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perf(tui): cache stringWidth/wrapText/sliceAnsi + skip-slice when line fits clip

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CPU profile (Apr 2026, real-user scroll on 11k-line session) showed three
hot loops in the per-frame render path:

  Output.get() per-frame walk:                 24% total
  └─ sliceAnsi(line, from, to) per write:     18% total
  stringWidth(line) chain (cached + JS):      14% total

All three were re-doing identical work every frame: same string → same
clipped slice → same width.

Fixes:

1. Memoize stringWidth (8k-entry LRU) for non-ASCII strings; ASCII fast-path
   skips the cache (inline scan beats Map.get for short ASCII, the >90%
   case). String.charCodeAt scan up to 64 chars is cheaper than the regex
   fallback.

2. Memoize wrapText (4k-entry LRU keyed by maxWidth|wrapType|text) — wrapAnsi
   is pure and the same content reflows identically every frame.

3. Memoize sliceAnsi (4k-entry LRU keyed by start|end|str) for the
   end-defined hot path used by Output.get().

4. Skip the slice entirely in Output.get() when the line already fits the
   clip box (startsBefore=false && endsAfter=false). Most transcript lines
   never exceed their container width, and tokenizing them just to slice
   (line, 0, width) was pure overhead. This single fast-path drops
   sliceAnsi from 18% → ~0% in the profile.

Also tighten virtualization constants (MAX_MOUNTED 260→120, OVERSCAN 40→20,
SLIDE_STEP 25→12) and cap historical-message render at 800 chars / 16
lines via HISTORY_RENDER_MAX_*; messages inside the FULL_RENDER_TAIL_ITEMS
window still render in full so reading-zone behavior is unchanged.

Validation, real-user CPU profile, page-up scroll on 11k-line session:

  Output.get() self-time:     24%   →   0.3%
  sliceAnsi total:            18%   →   not in top 25
  stringWidth family:         14%   →   ~3%
  idle:                     60.7%   →  77.3%

Frame timings (synthetic page-up profile harness):
  dur p95:   ~10ms   →  4.87ms
  dur p99:   25ms+   → 12.80ms
  yoga p99:  ~20ms   →  1.87ms

The remaining CPU in the profile is Yoga layoutNode + React commit,
which is the irreducible work for this UI tree size.
This commit is contained in:
Brooklyn Nicholson 2026-04-26 19:28:09 -05:00
parent 85e9a23efb
commit c370e2e1e5
14 changed files with 450 additions and 42 deletions
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2
ui-tui/src/components/appLayout.tsx
View file

@ -7,6 +7,7 @@ import type { AppLayoutProps } from '../app/interfaces.js'
import { $isBlocked, $overlayState, patchOverlayState } from '../app/overlayStore.js'
import { $uiState } from '../app/uiStore.js'
import { INLINE_MODE, SHOW_FPS } from '../config/env.js'
import { FULL_RENDER_TAIL_ITEMS } from '../config/limits.js'
import { PLACEHOLDER } from '../content/placeholders.js'
import { inputVisualHeight, stableComposerColumns } from '../lib/inputMetrics.js'
import { PerfPane } from '../lib/perfPane.js'
@ -51,6 +52,7 @@ const TranscriptPane = memo(function TranscriptPane({
compact={ui.compact}
detailsMode={ui.detailsMode}
detailsModeCommandOverride={ui.detailsModeCommandOverride}
limitHistoryRender={row.index < transcript.historyItems.length - FULL_RENDER_TAIL_ITEMS}
msg={row.msg}
sections={ui.sections}
t={ui.theme}