feat: frontend turn_html_replace SSE handler for regenerate live-swap (T86)

tests/test_streaming_ux.py

@@ -174,3 +174,74 @@ def test_chat_html_includes_stop_streaming_script(client, tmp_path):
     assert "stop-streaming" in body or "isStreaming" in body
     # Cancel route reference must be wired so the Stop button can call it.
     assert "/turns/cancel" in body
+
+
+def test_chat_html_has_turn_html_replace_listener(client, tmp_path):
+    """T86: the chat shell wires a JS handler for the ``turn_html_replace``
+    SSE event so regenerate-driven swaps land in connected tabs without a
+    page refresh.
+
+    This is a presence / string-check test: it verifies the handler is
+    embedded in the rendered template but does NOT drive a real browser
+    (no headless runner is wired into this test environment). The end-to-
+    end behaviour — receiving the event over SSE and replacing the prior
+    turn's DOM node — is therefore not exercised here; a manual smoke
+    check or future browser-driven test would close that gap.
+    """
+    _seed_chat(tmp_path / "test.db")
+    response = client.get("/chats/chat_bot_a")
+    assert response.status_code == 200
+    body = response.text
+    # The handler must be wired against the SSE event name the backend
+    # publishes (chat.services.regenerate -> "turn_html_replace").
+    assert "turn_html_replace" in body
+    # Confirm the handler reads the JSON payload's ``supersedes_id`` so
+    # it can locate the prior turn node. The exact lookup mechanism may
+    # vary, but the field name is part of the contract with the backend.
+    assert "supersedes_id" in body
+
+
+def test_rendered_turn_html_includes_event_id(client, tmp_path):
+    """T86 follow-up: the chat-detail Jinja loop stamps
+    ``id="turn-<event_id>"`` on every rendered turn DIV. Without this id
+    the ``turn_html_replace`` SSE handler's ``getElementById`` lookup
+    misses, falls through to ``insertAdjacentHTML('beforeend', …)``, and
+    the regenerated turn appears APPENDED instead of swapped in-place
+    (rendering the primary handler path dead code — exactly the gap the
+    T86 reviewer flagged).
+
+    Seed a user_turn + assistant_turn, GET the chat page, and assert the
+    response body carries both turns' event ids on the wrapper DIVs.
+    """
+    db_path = tmp_path / "test.db"
+    _seed_chat(db_path)
+    with open_db(db_path) as conn:
+        ut_id = append_event(
+            conn,
+            kind="user_turn",
+            payload={
+                "chat_id": "chat_bot_a",
+                "prose": "hello bot",
+                "segments": [],
+            },
+        )
+        at_id = append_event(
+            conn,
+            kind="assistant_turn",
+            payload={
+                "chat_id": "chat_bot_a",
+                "speaker_id": "bot_a",
+                "text": "Hi there.",
+                "truncated": False,
+                "user_turn_id": ut_id,
+            },
+        )
+        conn.commit()
+
+    response = client.get("/chats/chat_bot_a")
+    assert response.status_code == 200
+    body = response.text
+    # Both seeded turns must carry ``id="turn-<event_id>"`` so the SSE
+    # in-place swap can find them.
+    assert f'id="turn-{ut_id}"' in body
+    assert f'id="turn-{at_id}"' in body