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

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.

ui-tui/packages/hermes-ink/src/ink/output.ts

@@ -467,9 +467,21 @@ export default class Output {
 
             if (clipHorizontally) {
               lines = lines.map(line => {
-                const from = x < clip.x1! ? clip.x1! - x : 0
+                const startsBefore = x < clip.x1!
                 const width = stringWidth(line)
-                const to = x + width > clip.x2! ? clip.x2! - x : width
+                const endsAfter = x + width > clip.x2!
+
+                // Fast path: line fits entirely within the clip box — skip
+                // the tokenize/slice. This is the common case for transcript
+                // text where containers are wider than the rendered content.
+                // CPU profile (Apr 2026) showed sliceAnsi at 18% total time;
+                // most calls were no-op slices like (line, 0, width).
+                if (!startsBefore && !endsAfter) {
+                  return line
+                }
+
+                const from = startsBefore ? clip.x1! - x : 0
+                const to = endsAfter ? clip.x2! - x : width
                 let sliced = sliceAnsi(line, from, to)
 
                 // Wide chars (CJK, emoji) occupy 2 cells. When `to` lands

ui-tui/packages/hermes-ink/src/ink/stringWidth.ts

@@ -270,6 +270,58 @@ const bunStringWidth = typeof Bun !== 'undefined' && typeof Bun.stringWidth ===
 
 const BUN_STRING_WIDTH_OPTS = { ambiguousIsNarrow: true } as const
 
-export const stringWidth: (str: string) => number = bunStringWidth
+const rawStringWidth: (str: string) => number = bunStringWidth
   ? str => bunStringWidth(str, BUN_STRING_WIDTH_OPTS)
   : stringWidthJavaScript
+
+// Memoize stringWidth — it's pure, hot (~100k calls/frame per the comment
+// above), and the underlying impl scans every grapheme + tests EMOJI_REGEX.
+// CPU profile (Apr 2026) showed stringWidth dominating at 21% of total
+// runtime during scroll. Cache is global (vs per-frame) since the same
+// strings recur across frames in a stable transcript.
+//
+// Pure-ASCII short-strings (the >90% common case) skip the cache: the inline
+// loop in stringWidthJavaScript is already faster than a Map.get for them.
+const widthCache = new Map<string, number>()
+const WIDTH_CACHE_LIMIT = 8192
+
+export const stringWidth: (str: string) => number = str => {
+  if (!str) {
+    return 0
+  }
+
+  // ASCII fast-path detection — for short ASCII, skip the cache.
+  if (str.length <= 64) {
+    let asciiOnly = true
+
+    for (let i = 0; i < str.length; i++) {
+      const code = str.charCodeAt(i)
+
+      if (code >= 127 || code === 0x1b) {
+        asciiOnly = false
+        break
+      }
+    }
+
+    if (asciiOnly) {
+      return rawStringWidth(str)
+    }
+  }
+
+  const cached = widthCache.get(str)
+
+  if (cached !== undefined) {
+    return cached
+  }
+
+  const w = rawStringWidth(str)
+
+  if (widthCache.size >= WIDTH_CACHE_LIMIT) {
+    // Drop oldest entry — Map iteration order is insertion order.
+    widthCache.delete(widthCache.keys().next().value!)
+  }
+
+  widthCache.set(str, w)
+
+  return w
+}

ui-tui/packages/hermes-ink/src/ink/wrap-text.ts

@@ -6,6 +6,40 @@ import { wrapAnsi } from './wrapAnsi.js'
 
 const ELLIPSIS = '…'
 
