The slowest user-felt path is typing into the composer while the
assistant is streaming. Profile (scripts/profile-under-stream.mjs):
FadeText measureOverflow self time: 35.8 ms → 18.1 ms (-50%)
total active CPU during 7s window: ~150 ms → ~50 ms
Two changes in src/components/ui/fade-text.tsx:
1. Drop the `useEffect([children])` that re-ran `measureOverflow`
(reads scrollWidth + clientWidth — forced layout) on every parent
re-render. `useResizeObserver` already fires the same callback on
mount and whenever the host span's box size changes; that covers
the only case where overflow state can legitimately change. The
previous explicit useEffect was a forced-layout flush on every
parent render, which during streaming meant every token tick.
2. Wrap the component in `memo` with a custom comparator that
short-circuits the entire render when scalar string `children` and
the className/fadeWidth/style props are unchanged. The hot path
was tool-fallback's title chips being re-rendered by parent
streaming updates even though their text was stable; memo+
comparator skips that.
Also adds two harness scripts under apps/desktop/scripts/:
- latency-under-stream.mjs (key→paint latency while a turn streams)
- profile-under-stream.mjs (CPU profile while a turn streams)
Updates profile-typing-lag.md with the streaming numbers and confirms
the Enter→paint submit path is already fast (≤320ms on the populated
session; the 2s "stall after Enter" the user noticed once was a
one-time cold-start, not reproducible at the UI layer).
I'd guess the felt jank in real use is fast-burst typing during a
long-form streaming reply (code blocks + markdown lists multiply the
per-token render cost). The CPU savings here scale linearly with
token volume.
Empirical work via CDP harnesses under apps/desktop/scripts/ (see
profile-typing-lag.md):
jsListeners growth (per round of 200 chars + GC):
before: +35 (verified leak — listeners stuck after 1st trigger popover use)
after: +0
Four narrow edits in src/app/chat/composer/index.tsx:
1. Drop the per-keystroke `editorRef.current.scrollHeight` read used to
decide composer expansion. Replace with `draft.length > 60` heuristic;
the existing ResizeObserver still catches edge cases. `scrollHeight`
is a forced-layout call and was firing on every char until the first
wrap.
2. Bucket measured composer height to 8px before writing
`--composer-measured-height` / `--composer-surface-measured-height`
on `documentElement`. Without this, the editor grows ~1px per char,
setProperty fires every keystroke, computed style is invalidated tree-
wide.
3. Remove the dead `$composerDraft` two-way sync. Nothing outside the
composer subscribed to that atom (verified via grep). Two useEffects
on `[draft]` were pushing draft→atom and atom→aui per keystroke for
no consumer. Also drop the per-keystroke
`reconcileComposerTerminalSelections` call; it was pruning stale
labels for `terminalContextBlocksFromDraft`, but that helper already
ignores labels not in the current submitted text, so pruning per
keystroke was just bookkeeping.
4. `refreshTrigger` fast-bails when the draft contains neither `@` nor
`/`. Previously `textBeforeCaret(editor)` ran on every input/keyup
regardless; `range.toString()` inside is O(n) over draft length.
Synthetic typing latency p50/p90/p99 is similar before vs after on a
freshly-loaded session (Blink can already handle ~30cps typing into a
contentEditable on its own); the real win is the listener leak being
gone and the global computed-style invalidations dropping ~8× when the
composer is sitting at a fixed height row.
The `Enter → stall` follow-up (see profile-typing-lag.md §"Submit /
TTFT stall") is unmeasured here — needs a throwaway session because
the harness fires a real prompt. Not blocking this commit.
