5chan/.cursor/skills/profile-browsing/SKILL.md

name, description

name

description

profile-browsing

Profile app performance while browsing, collecting Web Vitals and React rerender data via react-scan. Orchestrates sequential profiler subagents via playwright-cli to capture navigation timing, long tasks, layout shifts, LCP, React commit counts, render bursts, and per-component render data without saturating the machine. Use when profiling browsing performance, finding bottlenecks, diagnosing excessive rerenders, or auditing page performance.

Profile Browsing Performance

Two-layer profiling: browser-level symptoms (Web Vitals, long tasks, scroll jank) and React-level diagnosis (commit counts, render bursts, per-component render data from react-scan). Each profiler subagent runs in its own browser session and context window, with only one profiler active at a time.

Prerequisites

• Dev server running at https://5chan.localhost (yarn start via Portless)
• playwright-cli installed (npm install -g @playwright/cli@latest)

IMPORTANT: The orchestrator (you) is responsible for ensuring exactly ONE dev server is running. Profiler subagents must NEVER start a dev server themselves.

react-scan (already configured)

src/lib/react-scan.ts runs react-scan in dev mode. It:

• Highlights rerendering components visually (toolbar + overlay)
• Accumulates per-component render counts and times via react-scan's onRender option
• Exposes window.__getReactScanReport() and window.__resetReactScanReport() for programmatic collection

__getReactScanReport() returns a plain object: { ComponentName: { count, time } }.

Do not use react-scan's own getReport(). It reads Store.legacyReportData, which react-scan 0.5.3 never writes to, so it always returns an empty Map. The live Store.reportData is no better: it is only populated while the toolbar is visible and a component is manually focused in the inspector, neither of which holds under automation. The app's onRender collector exists precisely because of this. Also note a Map cannot be serialized — JSON.stringify(new Map()) is "{}" regardless of contents — which is why the collector returns a plain object.

The profiler's addInitScript sets window.__PROFILING__ = true before the app loads, which tells react-scan to disable its toolbar and sounds during automated runs.

No additional setup needed — react-scan is already a devDependency and imported in the entry file.

Step 0: Ensure Dev Server is Running

Before running any profiler subagents, verify exactly one dev server is available:

# Check if the dev server is reachable
curl -sf https://5chan.localhost -o /dev/null && echo "OK" || echo "NOT RUNNING"

• If OK: proceed to Step 1.
• If NOT RUNNING: start one instance with yarn start (backgrounded), then poll until it responds. Do NOT start more than one.
• If a dev server is already running on a different port (check ps aux | grep vite), reuse it — do not start another.

Step 1: Define Route Batches

Split routes into batches of 2–4 for sequential profiling. Give every batch a short task-specific session name so unrelated profiling runs cannot collide.

Default batches (adjust boards as needed):

Batch	Session	Routes	Focus
1	prof-1	/all, /all/catalog	Multi-board feed + catalog
2	prof-2	/biz, /biz/catalog	Single board feed + catalog
3	prof-3	/pol, /pol/catalog, /g, /g/catalog	Board switching (feed reloads)

Keep batches balanced. Add thread views (/:boardIdentifier/thread/:cid) as needed.

Step 2: Run Profiler Subagents Sequentially

Read the profiler subagent definition at .cursor/agents/profiler.md. Then spawn one profiler Task for the first batch:

For each batch, create a Task:
  subagent_type: "profiler"
  prompt: |
    Session name: "prof-N"
    Routes to profile: /route1, /route2, ...
    Any non-default app URL or extra profiling constraints

Wait for that profiler to close its browser and return results before spawning the next batch. Never run profiler or browser-check subagents concurrently: competing browser sessions both saturate the machine and invalidate timing measurements.

Step 3: Merge Results

Collect structured output from each subagent and merge:

1. Concatenate all Critical / Warning / React Rerenders / Scroll Jank / Info items
2. Combine per-view summary tables into one
3. Merge react-scan component data across routes (same component appearing in multiple routes = sum counts)
4. Deduplicate shared issues (e.g., same slow resource across routes)
5. Sort by severity (Critical first)

Step 4: Final Report

## Performance Profile Results

### Critical
- [metric]: [value] at [route] — [what likely needs fixing]

### Warning
- [metric]: [value] at [route] — [what likely needs fixing]

### React Rerenders
- [route]: [N] commits during load, [M] during scroll — [likely cause]
- Render bursts detected at [routes] — suggests cascading state updates
- Top rerendering components (react-scan):
  - [ComponentName]: [total count] renders across [routes], [time]ms total
  - [ComponentName]: [total count] renders across [routes], [time]ms total

### Scroll Jank
- [route]: [N] long tasks during scroll (max [X]ms), [M] React commits — [likely cause]

### Info
- [observations]

### Per-View Summary
| View | Nav (ms) | Long Tasks | CLS | LCP (ms) | Commits | Scroll Commits | Bursts | Top Component |
|------|----------|-----------|-----|-----------|---------|----------------|--------|---------------|
| /all | ... | ... | ... | ... | ... | ... | ... | ... |

Interpreting React Metrics

Signal	Likely cause	Fix direction
High commits, no long tasks	Frequent cheap rerenders	React.memo, stabilize props
High commits + long tasks	Expensive rerenders	Profile render cost, split components
High scroll commits	Scroll/intersection observer triggering renders	Throttle handlers, memoize list items
Render bursts (>5 in 100ms)	Cascading state updates	Batch updates, review Zustand selectors
react-scan: component with >30 renders	Missing memoization or unstable references	useMemo/useCallback, check parent renders
react-scan: component with >50ms time	Expensive render function	Split component, move work out of render

Element-source follow-up

When react-scan identifies a rerender hotspot but you still need the exact file behind a concrete DOM node, hand off to $inspect-elements.

playwright-cli -s=prof-followup eval "async el => JSON.stringify(await window.__ELEMENT_SOURCE__.resolve(el))" e7

Use source.filePath as the direct edit target and stack to understand which parent components own the node.

Step 5: Cleanup

After profiling is complete and the report is delivered, verify no orphaned processes were left behind:

# Check for any Vite dev servers started during profiling
ps aux | grep 'vite.*--port' | grep -v grep

• If the dev server was already running before Step 0, leave it alone.
• If the orchestrator started the dev server in Step 0, kill it now.
• If there are multiple Vite processes (should never happen), kill the extras and warn the user.

Confirm the profiling session released the shared browser slot:

./scripts/pw-session.sh status

If a failed profiler still owns the slot, close that exact recorded session with ./scripts/pw-session.sh close <session>. A slot whose browser already died is reclaimed by the next open, so it needs no manual cleanup. Never use close-all or kill-all during concurrent agent work.

Notes

• Session isolation: Each subagent uses a short task-specific playwright-cli session (-s=prof-<task>-N).
• Context isolation: Each subagent runs in its own context window.
• Per-route collection: Data resets on each goto — the profiler collects before navigating away.
• addInitScript persistence: Instrumentation re-injects automatically in each new document.
• Tracing: Each subagent produces a trace.zip viewable in Trace Viewer.
• Board codes: biz, pol, g, a, v, etc. map to community addresses via the directory.
• Empty react-scan report: react-scan is a dynamic import, so __getReactScanReport() returns {} for the first moment after a goto. If it is empty, wait ~1s and re-read before falling back to commit counts + render bursts (still useful, just no component names).