Add info about empty Kino outputs (#1899)

lib/livebook_web/live/app_live.ex

@@ -119,6 +119,10 @@ defmodule LivebookWeb.AppLive do
               input_values={output_view.input_values}
             />
           </div>
+          <div :if={@data_view.output_views == []} class="info-box">
+            This deployed notebook is empty. Deployed apps only render Kino outputs.
+            Ensure you use Kino for interactive visualizations and dynamic content.
+          </div>
         </div>
         <div style="height: 80vh"></div>
       </div>

test/livebook_web/live/app_live_test.exs

@@ -0,0 +1,34 @@
+defmodule LivebookWeb.AppLiveTest do
+  use LivebookWeb.ConnCase, async: true
+
+  import Phoenix.LiveViewTest
+
+  alias Livebook.Session
+
+  test "render guidance when Kino output is empty", %{conn: conn} do
+    session = start_session()
+
+    Session.subscribe(session.id)
+
+    slug = Livebook.Utils.random_short_id()
+    app_settings = %{Livebook.Notebook.AppSettings.new() | slug: slug}
+    Session.set_app_settings(session.pid, app_settings)
+
+    Session.set_notebook_name(session.pid, "My app #{slug}")
+    Session.deploy_app(session.pid)
+
+    assert_receive {:operation, {:add_app, _, _, _}}
+    assert_receive {:operation, {:set_app_registered, _, _, true}}
+
+    {:ok, view, _} = live(conn, ~p"/apps/#{slug}")
+
+    assert render(view) =~
+             "This deployed notebook is empty. Deployed apps only render Kino outputs."
+  end
+
+  defp start_session() do
+    {:ok, session} = Livebook.Sessions.create_session()
+    on_exit(fn -> Session.close(session.pid) end)
+    session
+  end
+end