perf(cuesheet): add result capture + candidate lazy-autosize (Exp B, under measurement)

- Scaffold: snapshot()/getResults() so the benchmark returns structured
  metrics for automated before/after capture (PERF-METRICS).
- Candidate Exp B: defer AutoTextarea autosize() off the mount path to
  focus-time, removing the per-row forced reflows during scroll. Under
  evaluation against the metrics.

apps/client/src/common/components/input/auto-textarea/AutoTextarea.tsx

@@ -1,6 +1,6 @@
 // @ts-expect-error no types from library
 import autosize from 'autosize/dist/autosize';
-import { RefObject, useEffect } from 'react';
+import { FocusEvent, RefObject, useCallback, useEffect } from 'react';
 
 import { timeSync } from '../../../devtools/cuesheet-metrics/usePerfMark'; // PERF-METRICS
 import Textarea, { type TextareaProps } from '../textarea/Textarea';
@@ -10,18 +10,41 @@ interface AutoTextAreaProps extends TextareaProps {
 }
 
 /**
- * A textarea that automatically resizes based on its content
+ * A textarea that automatically resizes based on its content.
+ *
+ * `autosize()` forces a synchronous reflow (reads scrollHeight) and installs a MutationObserver.
+ * Doing that on mount makes it expensive when many of these are mounted at once (e.g. virtualised
+ * table rows during scroll), so we only attach autosize while the field is focused for editing and
+ * keep it in sync with the value during that time.
  */
-export function AutoTextarea({ value, inputref, ...textAreaProps }: AutoTextAreaProps) {
-  // when the value changes, we use the ref to reapply autosize
+export function AutoTextarea({ value, inputref, onFocus, onBlur, ...textAreaProps }: AutoTextAreaProps) {
+  const handleFocus = useCallback(
+    (event: FocusEvent<HTMLTextAreaElement>) => {
+      timeSync('cell.autosize', () => autosize(inputref.current)); // PERF-METRICS
+      onFocus?.(event);
+    },
+    [inputref, onFocus],
+  );
+
+  const handleBlur = useCallback(
+    (event: FocusEvent<HTMLTextAreaElement>) => {
+      if (inputref.current) {
+        autosize.destroy(inputref.current);
+      }
+      onBlur?.(event);
+    },
+    [inputref, onBlur],
+  );
+
+  // while focused, keep the height in sync as the value changes
   useEffect(() => {
     const node = inputref.current;
-    timeSync('cell.autosize', () => autosize(inputref.current)); // PERF-METRICS
-
-    return () => {
-      autosize.destroy(node);
-    };
+    if (node && document.activeElement === node) {
+      autosize(node);
+    }
   }, [inputref, value]);
 
-  return <Textarea ref={inputref} value={value} {...textAreaProps} />;
+  return (
+    <Textarea ref={inputref} value={value} onFocus={handleFocus} onBlur={handleBlur} {...textAreaProps} />
+  );
 }

apps/client/src/common/devtools/cuesheet-metrics/benchmark.ts

@@ -9,7 +9,7 @@ import type { RefObject } from 'react';
 import type { TableVirtuosoHandle } from 'react-virtuoso';
 
 import { PERF_ENABLED } from './perfConfig';
-import { dump, endSession, startSession } from './perfStore';
+import { dump, endSession, snapshot, startSession, type PerfSnapshot } from './perfStore';
 import { startFpsMonitor, stopFpsMonitor } from './scrollFpsMonitor';
 
 export interface BenchmarkOptions {
@@ -24,17 +24,17 @@ export interface BenchmarkOptions {
 export async function runScrollBenchmark(
   virtuosoRef: RefObject<TableVirtuosoHandle | null>,
   options: BenchmarkOptions = {},
-): Promise<void> {
+): Promise<PerfSnapshot | undefined> {
   if (!PERF_ENABLED) {
     // eslint-disable-next-line no-console
     console.warn('[cuesheet-perf] disabled — open the cuesheet with ?perf=1 in a dev build.');
-    return;
+    return undefined;
   }
   const handle = virtuosoRef.current;
   if (!handle) {
     // eslint-disable-next-line no-console
     console.warn('[cuesheet-perf] virtuoso handle not ready.');
-    return;
+    return undefined;
   }
 
   const { fromIndex = 0, toIndex = 200, stride = 8, stepMs = 16 } = options;
@@ -48,6 +48,7 @@ export async function runScrollBenchmark(
   stopFpsMonitor();
   endSession();
   dump();
+  return snapshot();
 }
 
 function scrollThrough(

apps/client/src/common/devtools/cuesheet-metrics/perfStore.ts

@@ -80,9 +80,21 @@ export function recordFrame(deltaMs: number): void {
   }
 }
 
-/** Dumps the collected metrics to the console as tables. */
-export function dump(): void {
-  const markRows = Object.entries(state.marks)
+export interface PerfSnapshot {
+  summary: {
+    rowsMounted: number;
+    frames: number;
+    longFrames: number;
+    avgFps: number;
+    minFps: number;
+    maxFrameMs: number;
+  };
+  marks: Array<{ mark: string; count: number; avgMs: number; maxMs: number; totalMs: number }>;
+}
+
+/** Returns the collected metrics as a structured object (for console or automated capture). */
+export function snapshot(): PerfSnapshot {
+  const marks = Object.entries(state.marks)
     .map(([name, m]) => ({
       mark: name,
       count: m.count,
@@ -95,19 +107,26 @@ export function dump(): void {
   const avgFps = state.totalFrameMs > 0 ? round((state.frameCount * 1000) / state.totalFrameMs) : 0;
   const minFps = state.maxFrameMs > 0 ? round(1000 / state.maxFrameMs) : 0;
 
-  const summary = {
-    rowsMounted: state.rowsMounted,
-    frames: state.frameCount,
-    longFrames: state.longFrames,
-    avgFps,
-    minFps,
-    maxFrameMs: round(state.maxFrameMs),
+  return {
+    summary: {
+      rowsMounted: state.rowsMounted,
+      frames: state.frameCount,
+      longFrames: state.longFrames,
+      avgFps,
+      minFps,
+      maxFrameMs: round(state.maxFrameMs),
+    },
+    marks,
   };
+}
 
+/** Dumps the collected metrics to the console as tables. */
+export function dump(): void {
+  const { summary, marks } = snapshot();
   /* eslint-disable no-console */
   console.group('[cuesheet-perf] benchmark result');
   console.table(summary);
-  console.table(markRows);
+  console.table(marks);
   console.groupEnd();
   /* eslint-enable no-console */
 }

apps/client/src/common/devtools/cuesheet-metrics/useCuesheetPerf.ts

@@ -13,7 +13,7 @@ import type { TableVirtuosoHandle } from 'react-virtuoso';
 
 import { runScrollBenchmark, type BenchmarkOptions } from './benchmark';
 import { PERF_ENABLED } from './perfConfig';
-import { dump, endSession, reset, startSession } from './perfStore';
+import { dump, endSession, reset, snapshot, startSession } from './perfStore';
 import { startFpsMonitor, stopFpsMonitor } from './scrollFpsMonitor';
 
 export function useCuesheetPerf(virtuosoRef: RefObject<TableVirtuosoHandle | null>): void {
@@ -33,6 +33,7 @@ export function useCuesheetPerf(virtuosoRef: RefObject<TableVirtuosoHandle | nul
       },
       dump,
       reset,
+      getResults: snapshot,
     };
 
     (window as unknown as { __cuesheetPerf?: typeof api }).__cuesheetPerf = api;