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.

6 files changed, 320 insertions, 110 deletions

diff --git a/src/features/chat/store.svelte.ts b/src/features/chat/store.svelte.ts
index 45f1ed1..7f8e113 100644
--- a/src/features/chat/store.svelte.ts
+++ b/src/features/chat/store.svelte.ts
@@ -69,6 +69,15 @@ export interface ChatStoreDependencies {
    * `reportError` → full-screen error modal. Absent → errors are logged only.
    */
   readonly onError?: (context: string, err: unknown) => void;
+  /**
+   * Schedules a deferred flush of buffered live events (e.g. once per animation
+   * frame). When provided, `handleDelta` QUEUES events and folds the whole batch
+   * in one pass on the scheduled callback — so a burst (the late-join replay on
+   * opening a conversation mid-turn, or a fast stream on a slow device) costs ONE
+   * derivation + render instead of one per WS frame. Absent → every event is
+   * folded synchronously (the original behavior; used by tests).
+   */
+  readonly scheduleFlush?: (flush: () => void) => void;
 }
 
 export interface ChatStore {
@@ -195,6 +204,63 @@ export function createChatStore(deps: ChatStoreDependencies): ChatStore {
 
   let chatLimit = normalizeChatLimit(deps.chatLimit);
 
+  // Memoized projections, keyed by state-object identity (the reducers are
+  // immutable, so identity == content). The getters are read several times per
+  // render (ChatPane + ChatView); returning the SAME array for an unchanged
+  // state keeps downstream `$derived`s (grouping, metrics placement) from
+  // recomputing on every read. Plain closures, not `$derived` — a store factory
+  // may run inside a component effect, which would own (and later destroy) it.
+  const memo = <S, R>(fn: (s: S) => R): ((s: S) => R) => {
+    let lastIn: S | undefined;
+    let lastOut: R | undefined;
+    return (s) => {
+      if (lastOut === undefined || s !== lastIn) {
+        lastIn = s;
+        lastOut = fn(s);
+      }
+      return lastOut;
+    };
+  };
+  const chunksOf = memo(selectChunks);
+  const turnMetricsOf = memo(selectOrderedTurnMetrics);
+
+  // Buffered live events awaiting a scheduled flush (see `scheduleFlush`).
+  let pendingEvents: ChatDeltaMessage["event"][] = [];
+  let flushScheduled = false;
+
+  /** Fold every buffered event in one pass, then trim + sync once. */
+  function flushEvents(): void {
+    flushScheduled = false;
+    if (disposed || pendingEvents.length === 0) return;
+    const batch = pendingEvents;
+    pendingEvents = [];
+    applyEvents(batch);
+  }
+
+  function applyEvents(events: readonly ChatDeltaMessage["event"][]): void {
+    let t = transcript;
+    let m = metrics;
+    let sealed = false;
+    let failed = false;
+    for (const event of events) {
+      t = foldEvent(t, event);
+      m = foldMetricsEvent(m, event);
+      if (t.sealedTurnId !== null) sealed = true;
+      else if (event.type === "error") failed = true;
+    }
+    transcript = t;
+    metrics = m;
+    maybeTrim();
+    if (sealed) {
+      void syncTail();
+      void syncMetrics();
+    } else if (failed) {
+      // A turn that FAILS never seals, but the backend still persists the
+      // metrics it accumulated — pull them so the turn gets its row.
+      void syncMetrics();
+    }
+  }
+
   /**
    * Enforce the chat limit after a transcript mutation — unless the injected
    * gate says the reader is scrolled up (then defer; the next mutation retries
@@ -295,10 +361,10 @@ export function createChatStore(deps: ChatStoreDependencies): ChatStore {
       return selectMessages(transcript);
     },
     get chunks(): readonly RenderedChunk[] {
-      return selectChunks(transcript);
+      return chunksOf(transcript);
     },
     get turnMetrics(): readonly TurnMetricsEntry[] {
-      return selectOrderedTurnMetrics(metrics);
+      return turnMetricsOf(metrics);
     },
     get currentContextSize(): number | undefined {
       return selectCurrentContextSize(metrics);
@@ -336,20 +402,19 @@ export function createChatStore(deps: ChatStoreDependencies): ChatStore {
       if (msg.event.conversationId !== deps.conversationId) {
         return;
       }
-      transcript = foldEvent(transcript, msg.event);
-      metrics = foldMetricsEvent(metrics, msg.event);
-      maybeTrim();
-      if (transcript.sealedTurnId !== null) {
-        void syncTail();
-        void syncMetrics();
-      } else if (msg.event.type === "error") {
-        // A turn that FAILS never seals, but the backend still persists the
-        // metrics it accumulated — pull them so the turn gets its row.
-        void syncMetrics();
+      if (deps.scheduleFlush === undefined) {
+        applyEvents([msg.event]);
+        return;
+      }
+      pendingEvents.push(msg.event);
+      if (!flushScheduled) {
+        flushScheduled = true;
+        deps.scheduleFlush(flushEvents);
       }
     },
 
