fix(app): preserve assistant order across outline jumps

Live Markdown block promotion discarded timeline positions, so merging a disjoint history window could sort the final block ahead of earlier blocks. Preserve the source position on every promoted block so timeline merges keep the response intact.

packages/app/src/timeline/session-stream-reducers.test.ts

@@ -2715,6 +2715,49 @@ describe("processTimelineResponse", () => {
     expect(result.sideEffects).not.toContainEqual(expect.objectContaining({ type: "catch_up" }));
   });
 
+  it("keeps live assistant blocks ordered when merging a disjoint prompt-jump window", () => {
+    const live = processAgentStreamEvents({
+      events: [
+        makeStreamReducerEvent(
+          makeAssistantTimelineEvent("First paragraph.\n\nSecond", "assistant-1"),
+          80,
+        ),
+        makeStreamReducerEvent(makeAssistantTimelineEvent(" paragraph.", "assistant-1"), 81),
+        makeStreamReducerEvent(makeTimelineEvent("newest prompt", "user_message"), 82),
+      ],
+      currentTail: [],
+      currentHead: [],
+      currentCursor: undefined,
+    });
+    expect(getAssistantTexts([...live.tail, ...live.head])).toEqual([
+      "First paragraph.",
+      "Second paragraph.",
+    ]);
+
+    const result = processTimelineResponse({
+      ...baseTimelineInput,
+      currentTail: live.tail,
+      currentHead: live.head,
+      currentCursor: live.cursor ?? undefined,
+      payload: {
+        ...baseTimelineInput.payload,
+        direction: "before",
+        mergeWindow: true,
+        epoch: "epoch-1",
+        startCursor: { seq: 1 },
+        endCursor: { seq: 40 },
+        hasOlder: false,
+        hasNewer: true,
+        entries: [makeTimelineEntry(1, "oldest prompt", "user_message")],
+      },
+    });
+
+    expect(getAssistantTexts([...result.tail, ...result.head])).toEqual([
+      "First paragraph.",
+      "Second paragraph.",
+    ]);
+  });
+
   it("fills the gap between a retained prompt window and the contiguous tail", () => {
     const currentTail: StreamItem[] = [
       {

packages/app/src/types/stream.test.ts

@@ -616,6 +616,30 @@ describe("stream reducer canonical tool calls", () => {
     expect(new Set(messages.map((message) => message.id)).size).toBe(messages.length);
   });
 
+  it("keeps the timeline position on every promoted assistant block", () => {
+    const timelineCursor = { epoch: "epoch-1", seq: 42 };
+    const result = applyStreamEvent({
+      tail: [],
+      head: [],
+      event: assistantTimeline("First paragraph.\n\nSecond paragraph.", undefined, "message-1"),
+      timestamp: new Date("2025-01-01T10:02:00Z"),
+      timelineCursor,
+    });
+
+    const messages = [...result.tail, ...result.head].filter(
+      (item): item is Extract<StreamItem, { kind: "assistant_message" }> =>
+        item.kind === "assistant_message",
+    );
+    expect(messages.map((message) => message.text)).toEqual([
+      "First paragraph.",
+      "Second paragraph.",
+    ]);
+    expect(messages.map((message) => message.timelineCursor)).toEqual([
+      timelineCursor,
+      timelineCursor,
+    ]);
+  });
+
   it("preserves old assistant merge behavior when message ids are absent", () => {
     const state = hydrateStreamState([
       {

packages/app/src/types/stream.ts

@@ -1573,16 +1573,14 @@ function promoteCompletedAssistantBlocks(params: { tail: StreamItem[]; head: Str
   const completedBlocks = blocks.slice(0, -1);
   const liveBlock = `${blocks[blocks.length - 1] ?? ""}${getTrailingNewlineSuffix(activeItem.text)}`;
   const promotedItems = completedBlocks.map<AssistantMessageItem>((block, offset) => ({
-    kind: "assistant_message",
+    ...activeItem,
     id: createAssistantBlockId({
       groupId: blockGroupId,
       blockIndex: firstBlockIndex + offset,
     }),
-    ...(activeItem.messageId ? { messageId: activeItem.messageId } : {}),
     blockGroupId,
     blockIndex: firstBlockIndex + offset,
     text: block,
-    timestamp: activeItem.timestamp,
   }));
 
   const nextTail = flushHeadToTail(params.tail, promotedItems);