perf(agent pane): a streamed chunk re-renders only its row; each frame is parsed once

webviews/src/agent-session/acpmux/App.tsx

@@ -193,6 +193,15 @@ import { createAcpmuxDebug, type AcpmuxDebug } from "./debug";
 import { type AutomationCompose, pageAction } from "./automation";
 import { acpWire } from "./wire";
 import { acpmuxPerf, type MountedSpan } from "./perf";
+import {
+  drawsAsIs,
+  drewStreamingRow,
+  setStreamingRowBlocked,
+  streamingRow,
+  streamingRowBlocked,
+  textOnlyRow,
+  useStreamingRow,
+} from "./streamingRow";
 import { followTrace } from "./followTrace";
 import { ScrollPacing } from "./pacing";
 import { AdaptiveRenderRate, reportScrollPacing } from "./renderPacing";
@@ -2355,6 +2364,7 @@ const RowFrame = React.memo(function RowFrame({
   children: React.ReactNode;
 }) {
   acpmuxPerf.span("row.renders", 1);
+  if (acpmuxPerf.enabled) acpmuxPerf.render(`row:${row.id}`);
   const t = useT();
   const ref = useRef<HTMLElement>(null);
   const [entering] = useState(enter);
@@ -2830,8 +2840,26 @@ function TranscriptContext({
   );
 }
 