     send(text: string, images?: readonly ImageInput[]): void {
+      flushEvents();
       transcript = appendUserMessage(transcript, text, images);
       maybeTrim();
       const msg: ChatSendMessage = {
@@ -444,6 +509,7 @@ export function createChatStore(deps: ChatStoreDependencies): ChatStore {
       // A turn may have sealed while we were disconnected (missed `turn-sealed`):
       // clear the now-stale spinner BEFORE re-subscribing, so a finished turn
       // doesn't spin forever. A still-running turn's replay re-asserts it.
+      flushEvents();
       transcript = clearGenerating(transcript);
       void syncTail();
       void syncMetrics();
@@ -451,6 +517,7 @@ export function createChatStore(deps: ChatStoreDependencies): ChatStore {
 
     dispose(): void {
       disposed = true;
+      pendingEvents = [];
     },
   };
 }
diff --git a/src/features/chat/store.test.ts b/src/features/chat/store.test.ts
index aa5560f..f9b4b38 100644
--- a/src/features/chat/store.test.ts
+++ b/src/features/chat/store.test.ts
@@ -1706,3 +1706,86 @@ describe("createChatStore", () => {
     expect(metricsSync.calls).toHaveLength(0);
   });
 });
+
+describe("createChatStore — batched delta application (scheduleFlush)", () => {
+  function setup() {
+    const scheduled: (() => void)[] = [];
+    const historySync = createFakeHistorySync();
+    const store = createChatStore({
+      conversationId: CONV_ID,
+      transport: createFakeTransport().impl,
+      historySync: historySync.impl,
+      metricsSync: createFakeMetricsSync().impl,
+      cache: createFakeCache().impl,
+      scheduleFlush: (flush) => scheduled.push(flush),
+    });
+    const runScheduled = (): void => {
+      for (const f of scheduled.splice(0)) f();
+    };
+    return { store, scheduled, runScheduled, historySync };
+  }
+  const text = (delta: string) =>
+    deltaEvent({ type: "text-delta", conversationId: CONV_ID, turnId: "t1", delta });
+
+  it("queues a burst and applies it in ONE scheduled flush", () => {
+    const { store, scheduled, runScheduled } = setup();
+    store.handleDelta(deltaEvent({ type: "turn-start", conversationId: CONV_ID, turnId: "t1" }));
+    for (let i = 0; i < 500; i++) store.handleDelta(text("x"));
+
+    expect(scheduled).toHaveLength(1);
+    expect(store.chunks).toHaveLength(0);
+
+    runScheduled();
+    expect(store.chunks).toHaveLength(1);
+    expect((store.chunks[0]?.chunk as { text: string }).text).toBe("x".repeat(500));
+    expect(store.generating).toBe(true);
+    store.dispose();
+  });
+
+  it("schedules a new flush for events arriving after a flush", () => {
+    const { store, scheduled, runScheduled } = setup();
+    store.handleDelta(text("a"));
+    runScheduled();
+    store.handleDelta(text("b"));
+    expect(scheduled).toHaveLength(1);
+    runScheduled();
+    expect((store.chunks[0]?.chunk as { text: string }).text).toBe("ab");
+    store.dispose();
+  });
+
+  it("a sealed turn in the batch still triggers a history sync", async () => {
+    const { store, runScheduled, historySync } = setup();
+    store.handleDelta(deltaEvent({ type: "turn-start", conversationId: CONV_ID, turnId: "t1" }));
+    store.handleDelta(text("done"));
+    store.handleDelta(deltaEvent({ type: "turn-sealed", conversationId: CONV_ID, turnId: "t1" }));
+    expect(historySync.calls).toHaveLength(0);
+    runScheduled();
+    await vi.waitFor(() => expect(historySync.calls.length).toBeGreaterThan(0));
+    store.dispose();
+  });
+
+  it("send() flushes queued events first so the user echo lands after them", () => {
+    const { store } = setup();
+    store.handleDelta(text("earlier reply"));
+    store.send("next question");
+    const roles = store.chunks.map((c) => c.role);
+    expect(roles).toEqual(["assistant", "user"]);
+    store.dispose();
+  });
+
+  it("chunks getter returns a stable array for an unchanged transcript", () => {
+    const { store, runScheduled } = setup();
+    store.handleDelta(text("a"));
+    runScheduled();
+    expect(store.chunks).toBe(store.chunks);
+    store.dispose();
+  });
+
+  it("a flush after dispose is a no-op", () => {
+    const { store, scheduled } = setup();
+    store.handleDelta(text("a"));
+    store.dispose();
+    for (const f of scheduled) f();
+    expect(store.chunks).toHaveLength(0);
+  });
+});
diff --git a/src/features/chat/ui.test.ts b/src/features/chat/ui.test.ts
index 189be2e..3c7fd0d 100644
--- a/src/features/chat/ui.test.ts
+++ b/src/features/chat/ui.test.ts
@@ -10,6 +10,14 @@ import Composer from "./ui/Composer.svelte";
 import ModelSelector from "./ui/ModelSelector.svelte";
 import ReasoningEffortSelector from "./ui/ReasoningEffortSelector.svelte";
 
