---
phase: 14-performance-optimization
plan: 03
subsystem: performance
tags: [event-delegation, queueMicrotask, closure-elimination, scroll-perf, store-batching]

# Dependency graph
requires:
  - phase: 14-01
    provides: CSS containment and GPU scroll promotion for scroll containers
provides:
  - Zero per-item closure allocation during scroll rendering in track-list and queue-panel
  - queueMicrotask notification batching in queue store
  - changeGeneration counter in library store for granular update skipping
affects: [14-04, ui-performance]

# Tech tracking
tech-stack:
  added: []
  patterns:
    - Event delegation via data-index attributes replacing per-item inline closures
    - queueMicrotask batching for store notifications (now consistent across all stores)
    - changeGeneration counter to skip requestUpdate on loading-only state transitions

key-files:
  created: []
  modified:
    - frontend/src/components/track-list/track-list.ts
    - frontend/src/components/queue-panel/queue-panel.ts
    - frontend/src/store/queue-store.ts
    - frontend/src/store/library-store.ts
    - frontend/src/store/controllers/library-controller.ts

key-decisions:
  - "Event delegation via data-index + closest() instead of bound-method-per-row pattern"
  - "changeGeneration counter in library store instead of typed per-data subscriptions"

patterns-established:
  - "Event delegation: attach handlers on virtualizer element, resolve item via closest + data-index"
  - "Store change generation: monotonic counter for data-only changes, skip updates on loading toggles"

requirements-completed: [PERF-RENDER-01, PERF-RENDER-02]

# Metrics
duration: 4min
completed: 2026-03-14
---

# Phase 14 Plan 03: Render & Store Optimization Summary

**Event delegation eliminates per-scroll-frame closure allocation; queueMicrotask batching and changeGeneration counter reduce unnecessary re-renders**

## Performance

- **Duration:** 4 min
- **Started:** 2026-03-14T17:49:45Z
- **Completed:** 2026-03-14T17:54:13Z
- **Tasks:** 2
- **Files modified:** 5

## Accomplishments
- Eliminated all inline arrow function closures from `renderTrackRow` (track-list) and `renderTrackItem` (queue-panel), removing GC pressure during rapid scrolling
- Added queueMicrotask-based notification batching to queue store, matching the library store pattern
- Added changeGeneration counter to library store so LibraryController skips requestUpdate when only loading flags toggle (no actual data change)
- Confirmed cover-grid already uses event delegation — no changes needed

## Task Commits

Each task was committed atomically:

1. **Task 1: Eliminate per-item closure allocation in render hot paths** - `2f7ed70` (perf)
2. **Task 2: Add notification batching to queue store and granular subscriptions to library store** - `d0c05dc` (perf)

## Files Created/Modified
- `frontend/src/components/track-list/track-list.ts` - Event delegation via data-index; removed 5 inline closures per row from renderTrackRow
- `frontend/src/components/queue-panel/queue-panel.ts` - Event delegation via data-index; removed 5 inline closures per item from renderTrackItem
- `frontend/src/store/queue-store.ts` - queueMicrotask batching for notify()
- `frontend/src/store/library-store.ts` - changeGeneration counter incremented on actual data changes
- `frontend/src/store/controllers/library-controller.ts` - Checks changeGeneration before requestUpdate

## Decisions Made
- **Event delegation via data-index + closest()** — chosen over per-row bound method references because it requires zero function objects in renderItem, not just stable ones. The virtualizer and its children share the same shadow root so event.target.closest() works correctly.
- **changeGeneration counter** — chosen over typed per-data subscriptions (subscribeTo('tracks')) because it's simpler, backward-compatible, and eliminates the biggest source of unnecessary re-renders (loading flag transitions during eagerFetch cause 8+ requestUpdate calls) without requiring store API changes for existing subscribers.

## Deviations from Plan

### Auto-fixed Issues

**1. [Rule 1 - Bug] Removed unused handleRemoveTrack method**
- **Found during:** Task 1 (queue-panel event delegation)
- **Issue:** After delegating remove-button click to the virtualizer handler, the old `handleRemoveTrack` method became unused, causing a TypeScript error (TS6133)
- **Fix:** Removed the unused method; remove logic is now in `onDelegatedClick` which checks `closest('.remove-button')`
- **Files modified:** frontend/src/components/queue-panel/queue-panel.ts
- **Verification:** Build passes, pre-commit typecheck passes
- **Committed in:** 2f7ed70 (Task 1 commit)

---

**Total deviations:** 1 auto-fixed (1 bug)
**Impact on plan:** Trivial cleanup of dead code after refactoring. No scope creep.

## Issues Encountered
None

## User Setup Required
None - no external service configuration required.

## Next Phase Readiness
- Plans 14-01, 14-02, 14-03 complete
- Ready for 14-04 (remaining rendering optimizations)

## Self-Check: PASSED

All 5 modified files verified on disk. Both task commits (2f7ed70, d0c05dc) verified in git history.

---
*Phase: 14-performance-optimization*
*Completed: 2026-03-14*