+/// The index of the streaming reply's row when `next` differs from `drawn` only in that row's
+/// text (HOT PATH AUDIT area F, row W1), else -1. Every other field compares by identity: the client
+/// keeps the same objects while they do not change (direct.ts `publishedSummary`, `NO_CATALOG`,
+/// subagentFold.ts `views`), so this is O(fields + rows), with no serialization. Drawn only while
+/// the transcript draws that row as the client holds it (`drawsAsIs`) and no view that reads the
+/// pane's rows is open (`streamingRowBlocked`).
+function streamedOnly(drawn: AcpmuxSnapshot, next: AcpmuxSnapshot): number {
+  const before = drawn as unknown as Record<string, unknown>;
+  const after = next as unknown as Record<string, unknown>;
+  const keys = Object.keys(after);
+  if (keys.length !== Object.keys(before).length) return -1;
+  for (const key of keys) if (key !== "rows" && !Object.is(before[key], after[key])) return -1;
+  if (streamingRowBlocked()) return -1;
+  const at = textOnlyRow(drawn.rows, next.rows);
+  return at >= 0 && drawsAsIs(drawn.rows[at]!) ? at : -1;
+}
+
 export function VirtualTranscript({
-  rows,
+  rows: paneRows,
+  streams = false,
   earlier,
   promptCount,
   loadThrough,
@@ -2871,6 +2899,9 @@ export function VirtualTranscript({
   topInset = 0,
 }: {
   rows: readonly AcpmuxRow[];
+  /// The pane's live transcript: it draws the streaming reply's row ahead of the pane's snapshot
+  /// (streamingRow.ts), so a streamed chunk re-renders this transcript and that row, not the pane.
+  streams?: boolean;
   /// The chat's prompts older than `rows` (the snapshot's `earlierPrompts`): the thread rail lays
   /// them out from the first frame, so it never grows or shifts as history pages in.
   earlier?: readonly AcpmuxRow[];
@@ -2963,6 +2994,44 @@ export function VirtualTranscript({
   /// above the composer. 0 for a scroller below its header.
   topInset?: number;
 }) {
+  // The streaming reply's newer text, when the pane has not redrawn for it (streamingRow.ts): that
+  // row only, in place, with its version moved on by the chunks it is ahead.
+  const live = useStreamingRow(streams);
+  const rows = useMemo(() => {
+    if (!streams || !live) return paneRows;
+    for (let index = paneRows.length - 1; index >= 0; index -= 1) {
+      const row = paneRows[index]!;
+      if (row.id !== live.row.id) continue;
+      if (row.text !== live.drawnText) return paneRows;
+      const next = paneRows.slice();
+      const version = row.version + live.ahead;
+      next[index] = { ...row, text: live.row.text, version };
+      // The live line under the reply stays quiet while the reply's text moves (turns.ts
+      // `quietKey` names the reply's id and version): it moves with the newer version here.
+      const stale = `${row.id}:${row.version}`;
+      for (let below = index + 1; below < next.length; below += 1) {
+        const line = next[below]!;
+        if (line.kind === THINKING && line.quietKey === stale)
+          next[below] = { ...line, quietKey: `${row.id}:${version}`, version: line.version + live.ahead };
+      }
+      return next;
+    }
+    return paneRows;
+  }, [paneRows, live, streams]);
+  // What this transcript drew of the pane's streaming reply: the pane takes the fast path only
+  // while that row is drawn as the client holds it.
+  useLayoutEffect(() => {
+    if (!streams) return;
+    for (let index = paneRows.length - 1; index >= 0; index -= 1) {
+      const row = paneRows[index]!;
+      if (row.kind === "assistant" && row.streaming) {
+        drewStreamingRow(row.id, row.text);
+        return;
+      }
+      if (row.kind === "user") break;
+    }
+    drewStreamingRow(undefined, undefined);
+  }, [paneRows, streams]);
   const registry = givenRegistry ?? (readOnly ? readOnlyRegistry : defaultRegistry);
   const t = useT();
   // Debug measurement (acpmuxPerf): off until the first debug call.
@@ -6168,6 +6237,7 @@ const NO_PROJECTS: Project[] = [];
 function AcpmuxPane({ initialChat }: { initialChat?: InitialChat }) {
   // Debug measurement (acpmuxPerf): the pane's own render, before its children.
   const paneRenderStart = acpmuxPerf.enabled ? performance.now() : 0;
+  acpmuxPerf.render("pane");
   const t = useT();
   const { scrollToBottomOnSend, newChatsIn } = useComposerSettings();
   const scrollToBottomOnSendRef = useRef(scrollToBottomOnSend);
@@ -6468,6 +6538,11 @@ function AcpmuxPane({ initialChat }: { initialChat?: InitialChat }) {
         : undefined,
     [artifact, artifactRows, snapshot.sessionId],
   );
+  // An open artifact follows its fence in the pane's rows: chunks draw through the pane meanwhile.
+  useLayoutEffect(() => {
+    setStreamingRowBlocked(artifact !== undefined, "artifact");
+    return () => setStreamingRowBlocked(false, "artifact");
+  }, [artifact]);
   const liveArtifact = liveDocument?.artifact ?? artifact;
   // Render cards by id (cx-ncc.94): a render that names an earlier one's id shows in that card.
   const pages = useMemo(() => renderPages(snapshot.rows), [snapshot.rows]);
@@ -9501,6 +9576,9 @@ function AcpmuxPane({ initialChat }: { initialChat?: InitialChat }) {
     void callNative(QUICK_MESSAGES.dismiss).catch(() => undefined);
   });
   const rowsRef = useRef<readonly AcpmuxRow[]>(initialChat?.snapshot.rows ?? []);
+  /// The direct client's snapshot the pane last drew; a newer one that only grows the streaming
+  /// reply's text goes to that row instead (streamingRow.ts).
+  const drawnSnapshot = useRef<AcpmuxSnapshot | undefined>(undefined);
   /// The newest snapshot, for host requests that read it (pane.context).
   const snapshotRef = useRef<AcpmuxSnapshot | undefined>(initialChat?.snapshot);
   /// The chat menu shows New side chat: the right-click menu then opens one from any turn that forks.
@@ -11042,6 +11120,16 @@ function AcpmuxPane({ initialChat }: { initialChat?: InitialChat }) {
                 showsNewTabRef.current,
               );
             }
+            // Only the streaming reply's text grew: its row draws it (streamingRow.ts); the pane,
+            // the composer and Session Info keep the snapshot they drew.
+            const drawn = drawnSnapshot.current;
+            const at = drawn ? streamedOnly(drawn, next) : -1;
+            if (drawn && at >= 0) {
+              streamingRow.set(next.rows[at]!, drawn.rows[at]!);
+              return;
+            }
+            streamingRow.clear();
+            drawnSnapshot.current = next;
             setSnapshot((previous) => {
               if (
                 next.canHandoff &&
@@ -13438,6 +13526,7 @@ function AcpmuxPane({ initialChat }: { initialChat?: InitialChat }) {
                                                         key={chatPresentation}
                                                         topInset={soloUnder ? tabRow.height : 0}
                                                         rows={transcriptRows}
+                                                        streams={!childRows}
                                                         earlier={!childRows ? snapshot.earlierPrompts : undefined}
                                                         promptCount={!childRows ? snapshot.promptCount : undefined}
                                                         loadThrough={

