Preserve live markdown block newlines

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

@@ -955,6 +955,38 @@ describe("processAgentStreamEvents", () => {
     });
   });
 
+  it("preserves a live block trailing newline after promoting completed markdown blocks", () => {
+    const result = processAgentStreamEvents({
+      events: [
+        makeStreamReducerEvent(
+          makeTimelineEvent(
+            "Done. I added `[TimelineMerge] ...` logging around the suspicious merge",
+          ),
+          238,
+        ),
+        makeStreamReducerEvent(makeTimelineEvent("/reconcile paths.\n\nChanged:\n"), 239),
+        makeStreamReducerEvent(makeTimelineEvent("- [timeline-debug.ts]"), 240),
+      ],
+      currentTail: [],
+      currentHead: [],
+      currentCursor: undefined,
+      currentAgent: null,
+    });
+
+    expect(result.changedTail).toBe(true);
+    expect(result.changedHead).toBe(true);
+    expect(result.tail).toHaveLength(1);
+    expect(result.tail[0]).toMatchObject({
+      kind: "assistant_message",
+      text: "Done. I added `[TimelineMerge] ...` logging around the suspicious merge/reconcile paths.",
+    });
+    expect(result.head).toHaveLength(1);
+    expect(result.head[0]).toMatchObject({
+      kind: "assistant_message",
+      text: "Changed:\n- [timeline-debug.ts]",
+    });
+  });
+
   it("does not promote a markdown block that is still inside an open code fence", () => {
     const result = processAgentStreamEvents({
       events: [

packages/app/src/types/stream.ts

@@ -793,6 +793,10 @@ function createAssistantBlockId(params: { groupId: string; blockIndex: number })
   return `${params.groupId}:block:${params.blockIndex}`;
 }
 
+function getTrailingNewlineSuffix(text: string): string {
+  return /\n+$/.exec(text)?.[0] ?? "";
+}
+
 function getActiveAssistantHeadIndex(head: StreamItem[]): number {
   for (let index = head.length - 1; index >= 0; index -= 1) {
     if (head[index]?.kind === "assistant_message") {
@@ -832,7 +836,7 @@ function promoteCompletedAssistantBlocks(params: { tail: StreamItem[]; head: Str
   const blockGroupId = activeItem.blockGroupId ?? activeItem.id;
   const firstBlockIndex = activeItem.blockIndex ?? 0;
   const completedBlocks = blocks.slice(0, -1);
-  const liveBlock = blocks[blocks.length - 1] ?? "";
+  const liveBlock = `${blocks[blocks.length - 1] ?? ""}${getTrailingNewlineSuffix(activeItem.text)}`;
   const promotedItems = completedBlocks.map<AssistantMessageItem>((block, offset) => ({
     kind: "assistant_message",
     id: createAssistantBlockId({