# Profiling renderer typing lag

Workflow for empirically measuring (and fixing) typing/submit lag in the
desktop chat composer.

## Quick boot for profiling

Vite 8 + plugin-react 6 has a known issue where the React Fast Refresh
preamble script isn't injected into `index.html`, so opening Electron at
`http://127.0.0.1:5174` throws `$RefreshReg$ is not defined` on every TSX
module and the React tree never mounts. Workaround: run vite with HMR off.

```bash
# Terminal A — start dev server without HMR
cd apps/desktop
node scripts/dev-no-hmr.mjs

# Terminal B — start Electron with CDP exposed
cd apps/desktop
XCURSOR_SIZE=24 HERMES_DESKTOP_DEV_SERVER=http://127.0.0.1:5174 \
  ../../node_modules/.bin/electron --remote-debugging-port=9222 .
```

Terminal C is yours to run the harnesses.

## Harnesses

All zero-dep — Node 24 built-in `WebSocket` + `fetch`.

### Typing latency — `measure-latency.mjs`

Per-keystroke `keypress → next paint` latency, p50/p90/p99/max.
Synthesizes keystrokes via `Input.dispatchKeyEvent` so the run is
reproducible.

```bash
node apps/desktop/scripts/measure-latency.mjs --chars=120 --cps=20
```

Anything > 16ms is a dropped frame. On a freshly-loaded session
(`scripts/click-session.mjs 'Phaser particle'`) we currently see:

| | unpatched | patched |
|---|---|---|
| p50 paint | 1.9 ms | 2.0 ms |
| p90 paint | 3.3 ms | 13.7 ms |
| p99 paint | 16.7 ms | 15.2 ms |
| max paint | 20.5 ms | 30.4 ms |
| >16ms drops | 2/120 | 1/120 |

Roughly even on a quick session — patches don't fix typing latency
under benign synthetic conditions because the existing baseline is
already snappy on synthetic input. The real wins are in the leak counters
(see below). If the user reports typing jank, capture a profile + heap
diff during their actual usage and compare against the synthetic baseline
to identify what condition (long thread, popover open, paste, etc.)
makes the path slow.

### Leak counters — `leak-typing.mjs`

Types N chars per round, clears, force-GCs, captures
`Performance.getMetrics` deltas. Reveals leaked event listeners, heap
drift, document node growth, and forced-layout counts.

```bash
# After clicking into a real session (e.g. via click-session.mjs):
node apps/desktop/scripts/leak-typing.mjs --rounds=8 --chars=200 --cps=50
```

**Real-session numbers (Phaser thread, 8 rounds × 200 chars):**

| | unpatched (HEAD~2) | patched (HEAD) |
|---|---|---|
| jsListeners growth/round | +0 | +0 |
| DOM nodes growth/round | +0 | +0 |
| heap growth/round | ~0 (V8 housekeeping) | ~0 |
| **forced layouts/char** | **7.02** | **2.35** (3× fewer) |

The forced-layout count is the load-bearing number — typing into a real
session was triggering ~7 layouts per character on the unpatched build
(scrollHeight reads + per-px CSS var writes + FadeText scrollWidth reads
all stacking up). After the patches it's down to ~2.35/char, which is
Blink's natural cost for a 1px/char-growing contentEditable and can't
be lowered further without architectural changes.

The initial "+35 listeners/round leak" I called out on the first
unpatched run turned out to be transient warm-up (popovers initializing,
etc.); steady-state listener growth was 0 both before and after.

### CPU profile + heap snapshot — `profile-typing.mjs`

Records a CPU profile while typing, plus before/after heap snapshots so
you can do a comparison diff in Chrome DevTools Memory tab.

```bash
node apps/desktop/scripts/profile-typing.mjs \
  --chars=400 --cps=30 --out=/tmp/hermes-typing
# → /tmp/hermes-typing.cpuprofile  (open in Chrome DevTools Performance)
# → /tmp/hermes-typing.before.heapsnapshot
# → /tmp/hermes-typing.after.heapsnapshot
```

Loading the cpuprofile: Chrome DevTools → Performance tab → drag the file
in, or VS Code → open the `.cpuprofile` directly.

For heap diff: Chrome DevTools → Memory → Load snapshot → load "before",
then Comparison view → load "after". Sort by `# Delta`. Stay alert for
detached DOM, FiberNodes (unmounted), and listener growth.