webviews/src/agent-session/acpmux/Composer.tsx

@@ -1,3 +1,4 @@
+import { acpmuxPerf } from "./perf";
 import React, { useCallback, useEffect, useImperativeHandle, useLayoutEffect, useMemo, useRef, useState } from "react";
 import { createPortal } from "react-dom";
 // Send's iMessage colors (--cmux-messages-blue); build-agent-pane-web.sh inlines the sheet.
@@ -496,6 +497,7 @@ export function Composer({
   placeholder,
   onOpenContext,
 }: Props) {
+  acpmuxPerf.render("composer");
   const t = useT();
   const chrome = useComposerChrome();
   // Off (a queued row's ⋯ menu, queueing.ts), what is sent mid-turn steers on every key.

webviews/src/agent-session/acpmux/conversation/ChatFindScope.tsx

@@ -1,6 +1,7 @@
 // Find in Chat's state (cx-ncc.86): it lives here, under the pane, so a keystroke in the find bar
 // renders the transcript alone, not the composer and header beside it. The pane's page actions
 // reach it through `findRef`.
+import { setStreamingRowBlocked } from "../streamingRow";
 import { type ReactNode, type RefObject, useEffect, useLayoutEffect } from "react";
 import type { AcpmuxRow } from "../model";
 import { type ChatFind, isFindRow, useChatFind } from "./chatFind";
@@ -24,6 +25,12 @@ export function ChatFindScope({
 }) {
   const find = useChatFind(rows);
   findRef.current = find;
+  // While Find is open it searches the pane's rows, so a streamed chunk draws through the pane
+  // (streamingRow.ts) and new text is found as it comes.
+  useLayoutEffect(() => {
+    setStreamingRowBlocked(find.open, "find");
+    return () => setStreamingRowBlocked(false, "find");
+  }, [find.open]);
   // A view without the transcript (New Tab, replay) has no Find; the page actions then do nothing.
   useEffect(
     () => () => {

webviews/src/agent-session/acpmux/direct.ts

@@ -255,6 +255,8 @@ type Reply = {
 };
 type Notification = { method: string; params?: any };
 type Listener = (snapshot: AcpmuxSnapshot) => void;
+/// The snapshot's catalog: one empty array, so equal snapshots compare by identity.
+const NO_CATALOG: AcpmuxSnapshot["catalog"] = Object.freeze([]) as unknown as AcpmuxSnapshot["catalog"];
 
 /// A hint that a harness is likely next, so acpmux can warm its adapter (cmux-tui acpmux lane).
 export const PREWARM_METHOD = "_acpmux/prewarm";
@@ -1745,6 +1747,7 @@ export class AcpmuxDirectClient {
   private sessionEntriesOf?: { sessions: Session[]; entries: AcpmuxSessionEntry[] };
   /// The snapshot summary's `usageStats`, kept while acpmux's numbers are the same (sameUsageStats).
   private summaryUsage?: SessionUsageStats;
+  private publishedSummaryOf?: { summary: unknown; usage: unknown; value: AcpmuxSnapshot["summary"] };
   /// The usage read in flight (refreshUsage).
   private usageRead?: Promise<void>;
   private selectedSessionId?: string;
@@ -3330,9 +3333,11 @@ export class AcpmuxDirectClient {
   }
 
