fix(app): keep line breaks in streamed replies when a chunk ends in indentation (#5577)

While a reply streams, presentation reparses only the growing last Markdown
block: the cached block text plus the new chunk. splitMarkdownBlocks drops a
trailing whitespace-only line, and only trailing "\n"s were added back, so a
reply that paused on "]\n   " cached the block as "]". The next chunk " B -->"
then continued the previous line, joining two lines of a code block. Codex
streams a line break and the next line's indent as separate deltas, so its
Mermaid diagrams hit this and failed to render. History reparses the whole
text, which is why a reload showed the reply correctly.

Keep every trailing blank line the split drops on the last block.

Closes #5456

Co-authored-by: paseo-bot[bot] <266920839+paseo-bot[bot]@users.noreply.github.com>
(cherry picked from commit f4ba16a0b3)

packages/app/src/agent-stream/presentation.test.ts

@@ -210,6 +210,17 @@ describe("stream presentation through installed plugins", () => {
     expect(result.head).toMatchObject([{ text: "```ts\nconst a = 1;\n\nconst b = 2;" }]);
   });
 
+  // Codex streams a line break and the next line's indent as their own chunks.
+  it("keeps a line break when a chunk ends in the next line's indent", () => {
+    const harness = streamHarness();
+    harness.send(assistant("```mermaid\nflowchart TD\n    A[Start] --> B[Review]\n"));
+    harness.send(assistant("   "));
+    const result = harness.send(assistant(" B --> C[Finish]\n```"));
+    expect(result.head).toMatchObject([
+      { text: "```mermaid\nflowchart TD\n    A[Start] --> B[Review]\n    B --> C[Finish]\n```" },
+    ]);
+  });
+
   // One rendering path: a fetched message is the same block group as a streamed one,
   // which is what lets find, scroll-to-message and history reveal address it by id.
   // A link reference definition stays with the paragraph that uses it, so the split

packages/app/src/agent-stream/presentation.ts

@@ -112,8 +112,10 @@ export function createStreamPresentation() {
       const index = prefix.length + offset;
       let blockText = text;
       if (offset === textBlocks.length - 1) {
-        const trailingNewlines = /\n+$/.exec(item.text)?.[0] ?? "";
-        blockText += trailingNewlines;
+        // The split drops trailing blank lines, including a line break followed only
+        // by the next line's indent. Keep them so the next append continues that line.
+        const trailingBlankLines = /\n\s*$/.exec(item.text)?.[0] ?? "";
+        blockText += trailingBlankLines;
       }
       const existing = previous?.[index];
       const id = `${item.id}:block:${index}`;