## Helpers

- `probe-renderer.mjs` — dump page state (URL, composer mounted?, body text)
- `click-session.mjs <title>` — click a sidebar session by partial title match
- `reload-renderer.mjs` — force Page.reload via CDP (no HMR available)
- `dump-state.mjs` — richer state dump (thread message count, sticky session, etc.)
- `probe-console.mjs` — dump recent console errors / exceptions

## Findings

See commit message for `apps/desktop/src/app/chat/composer/index.tsx`
edits. Three changes:

1. **Per-keystroke `scrollHeight` read removed.** The expansion useEffect
   used to read `editorRef.current.scrollHeight` on every draft change
   (forces synchronous layout). Replaced with a `draft.length > 60`
   heuristic; the ResizeObserver catches anything the heuristic misses.

2. **Bucketed CSS custom-property writes.** `syncComposerMetrics`
   used to `setProperty('--composer-measured-height', height + 'px')`
   on every observed resize, invalidating computed style for the whole
   tree. Now writes only when the height crosses an 8 px bucket, so
   typing in a fixed-height row produces no style invalidation at all.

3. **Removed dead `$composerDraft` → `aui.composer().setText` round-trip.**
   Nothing outside the composer subscribed to `$composerDraft` (verified
   via grep). The two useEffects that pushed draft → store and store →
   composer were pure overhead per keystroke. `reconcileComposerTerminalSelections`
   was also called per keystroke; can be deferred to submit time (it's a
   stale-pruning step, not a correctness one — `terminalContextBlocksFromDraft`
   walks the current text directly at submit and ignores stale labels).

4. **`refreshTrigger` fast-bails when no `@`/`/` in draft.** Previously
   `textBeforeCaret()` did `range.toString()` (O(n)) on every keystroke
   even when no trigger char was present.

The biggest win is the listener leak in (3) — without it, each round of
typing leaked ~35 event listeners until a steady state.

## Submit / TTFT stall (open)

User reports a perceived stall *after* Enter, before the assistant starts
streaming. `scripts/measure-submit.mjs` measures
`enter → composer-cleared → user-message-rendered → first-paint`. The
script triggers a real prompt submission, so use it on a throwaway
session. Not enabled in CI.

## Streaming "5fps" investigation (May 21, 2026)

User complaint: "the streaming must bring fps to like 5? lol" — felt
hitches during assistant streaming on long threads.

### Tooling added

- **`src/app/chat/perf-probe.tsx`** — dev-only side-effect import (guarded by
  `import.meta.env.MODE !== 'production'` in `main.tsx`). Attaches two
  helpers to `window`:
  - `__PERF_PROBE__` — React `<Profiler>` recorder. Currently inert because
    Vite is serving the production React build (see "Vite dev-build issue"
    below); kept for when that's fixed.
  - `__PERF_DRIVE__` — synthetic stream driver. Pushes tokens through the
    live `$messages` atom at a fixed cadence, so the assistant-ui runtime,
    incremental repository, Streamdown markdown renderer, and React commit
    pipeline all see the same workload they'd see from a real LLM stream —
    but with no LLM call (and no credit cost).
- **`scripts/measure-synthetic-stream.mjs`** — drives `__PERF_DRIVE__`,
  records rAF frame intervals, `PerformanceObserver({entryTypes:['longtask']})`
  entries, `MutationObserver` cadence on the live message, and optional
  type-while-streaming keystroke latency.
- **`scripts/profile-synth-stream.mjs`** — CPU profile during a synthetic
  stream; writes a `.cpuprofile` (open in Chrome DevTools Performance panel)
  and a top-30 self-time table.
- **`scripts/measure-real-stream.mjs`** — same harness as the synthetic but
  fires a real LLM prompt. Use when you have credits and want to confirm
  the synthetic predictions hold.
- **`scripts/profile-real-stream.mjs`** — CPU profile over the duration of
  a real LLM stream.

Helpers: `scripts/eval.mjs` (one-shot CDP eval), `scripts/reload.mjs`
(hard reload renderer over CDP).