   private receive(raw: string, stale = false): void {
-    this.wire.received(raw);
     // A frame that is not a JSON object is dropped like one that is not JSON (`null` would throw at `in`).
-    const message = parseJsonObjectFrame(raw) as Reply | Notification | null;
+    // Parsed once: the wire log takes the parsed frame.
+    const parsed = parseJsonObjectFrame(raw);
+    this.wire.received(raw, parsed);
+    const message = parsed as Reply | Notification | null;
     if (!message) return;
     if ("id" in message && (typeof message.id === "number" || typeof message.id === "string")) {
       const request = this.pending.get(message.id);
@@ -6738,6 +6743,73 @@ export class AcpmuxDirectClient {
     return entries;
   }
 
+  /// The snapshot's `summary`: the same object while the chat's summary and usage are the same
+  /// objects (every change assigns a new one; `effort` and `limits` are read from the summary), so
+  /// the pane compares it by identity. A streamed chunk changes neither (HOT PATH AUDIT area F,
+  /// row W1; review of #34004: comparing it by its JSON cost O(summary) per chunk).
+  private publishedSummary(
+    summary: Record<string, any> | undefined,
+    effort: any,
+    limits: ReturnType<typeof subsessionLimits>,
+  ): AcpmuxSnapshot["summary"] {
+    const kept = this.publishedSummaryOf;
+    if (kept && kept.summary === summary && kept.usage === this.usage) return kept.value;
+    const value: AcpmuxSnapshot["summary"] = summary
+      ? {
+          sessionId: summary.sessionId,
+          cwd: summary.cwd,
+          turnCount: summary.turnCount,
+          usage: this.usage,
+          contextBreakdown: contextBreakdownOf(summary.contextBreakdown),
+          usageStats: (this.summaryUsage = sameUsageStats(summary.usageStats, this.summaryUsage)),
+          host: text(summary.host),
+          machine: text(summary.machine),
+          peer: text(summary.peer),
+          hostKind: hostKind(summary.hostKind),
+          // acpmux's session detail names the folder's branch `gitBranch` (when the session started).
+          branch: text(summary.branch) ?? text(summary.gitBranch),
+          worktree: text(summary.worktree),
+          title: summary.title,
+          name: summary.name,
+          goal: taskGoal(summary.goal),
+          harness: summary.harness,
+          family: typeof summary.family === "string" ? summary.family : undefined,
+          agentInfo:
+            summary.agentInfo && typeof summary.agentInfo === "object" && !Array.isArray(summary.agentInfo)
+              ? summary.agentInfo
+              : undefined,
+          authSource: text(summary.authSource),
+          baseUrl: text(summary.baseUrl),
+          route: text(summary.route),
+          settingsSources: summary.settingsSources,
+          usageLimits: usageLimitsOf(summary.usageLimits) ?? text(summary.usageLimits),
+          ...(summary.usageLimitsApply === false ? { usageLimitsApply: false } : {}),
+          // acpmux reports `model: null` for a harness with no model (one known only from its config).
+          model: text(summary.model),
+          effort: effort?.currentValue,
+          // `audio` only while the picked model hears it (takesAudio).
+          promptCapabilities: summary.agentCapabilities?.promptCapabilities && {
+            ...summary.agentCapabilities.promptCapabilities,
+            audio: takesAudio(summary.agentCapabilities.promptCapabilities, summary.configOptions),
+          },
+          steering: summary.steering === true,
+          backgroundsWork: summary.backgroundsWork === true,
+          queuePaused: summary.queuePaused === true,
+          ...(text(summary.watching) ? { watching: text(summary.watching) } : {}),
+          ...(Array.isArray(summary.mcpServers) ? { mcpServers: summary.mcpServers } : {}),
+          ...(limits ? { subsessionLimits: limits } : {}),
+          status: summary.status,
+          enforcement: sessionEnforcement(summary.enforcement),
+          modes: summary.modes,
+          configOptions: summary.configOptions,