+/** Collapsed card bodies render lazily — open every card so its body mounts. */
+async function openAllCollapses(container: HTMLElement): Promise<void> {
+  const boxes = container.querySelectorAll<HTMLInputElement>('.collapse > input[type="checkbox"]');
+  for (const box of boxes) {
+    if (!box.checked) await userEvent.click(box);
+  }
+}
+
 describe("ChatView", () => {
   it("renders a message's text chunk", () => {
     const chunks: RenderedChunk[] = [
@@ -82,7 +90,7 @@ describe("ChatView", () => {
     expect(screen.queryByRole("button", { name: /show earlier/i })).not.toBeInTheDocument();
   });
 
-  it("renders tool-call chunks", () => {
+  it("renders tool-call chunks", async () => {
     const chunks: RenderedChunk[] = [
       {
         seq: 1,
@@ -97,7 +105,8 @@ describe("ChatView", () => {
       },
     ];
 
-    render(ChatView, { props: { chunks } });
+    const { container } = render(ChatView, { props: { chunks } });
+    await openAllCollapses(container);
 
     expect(screen.getByText("read_file")).toBeInTheDocument();
     const pre = screen.getByText((content, element) => {
@@ -106,7 +115,7 @@ describe("ChatView", () => {
     expect(pre).toBeInTheDocument();
   });
 
-  it("renders tool-result chunks", () => {
+  it("renders tool-result chunks", async () => {
     const chunks: RenderedChunk[] = [
       {
         seq: 1,
@@ -122,7 +131,8 @@ describe("ChatView", () => {
       },
     ];
 
-    render(ChatView, { props: { chunks } });
+    const { container } = render(ChatView, { props: { chunks } });
+    await openAllCollapses(container);
 
     expect(screen.getByText("read_file")).toBeInTheDocument();
     expect(screen.getByText("file contents here")).toBeInTheDocument();
@@ -200,7 +210,7 @@ describe("ChatView", () => {
     expect(log.children).toHaveLength(0);
   });
 