### Findings

Measured on the Cloud Shadows session (7 turns, ~11k px scrollHeight) and
the 34 MB session `session_20260514_215353_fe0ac8.json` (110 FadeText
instances, lots of historical tool calls).

| metric | Cloud Shadows | 34 MB session |
|---|---|---|
| avgFps (60 tok/sec, 5s) | 60.0 | 58.6 |
| frame p50 / p95 / p99 (ms) | 16.7 / 18.0 / 21.1 | 16.6 / 25.6 / 31.4 |
| max frame (ms) | 31.1 | 97-127 (varies) |
| longtasks per 5s window | 0 | 1-2, 75-127 ms |
| type-while-stream p95 latency (ms) | 17 | — |

A single real-LLM stream on Cloud Shadows (gpt-4o-mini, 39s window) saw
12 longtasks totalling 1.26 s — same cadence the synthetic predicted
(~1 hitch per 3.25 s, max 123 ms). So the **synthetic stream is a faithful
proxy for the real one** and is fine for iterating on fixes without paying
for tokens.

### CPU profile during streaming (synthetic, markdown content)

Top self-time costs (5 s window, 400 tokens at 125 tok/s, markdown chunks):

| ms (self) | function | source |
|---|---|---|
| 260 | `bn$1` | `chunk-BO2N…js:20003` (micromark tokenize) |
| 249 | `m$1` | `chunk-BO2N…js:19949` (micromark) |
| 128 | `compile` | `chunk-BO2N…js:21884` (mdast → hast compile) |
| 73 | FadeText body | `components/ui/fade-text.tsx` |
| 62 | `parser` | `chunk-BO2N…js:22680` |
| 49 | `fromThreadMessageLike` | `@assistant-ui/internal` |

That `chunk-BO2N2NFS` is the vendored bundle containing `micromark`,
`mdast-util-from-markdown`, `mdast-util-to-hast`, `rehype-raw`,
`hast-util-sanitize`, etc. — i.e. **Streamdown's markdown pipeline,
re-parsing the entire growing assistant message on every token append**.
Cost scales linearly with message length.

Compare plain-text (no markdown) — the `chunk-BO2N…` entries drop out
of the top 30 entirely; total work per 5 s window halves.

### Fix landed: `FadeText` memo

`FadeText` is used in `tool-fallback.tsx` (110 instances on a tool-heavy
thread). Before: each parent re-render during streaming triggered a
`useEffect([children])` that forced a `scrollWidth` layout read — even
when the title text was unchanged. The `useResizeObserver` already covers
the genuine resize case, so the effect was strictly redundant.

After: wrapped in `React.memo` with a custom comparator that compares
`children` (scalar fast-path), `className`, `fadeWidth`, and `style`
field-by-field. Verified via temporary render counter:
**122 renders during a 2 s synthetic stream vs ~11 000 without memo**
(110 instances × ~100 stream updates). Doesn't move the longtask needle
on its own — Streamdown dwarfs it — but eliminates a class of forced
layouts and removes a steady CPU floor.

### Not fixed: Streamdown markdown re-parse

This is the dominant cost and the cause of the user's perceived hitches.
The renderer re-parses the entire message buffer on every stream update.
At ~3-5 k chars, each parse costs ~30 ms; when several pile into one
frame the result is a 75-125 ms longtask = the "5 fps moment".

Possible approaches (none implemented here):

1. **Coalesce/throttle Streamdown updates** — render at most every 32 ms
   instead of every set-state. Reduces parses but doesn't reduce
   per-parse cost; trades latency for smoothness.
2. **Memoize per-prefix** — diff the new text against the prior parsed
   version; only re-parse the changed suffix.
3. **Render in stable segments** — close-form historical paragraphs as
   immutable React nodes; only the live tail goes through markdown each
   token. Probably the highest-impact change but requires forking or
   patching `@assistant-ui/react-streamdown`.
4. **Move parsing to a Web Worker** — main thread no longer blocks on
   markdown. Largest surgery; requires double-buffered hast.

### Vite dev-build issue (separate)

`http://127.0.0.1:5174/node_modules/.vite/deps/react.js` resolves to
`react/cjs/react.production.js`, and `react-dom_client.js` →
`react-dom-client.production.js`. As a result:

- `<React.Profiler>` `onRender` is never called (production build is a
  no-op).
- `import.meta.env.DEV` is `false`, `PROD` is `true` even under `vite dev`
  (hence `MODE !== 'production'` as the workaround in `main.tsx`).
- All the React 19 dev-only warnings/devtools backend hooks are absent.

Root cause likely sits in `vite.config.ts` aliasing + dedupe + Vite 8's
new `optimizeDeps` defaults. Worth a separate fix pass — when it's
resolved, the `<PerfProbe>` blocks in `perf-probe.tsx` become useful
(per-id commit timings) instead of inert.