---
phase: 14-performance-optimization
plan: 01
subsystem: ui
tags: [css-containment, gpu-compositing, will-change, content-visibility, scroll-performance]

# Dependency graph
requires: []
provides:
  - CSS containment on all app shell layout boundaries
  - GPU-composited scrolling on all 6 scroll-heavy components
  - content-visibility auto on album cards for off-screen rendering skip
affects: [14-performance-optimization]

# Tech tracking
tech-stack:
  added: []
  patterns: [CSS contain for layout isolation, will-change transform for GPU scroll promotion, content-visibility auto for off-screen rendering skip]

key-files:
  created: []
  modified:
    - frontend/index.css
    - frontend/src/components/cover-grid/cover-grid-styles.ts
    - frontend/src/components/track-list/track-list.ts
    - frontend/src/components/queue-panel/queue-panel.ts
    - frontend/src/components/artists-view/artists-view.ts
    - frontend/src/components/genres-view/genres-view.ts
    - frontend/src/components/playlist-view/playlist-view.ts

key-decisions:
  - "contain: strict on .main-panel (has explicit dimensions), layout style elsewhere (avoids breaking flex)"
  - "will-change: transform only on scroll containers (not :host) to avoid wasting GPU memory"
  - "content-visibility: auto on album cards with contain-intrinsic-size hint to prevent layout shift"

patterns-established:
  - "CSS containment pattern: :host gets contain: layout style; scroll container gets contain: paint + will-change: transform"
  - "content-visibility: auto with contain-intrinsic-size for cards in virtualized grids"

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

# Metrics
duration: 3min
completed: 2026-03-14
---

# Phase 14 Plan 01: CSS Containment & GPU Scroll Promotion Summary

**CSS containment on app shell boundaries + GPU-composited scrolling on all 6 scroll-heavy components with content-visibility on album cards**

## Performance

- **Duration:** 3 min
- **Started:** 2026-03-14T17:43:01Z
- **Completed:** 2026-03-14T17:46:24Z
- **Tasks:** 2
- **Files modified:** 7

## Accomplishments
- App shell layout boundaries (content-area, main-panel, sidebar, bottom-bar) isolated with CSS containment to prevent cross-boundary layout recalculation
- All 6 scroll-heavy components (cover-grid, track-list, queue-panel, artists-view, genres-view, playlist-view) now have CSS containment on :host and GPU layer promotion on scroll containers
- Album cards skip rendering when off-screen via content-visibility: auto with intrinsic size hints

## Task Commits

Each task was committed atomically:

1. **Task 1: Add CSS containment to app shell layout boundaries** - `efa06f7` (perf)
2. **Task 2: Add GPU promotion and containment to all scroll containers** - `ac8a52e` (perf)

## Files Created/Modified
- `frontend/index.css` - CSS containment on .content-area, .main-panel, .main-panel > *, sidebar, .bottom-bar
- `frontend/src/components/cover-grid/cover-grid-styles.ts` - contain + will-change on scroll container, content-visibility on album cards
- `frontend/src/components/track-list/track-list.ts` - contain on :host, contain + will-change on lit-virtualizer
- `frontend/src/components/queue-panel/queue-panel.ts` - contain on :host, contain + will-change on lit-virtualizer
- `frontend/src/components/artists-view/artists-view.ts` - contain on :host, contain + will-change on grid scroll container
- `frontend/src/components/genres-view/genres-view.ts` - contain on :host, contain + will-change on grid scroll container
- `frontend/src/components/playlist-view/playlist-view.ts` - contain on :host, contain + will-change on playlist-list

## Decisions Made
- Used `contain: strict` only on `.main-panel` (has explicit flex: 1 + overflow: hidden), used `contain: layout style` elsewhere to avoid breaking flex layouts
- Applied `will-change: transform` only to actual scroll containers (not :host elements) to avoid wasting GPU memory on non-scrolling elements
- Added `content-visibility: auto` with `contain-intrinsic-size` hints on album cards to prevent layout shift during scroll

## Deviations from Plan

None - plan executed exactly as written.

## Issues Encountered
None

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

## Next Phase Readiness
- CSS containment foundation in place for all components
- Ready for Plan 02 (next performance optimization plan in phase 14)

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

## Self-Check: PASSED