-  it("groups batched tool calls (shared stepId) into one DaisyUI list", () => {
+  it("groups batched tool calls (shared stepId) into one DaisyUI list", async () => {
     const chunks: RenderedChunk[] = [
       {
         seq: 1,
@@ -242,6 +252,7 @@ describe("ChatView", () => {
     ];
 
     const { container } = render(ChatView, { props: { chunks } });
+    await openAllCollapses(container);
 
     // Batched calls render as collapsible cards (one per call), not a list.
     const collapses = container.querySelectorAll(".collapse");
@@ -735,7 +746,7 @@ describe("ChatView", () => {
     expect(container.querySelector("img")?.getAttribute("src")).toBe(url);
   });
 
-  it("renders a consult_vision tool call/result like any other tool", () => {
+  it("renders a consult_vision tool call/result like any other tool", async () => {
     // read_image is GONE — replaced by consult_vision (opens a vision-model
     // conversation, attaches the image + question, returns the answer). It is
     // a normal tool call: rendered generically by toolName.
@@ -765,7 +776,8 @@ describe("ChatView", () => {
       },
     ];
 
-    render(ChatView, { props: { chunks } });
+    const { container } = render(ChatView, { props: { chunks } });
+    await openAllCollapses(container);
 
     expect(screen.getAllByText("consult_vision").length).toBeGreaterThan(0);
     expect(screen.getByText("a red square on a white background")).toBeInTheDocument();
diff --git a/src/features/chat/ui/ChatView.svelte b/src/features/chat/ui/ChatView.svelte
index 8f153bf..5e4513c 100644
--- a/src/features/chat/ui/ChatView.svelte
+++ b/src/features/chat/ui/ChatView.svelte
@@ -10,6 +10,7 @@
     type TurnMetricsEntry,
   } from "../../../core/metrics";
   import { Markdown } from "../../markdown";
+  import LazyCollapse from "./LazyCollapse.svelte";
 
   const badgeClass = {
     success: "badge-success",
@@ -102,6 +103,22 @@
   });
 </script>
 
