createDeferredStorage when the 300ms debounce fires or on beforeunload flush.

createDebouncedStorage is replaced by createDeferredStorage, which debounces typed values and applies a injected serialize at write time. removeItem cancels the debouncer and reset() so canceled writes cannot flush stale snapshots.

Tests call the exported partializeComposerDraftStoreState directly, exercise real store rehydrate with fake timers, and assert rapid typing does not read heavy attachment data or stringify until one flush.

^{Reviewed by Cursor Bugbot for commit 17e2c91. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Defer composer draft serialization to reduce writes during rapid typing

• Introduces a deferred storage wrapper (createDeferredStorage) that serializes and writes to base storage only when the debounce fires, not on every draft update
• Rapid prompt updates with large image attachments no longer trigger repeated attachment traversal or JSON serialization per keystroke
• Exports partializeComposerDraftStoreState so tests call the partializer directly instead of reaching into persistence middleware options
• Behavioral Change: pending writes that are canceled by removeItem or superseded by a new value skip serialization entirely; flush serializes and writes synchronously

📊 Macroscope summarized 17e2c91. 2 files reviewed, 1 issue evaluated, 1 issue filtered, 0 comments posted

🗂️ Filtered Issues

apps/web/src/composerDraftStore.ts — 0 comments posted, 1 evaluated, 1 filtered

• line 103: composerPersistStorage.getItem calls JSON.parse without handling malformed localStorage. The previous createJSONStorage path intentionally treated malformed composer storage as empty (and the existing test covers that case); now a truncated/corrupt t3code:composer-drafts:v1 value throws during persist hydration rather than falling back to an empty draft store, preventing drafts from hydrating until the user manually clears site storage. [ Out of scope (triage) ]

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.7 KiB	13.6 KiB	−177 B (−1.3%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.0 KiB	7.0 KiB	−2 B (−0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.7 KiB	6.5 KiB	−175 B (−2.5%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	58.5 KiB	57.0 KiB	−1.5 KiB (−2.5%)	66.4 KiB	✅
Codex	Live turn messages	10	9	−1 (−10.0%)	21	✅
Claude	Total thread wire	13.5 KiB	13.6 KiB	+77 B (+0.6%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.0 KiB	7.0 KiB	−2 B (−0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.5 KiB	6.6 KiB	+79 B (+1.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.8 KiB	57.9 KiB	+88 B (+0.1%)	66.4 KiB	✅
Claude	Live turn messages	8	10	+2 (+25.0%)	21	✅

Baseline: 2460649 · PR result: 17e2c91 · 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: 113.8 KiB
• Claude decoded thread snapshot: 114.5 KiB

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

[macroscopeapp]

Approvability

Verdict: Approved at 17e2c91

Macroscope's review found this PR approvable — The PR narrowly defers existing composer draft traversal and JSON serialization without changing the persisted format, debounce interval, or user-facing capabilities. Its runtime impact is isolated to local draft persistence and is covered by focused round-trip, flush, cancellation, and deferred-work tests.

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