fix(cli): re-render assistant messages after streaming to fix fenced code disappearance (#3293)

Adds a full `Markdown.update()` pass once streaming finishes to work
around Textualize/textual#6518, where incremental
`MarkdownFence._update_from_block` refreshes the visible `Label` but
leaves `_highlighted_code` pinned to the first chunk. Any later
recompose (click, focus change, theme update) re-yields the stale value
and wrapped fenced-code bodies vanish. A full re-parse rebuilds every
fence with correct internal state.

libs/cli/deepagents_cli/widgets/messages.py

@@ -584,7 +584,13 @@ class AssistantMessage(_TimestampClickMixin, Vertical):
     """Widget displaying an assistant message with markdown support.
 
     Uses MarkdownStream for smoother streaming instead of re-rendering
-    the full content on each update.
+    the full content on each update. Once a stream finishes, the message
+    is re-rendered from the complete source via `Markdown.update()` to
+    work around Textualize/textual#6518: `MarkdownFence._update_from_block`
+    refreshes the visible `Label` but leaves `_highlighted_code` pinned to
+    the first chunk, so any later recompose (click, focus change, theme
+    update) re-yields the stale value and wrapped fenced-code bodies vanish.
+    A full re-parse rebuilds every fence with correct internal state.
     """
 
     DEFAULT_CSS = """
@@ -670,24 +676,28 @@ class AssistantMessage(_TimestampClickMixin, Vertical):
     async def write_initial_content(self) -> None:
         """Write initial content if provided at construction time."""
         if self._content:
-            stream = self._ensure_stream()
-            await stream.write(self._content)
+            await self._get_markdown().update(self._content)
 
     async def stop_stream(self) -> None:
         """Stop the streaming and finalize the content."""
         if self._stream is not None:
             await self._stream.stop()
             self._stream = None
+            await self._get_markdown().update(self._content)
 
     async def set_content(self, content: str) -> None:
         """Set the full message content.
 
-        This stops any active stream and sets content directly.
+        Cancels any active stream and renders the new content with a
+        single `Markdown.update()` (avoiding a redundant intermediate
+        update of the in-flight content).
 
         Args:
             content: The markdown content to display
         """
-        await self.stop_stream()
+        if self._stream is not None:
+            await self._stream.stop()
+            self._stream = None
         self._content = content
         if self._markdown:
             await self._markdown.update(content)

libs/cli/tests/unit_tests/test_messages.py

@@ -1,6 +1,6 @@
 """Unit tests for message widgets markup safety."""
 
-from unittest.mock import MagicMock, patch
+from unittest.mock import AsyncMock, MagicMock, patch
 
 import pytest
 from rich.style import Style
@@ -208,16 +208,10 @@ class TestMutedRichMarkdown:
 
     def test_strips_heading_and_table_colors(self) -> None:
         """Muted wrapper should drop magenta/cyan from headings and tables."""
-        from rich.markdown import Markdown as RichMarkdown
-
-        baseline = self._render(RichMarkdown(self._DOC))
         muted = self._render(_MutedRichMarkdown(self._DOC))
 
-        # Default Rich theme paints `markdown.h3` magenta (ANSI code 35)
-        # and `markdown.table.*` cyan (ANSI code 36).
-        assert "\x1b[1;35m" in baseline
-        assert "\x1b[36m" in baseline
-
+        # Some Rich versions paint headings/tables magenta/cyan by default.
+        # The wrapper should not emit those hues regardless of Rich's baseline.
         assert "\x1b[35m" not in muted
         assert ";35m" not in muted
         assert "\x1b[36m" not in muted
@@ -248,6 +242,59 @@ class TestMutedRichMarkdown:
         assert "body" in rendered
 
 
+class TestAssistantMessageMarkdownRendering:
+    """Tests for assistant markdown render lifecycle."""
+
+    async def test_write_initial_content_uses_full_markdown_update(self) -> None:
+        """Preloaded assistant messages should not keep stream state alive."""
+        msg = AssistantMessage("```python\nprint('hello')\n```")
+        markdown = MagicMock()
+        markdown.update = AsyncMock()
+        msg._markdown = markdown
+
+        await msg.write_initial_content()
+
+        markdown.update.assert_awaited_once_with("```python\nprint('hello')\n```")
+        assert msg._stream is None
+
+    async def test_stop_stream_rerenders_complete_markdown(self) -> None:
+        """Completed streams should get a full parse after incremental updates."""
+        msg = AssistantMessage()
+        markdown = MagicMock()
+        markdown.update = AsyncMock()
+        stream = MagicMock()
+        stream.stop = AsyncMock()
+        msg._markdown = markdown
+        msg._stream = stream
+        msg._content = "```python\nprint('wrapped text')\n```"
+
+        await msg.stop_stream()
+
+        stream.stop.assert_awaited_once_with()
+        markdown.update.assert_awaited_once_with(
+            "```python\nprint('wrapped text')\n```"
+        )
+        assert msg._stream is None
+
+    async def test_set_content_replaces_stream_with_single_update(self) -> None:
+        """Replacing content should cancel the stream and update exactly once."""
+        msg = AssistantMessage()
+        markdown = MagicMock()
+        markdown.update = AsyncMock()
+        stream = MagicMock()
+        stream.stop = AsyncMock()
+        msg._markdown = markdown
+        msg._stream = stream
+        msg._content = "old streamed content"
+
+        await msg.set_content("```python\nnew content\n```")
+
+        stream.stop.assert_awaited_once_with()
+        markdown.update.assert_awaited_once_with("```python\nnew content\n```")
+        assert msg._stream is None
+        assert msg._content == "```python\nnew content\n```"
+
+
 class TestSummarizationMessage:
     """Tests for summarization notification widget."""