+{#snippet wrench()}
+  <svg
+    class="h-4 w-4 opacity-60"
+    xmlns="http://www.w3.org/2000/svg"
+    viewBox="0 0 24 24"
+    fill="none"
+    stroke="currentColor"
+    stroke-width="2"
+    stroke-linecap="round"
+    stroke-linejoin="round"
+    ><path
+      d="M14.7 6.3a1 1 0 0 0 0 1.4l1.6 1.6a1 1 0 0 0 1.4 0l3.77-3.77a6 6 0 0 1-7.94 7.94l-6.91 6.91a2.12 2.12 0 0 1-3-3l6.91-6.91a6 6 0 0 1 7.94-7.94l-3.76 3.76z"
+    /></svg
+  >
+{/snippet}
+
 {#snippet chunkRow(rendered: RenderedChunk)}
   {#if rendered.role === "user"}
     <!-- User: a speech bubble, left-aligned. A user message may be multi-chunk
@@ -130,18 +147,15 @@
          while generating, then "Thoughts" with no dots once complete. -->
     <div class="chat chat-start [&>.chat-bubble]:max-w-5xl [&>.chat-bubble]:p-0">
       <div class="chat-bubble w-full bg-transparent">
-        <div class="collapse w-full rounded-box bg-base-200 text-sm">
-          <input type="checkbox" aria-label="Toggle thoughts" />
-          <div class="collapse-title flex min-h-0 items-center gap-2 py-2 font-medium">
+        <LazyCollapse label="Toggle thoughts">
+          {#snippet title()}
             <span>{rendered.streaming ? "Thinking" : "Thoughts"}</span>
             {#if rendered.streaming}
               <span class="loading loading-dots loading-sm" aria-label="Generating"></span>
             {/if}
-          </div>
-          <div class="collapse-content">
-            <p class="whitespace-pre-wrap">{rendered.chunk.text}</p>
-          </div>
-        </div>
+          {/snippet}
+          <p class="whitespace-pre-wrap">{rendered.chunk.type === "thinking" ? rendered.chunk.text : ""}</p>
+        </LazyCollapse>
       </div>
     </div>
   {:else if rendered.chunk.type === "tool-call" || rendered.chunk.type === "tool-result"}
@@ -151,61 +165,26 @@
     <div class="chat chat-start [&>.chat-bubble]:max-w-5xl [&>.chat-bubble]:p-0">
       <div class="chat-bubble w-full bg-transparent">
         {#if rendered.chunk.type === "tool-call"}
-          <div class="collapse w-full rounded-box bg-base-200 text-sm">
-            <input type="checkbox" aria-label="Toggle tool call" />
-            <div class="collapse-title flex min-h-0 items-center gap-2 py-2 font-medium">
-              <svg
-                class="h-4 w-4 opacity-60"
-                xmlns="http://www.w3.org/2000/svg"
-                viewBox="0 0 24 24"
-                fill="none"
-                stroke="currentColor"
-                stroke-width="2"
-                stroke-linecap="round"
-                stroke-linejoin="round"
-                ><path
-                  d="M14.7 6.3a1 1 0 0 0 0 1.4l1.6 1.6a1 1 0 0 0 1.4 0l3.77-3.77a6 6 0 0 1-7.94 7.94l-6.91 6.91a2.12 2.12 0 0 1-3-3l6.91-6.91a6 6 0 0 1 7.94-7.94l-3.76 3.76z"
-                /></svg
-              >
-              <span>{rendered.chunk.toolName}</span>
-            </div>
-            <div class="collapse-content">
-              <pre class="overflow-x-auto text-xs">{JSON.stringify(
-                  rendered.chunk.input,
-                  null,
-                  2,
-                )}</pre>
-            </div>
-          </div>
+          {@const call = rendered.chunk}
+          <LazyCollapse label="Toggle tool call">
+            {#snippet title()}
+              {@render wrench()}
+              <span>{call.toolName}</span>
+            {/snippet}
+            <pre class="overflow-x-auto text-xs">{JSON.stringify(call.input, null, 2)}</pre>
+          </LazyCollapse>
         {:else}
-          <div class="collapse w-full rounded-box bg-base-200 text-sm">
-            <input type="checkbox" aria-label="Toggle tool result" />
-            <div
-              class="collapse-title flex min-h-0 items-center gap-2 py-2 font-medium"
-              class:text-error={rendered.chunk.isError}
-            >
-              <svg
-                class="h-4 w-4 opacity-60"
-                xmlns="http://www.w3.org/2000/svg"
-                viewBox="0 0 24 24"
-                fill="none"
-                stroke="currentColor"
-                stroke-width="2"
-                stroke-linecap="round"
-                stroke-linejoin="round"
-                ><path
-                  d="M14.7 6.3a1 1 0 0 0 0 1.4l1.6 1.6a1 1 0 0 0 1.4 0l3.77-3.77a6 6 0 0 1-7.94 7.94l-6.91 6.91a2.12 2.12 0 0 1-3-3l6.91-6.91a6 6 0 0 1 7.94-7.94l-3.76 3.76z"
-                /></svg
-              >