+// CPU profile (Apr 2026) showed `wrap-ansi` → `string-width` consuming 30% of
+// total runtime during fast scroll: every layout pass re-wraps every visible
+// line via wrap-ansi, which calls string-width once per grapheme. The output
+// is pure of (text, maxWidth, wrapType), so memoize it. LRU-bounded so long
+// sessions don't accrete unbounded cache.
+const WRAP_CACHE_LIMIT = 4096
+const wrapCache = new Map<string, string>()
+
+function memoizedWrap(text: string, maxWidth: number, wrapType: Styles['textWrap']): string {
+  // Key folds maxWidth + wrapType into the prefix so the same text re-wrapped
+  // at a different width doesn't collide. Width prefix bounded by viewport
+  // (~10 distinct widths in a session); wrapType bounded by enum (~6 values).
+  const key = `${maxWidth}|${wrapType}|${text}`
+  const cached = wrapCache.get(key)
+
+  if (cached !== undefined) {
+    // LRU touch
+    wrapCache.delete(key)
+    wrapCache.set(key, cached)
+
+    return cached
+  }
+
+  const result = computeWrap(text, maxWidth, wrapType)
+
+  if (wrapCache.size >= WRAP_CACHE_LIMIT) {
+    wrapCache.delete(wrapCache.keys().next().value!)
+  }
+
+  wrapCache.set(key, result)
+
+  return result
+}
+
 // sliceAnsi may include a boundary-spanning wide char (e.g. CJK at position
 // end-1 with width 2 overshoots by 1). Retry with a tighter bound once.
 function sliceFit(text: string, start: number, end: number): string {
@@ -42,12 +76,9 @@ function truncate(text: string, columns: number, position: 'start' | 'middle' |
   return sliceFit(text, 0, columns - 1) + ELLIPSIS
 }
 
-export default function wrapText(text: string, maxWidth: number, wrapType: Styles['textWrap']): string {
+function computeWrap(text: string, maxWidth: number, wrapType: Styles['textWrap']): string {
   if (wrapType === 'wrap') {
-    return wrapAnsi(text, maxWidth, {
-      trim: false,
-      hard: true
-    })
+    return wrapAnsi(text, maxWidth, { trim: false, hard: true })
   }
 
   if (wrapType === 'wrap-char') {
@@ -55,25 +86,24 @@ export default function wrapText(text: string, maxWidth: number, wrapType: Style
   }
 
   if (wrapType === 'wrap-trim') {
-    return wrapAnsi(text, maxWidth, {
-      trim: true,
-      hard: true
-    })
+    return wrapAnsi(text, maxWidth, { trim: true, hard: true })
   }
 
   if (wrapType!.startsWith('truncate')) {
-    let position: 'end' | 'middle' | 'start' = 'end'
-
-    if (wrapType === 'truncate-middle') {
-      position = 'middle'
-    }
-
-    if (wrapType === 'truncate-start') {
-      position = 'start'
-    }
+    const position: 'end' | 'middle' | 'start' =
+      wrapType === 'truncate-middle' ? 'middle' : wrapType === 'truncate-start' ? 'start' : 'end'
 
     return truncate(text, maxWidth, position)
   }
 
   return text
 }
+
+export default function wrapText(text: string, maxWidth: number, wrapType: Styles['textWrap']): string {
+  // Skip cache for trivial inputs (faster than Map lookup).
+  if (!text || maxWidth <= 0) {
+    return computeWrap(text, maxWidth, wrapType)
+  }
+
+  return memoizedWrap(text, maxWidth, wrapType)
+}

ui-tui/packages/hermes-ink/src/utils/sliceAnsi.ts

@@ -10,7 +10,42 @@ function filterStartCodes(codes: AnsiCode[]): AnsiCode[] {
   return codes.filter(c => !isEndCode(c))
 }
 
+// LRU cache: same (string, start, end) → same output. Output.get() re-emits
+// identical writes every frame for stable transcript content; this avoids
+// re-tokenizing them. CPU profile (Apr 2026) showed sliceAnsi at 18% total
+// time during scroll. Bounded at 4096 entries — entries are short clipped
+// lines so memory cost is small.
+const sliceCache = new Map<string, string>()
+const SLICE_CACHE_LIMIT = 4096
+
 export default function sliceAnsi(str: string, start: number, end?: number): string {
+  if (!str) return ''
+
+  // Hot-path: only cache when end is defined (the Output.get() use-case).
+  if (end !== undefined) {
+    const key = `${start}|${end}|${str}`
+    const cached = sliceCache.get(key)
+
+    if (cached !== undefined) {
+      sliceCache.delete(key)
+      sliceCache.set(key, cached)
+      return cached
+    }
+
+    const result = computeSlice(str, start, end)
+
+    if (sliceCache.size >= SLICE_CACHE_LIMIT) {
+      sliceCache.delete(sliceCache.keys().next().value!)
+    }
+
+    sliceCache.set(key, result)
+    return result
+  }
+
+  return computeSlice(str, start, end)
+}
+
+function computeSlice(str: string, start: number, end?: number): string {
   const tokens = tokenize(str)
   let activeCodes: AnsiCode[] = []
   let position = 0