+          tools: chatTools(summary.tools),
+          toolsMutable: summary.toolsMutable === true,
+        }
+      : undefined;
+    this.publishedSummaryOf = { summary, usage: this.usage, value };
+    return value;
+  }
+
   private emitNow(connection: string): void {
     if (this.closed && !this.replayMode && !(this.fedMode && !this.disposed)) return;
     if (this.snapshotHold) {
@@ -6786,58 +6858,7 @@ export class AcpmuxDirectClient {
       peerStates: this.peerStates,
       peerResources: this.peerResources,
       ...(this.forkedAtSeq !== undefined ? { forkedAtSeq: this.forkedAtSeq } : {}),
-      summary: summary
-        ? {
-            sessionId: summary.sessionId,
-            cwd: summary.cwd,
-            turnCount: summary.turnCount,
-            usage: this.usage,
-            contextBreakdown: contextBreakdownOf(summary.contextBreakdown),
-            usageStats: (this.summaryUsage = sameUsageStats(summary.usageStats, this.summaryUsage)),
-            host: text(summary.host),
-            machine: text(summary.machine),
-            peer: text(summary.peer),
-            hostKind: hostKind(summary.hostKind),
-            // acpmux's session detail names the folder's branch `gitBranch` (when the session started).
-            branch: text(summary.branch) ?? text(summary.gitBranch),
-            worktree: text(summary.worktree),
-            title: summary.title,
-            name: summary.name,
-            goal: taskGoal(summary.goal),
-            harness: summary.harness,
-            family: typeof summary.family === "string" ? summary.family : undefined,
-            agentInfo:
-              summary.agentInfo && typeof summary.agentInfo === "object" && !Array.isArray(summary.agentInfo)
-                ? summary.agentInfo
-                : undefined,
-            authSource: text(summary.authSource),
-            baseUrl: text(summary.baseUrl),
-            route: text(summary.route),
-            settingsSources: summary.settingsSources,
-            usageLimits: usageLimitsOf(summary.usageLimits) ?? text(summary.usageLimits),
-            ...(summary.usageLimitsApply === false ? { usageLimitsApply: false } : {}),
-            // acpmux reports `model: null` for a harness with no model (one known only from its config).
-            model: text(summary.model),
-            effort: effort?.currentValue,
-            // `audio` only while the picked model hears it (takesAudio).
-            promptCapabilities: summary.agentCapabilities?.promptCapabilities && {
-              ...summary.agentCapabilities.promptCapabilities,
-              audio: takesAudio(summary.agentCapabilities.promptCapabilities, summary.configOptions),
-            },
-            steering: summary.steering === true,
-            backgroundsWork: summary.backgroundsWork === true,
-            queuePaused: summary.queuePaused === true,
-            ...(text(summary.watching) ? { watching: text(summary.watching) } : {}),
-            ...(Array.isArray(summary.mcpServers) ? { mcpServers: summary.mcpServers } : {}),
-            ...(limits ? { subsessionLimits: limits } : {}),
-            status: summary.status,
-            enforcement: sessionEnforcement(summary.enforcement),
-            modes: summary.modes,
-            configOptions: summary.configOptions,
-            tools: chatTools(summary.tools),
-            toolsMutable: summary.toolsMutable === true,
-          }
-        : undefined,
+      summary: this.publishedSummary(summary, effort, limits),
       connection,
       origin: this.origin,
       ...(this.listedSessions ? { listedSessions: true as const } : {}),
@@ -6888,7 +6909,7 @@ export class AcpmuxDirectClient {
         : {}),
       secret: this.pendingSecret,
       asks: this.asks.list(),
-      catalog: [],
+      catalog: NO_CATALOG,
       commands: this.commands,
       canLoadOlder: this.seqHistoryLeft() || this.nativeBefore() !== undefined,
       ...(this.shownEarlierPrompts().length ? { earlierPrompts: this.shownEarlierPrompts() } : {}),