-              <span>{rendered.chunk.toolName}</span>
-              {#if rendered.chunk.isError}
+          {@const res = rendered.chunk}
+          <LazyCollapse label="Toggle tool result" titleClass={res.isError ? "text-error" : ""}>
+            {#snippet title()}
+              {@render wrench()}
+              <span>{res.toolName}</span>
+              {#if res.isError}
                 <span class="badge badge-error badge-xs">error</span>
               {/if}
-            </div>
-            <div class="collapse-content">
-              <pre class="max-h-96 overflow-auto text-xs">{rendered.chunk.content}</pre>
-            </div>
-          </div>
+            {/snippet}
+            <pre class="max-h-96 overflow-auto text-xs">{res.content}</pre>
+          </LazyCollapse>
         {/if}
       </div>
     </div>
@@ -246,6 +225,9 @@
     </div>
   {/if}
   {#each keyedRows as { row, key } (key)}
+    <!-- `.chat-row`: content-visibility:auto — the browser skips layout/paint
+         for rows outside the viewport (see app.css). -->
+    <div class="chat-row">
     {#if row.kind === "step-metrics"}
       {@const sv = viewStepMetrics(row.step, row.index)}
       {@const stepCache = viewCacheRate(row.step.usage)}
@@ -311,47 +293,29 @@
         <div class="chat-bubble w-full bg-transparent">
           <div class="flex flex-col gap-1">
             {#each row.group.entries as entry, entryIndex (entryIndex)}
-              <div class="collapse w-full rounded-box bg-base-200 text-sm">
-                <input type="checkbox" aria-label="Toggle tool call" />
-                <div class="collapse-title flex min-h-0 items-center gap-2 py-2 font-medium">
-                  <svg
-                    class="h-4 w-4 opacity-60"
-                    xmlns="http://www.w3.org/2000/svg"
-                    viewBox="0 0 24 24"
-                    fill="none"
-                    stroke="currentColor"
-                    stroke-width="2"
-                    stroke-linecap="round"
-                    stroke-linejoin="round"
-                    ><path
-                      d="M14.7 6.3a1 1 0 0 0 0 1.4l1.6 1.6a1 1 0 0 0 1.4 0l3.77-3.77a6 6 0 0 1-7.94 7.94l-6.91 6.91a2.12 2.12 0 0 1-3-3l6.91-6.91a6 6 0 0 1 7.94-7.94l-3.76 3.76z"
-                    /></svg
-                  >
+              <LazyCollapse label="Toggle tool call">
+                {#snippet title()}
+                  {@render wrench()}
                   <span>{entry.call.toolName}</span>
                   {#if entry.result?.isError}
                     <span class="badge badge-error badge-xs">error</span>
                   {:else if entry.result === null}
                     <span class="loading loading-spinner loading-xs" aria-label="Running"></span>
                   {/if}
-                </div>
-                <div class="collapse-content">
-                  <pre class="overflow-x-auto text-xs">{JSON.stringify(
-                      entry.call.input,
-                      null,
-                      2,
-                    )}</pre>
-                  {#if entry.result}
-                    <pre
-                      class="mt-1 max-h-96 overflow-auto text-xs"
-                      class:text-error={entry.result.isError}>{entry.result.content}</pre>
-                  {/if}
-                </div>
-              </div>
+                {/snippet}
+                <pre class="overflow-x-auto text-xs">{JSON.stringify(entry.call.input, null, 2)}</pre>
+                {#if entry.result}
+                  <pre
+                    class="mt-1 max-h-96 overflow-auto text-xs"
+                    class:text-error={entry.result.isError}>{entry.result.content}</pre>
+                {/if}
+              </LazyCollapse>
             {/each}
           </div>
         </div>
       </div>
     {/if}
+    </div>
   {/each}
   {#if providerRetry}
     {@const rv = viewProviderRetry(providerRetry)}
diff --git a/src/features/chat/ui/ChatView.test.ts b/src/features/chat/ui/ChatView.test.ts
index eb807c1..fcac312 100644
--- a/src/features/chat/ui/ChatView.test.ts
+++ b/src/features/chat/ui/ChatView.test.ts
@@ -1,16 +1,25 @@
 import type { StepId } from "@dispatch/wire";
 import { render, screen } from "@testing-library/svelte";
+import userEvent from "@testing-library/user-event";
 import { describe, expect, it } from "vitest";
 import type { RenderedChunk } from "../../../core/chunks";
 import ChatView from "./ChatView.svelte";
 
+/** Collapsed card bodies render lazily — open every card so its body mounts. */
+async function openAllCollapses(container: HTMLElement): Promise<void> {
+  const boxes = container.querySelectorAll<HTMLInputElement>('.collapse > input[type="checkbox"]');
+  for (const box of boxes) {
+    if (!box.checked) await userEvent.click(box);
+  }
+}
+
 // Defensive rendering: a conversation transcript can contain duplicate
 // `toolCallId`s (provider double-emission, persisted — e.g. PII_REDACTED_CONVERSATION_SHORT_01). The
 // grouped render path MUST dedupe them so the keyed {#each} never throws
 // Svelte `each_key_duplicate` (which aborts the reactive flush and makes the
 // tab appear dead). See fix-toolcall-duplicate-keys plan Part 1.
 describe("ChatView — duplicate toolCallIds", () => {
-  it("renders a tool batch with duplicated toolCallIds without throwing, showing one card per unique call", () => {
+  it("renders a tool batch with duplicated toolCallIds without throwing, showing one card per unique call", async () => {
     const chunks: RenderedChunk[] = [
       {
         seq: 1,
@@ -117,6 +126,7 @@ describe("ChatView — duplicate toolCallIds", () => {
     ];
 
     const { container } = render(ChatView, { props: { chunks } });
+    await openAllCollapses(container);
 
     // No each_key_duplicate crash. Batched calls render as collapsible cards —
     // exactly ONE per unique toolCallId (deduped), not 4.
@@ -130,3 +140,33 @@ describe("ChatView — duplicate toolCallIds", () => {
     expect(screen.getByText("contents-of-b")).toBeInTheDocument();
   });
 });
+
+describe("ChatView — lazy collapse bodies", () => {
+  it("does not mount a collapsed card's body until it is first opened, then keeps it", async () => {
+    const chunks: RenderedChunk[] = [
+      {
+        seq: 1,
+        role: "tool",
+        chunk: {
+          type: "tool-result",
+          toolCallId: "x",
+          toolName: "run_shell",
+          content: "HUGE-OUTPUT",
+          isError: false,
+        },
+        provisional: false,
+      },
+    ];
+    const { container } = render(ChatView, { props: { chunks } });
+    expect(screen.getByText("run_shell")).toBeInTheDocument();
+    expect(screen.queryByText("HUGE-OUTPUT")).toBeNull();
+
+    const box = screen.getByRole("checkbox", { name: "Toggle tool result" });
+    await userEvent.click(box);
+    expect(screen.getByText("HUGE-OUTPUT")).toBeInTheDocument();
+
+    await userEvent.click(box); // close: body stays mounted (close animation)
+    expect(screen.getByText("HUGE-OUTPUT")).toBeInTheDocument();
+    expect(container.querySelectorAll(".chat-row")).toHaveLength(1);
+  });
+});
diff --git a/src/features/chat/ui/LazyCollapse.svelte b/src/features/chat/ui/LazyCollapse.svelte
new file mode 100644
index 0000000..0f66451
--- /dev/null
+++ b/src/features/chat/ui/LazyCollapse.svelte
@@ -0,0 +1,44 @@
+<script lang="ts">
+  import type { Snippet } from "svelte";
+
+  /**
+   * A daisyUI checkbox collapse whose BODY is rendered lazily: nothing inside
+   * `.collapse-content` is created until the user first opens it. A long turn
+   * has hundreds of collapsed tool/thinking cards, each holding a full JSON
+   * input / tool output / thought — building all of that DOM up front (only to
+   * be hidden) dominated first paint on slow devices. Once opened the body
+   * stays mounted, so the close animation and any inner scroll state work as
+   * before. Same markup + classes as the inline collapses it replaces.
+   */
+  let {
+    label,
+    titleClass = "",
+    title,
+    children,
+  }: {
+    /** aria-label of the toggle checkbox. */
+    label: string;
+    /** Extra classes on the `.collapse-title` row. */
+    titleClass?: string;
+    title: Snippet;
+    children: Snippet;
+  } = $props();
+
+  let open = $state(false);
+  let everOpened = $state(false);
+  $effect(() => {
+    if (open) everOpened = true;
+  });
+</script>
+
+<div class="collapse w-full rounded-box bg-base-200 text-sm">
+  <input type="checkbox" aria-label={label} bind:checked={open} />
+  <div class="collapse-title flex min-h-0 items-center gap-2 py-2 font-medium {titleClass}">
+    {@render title()}
+  </div>
+  <div class="collapse-content">
+    {#if open || everOpened}
+      {@render children()}
+    {/if}
+  </div>
+</div>