Fix dbg pipeline when result renders as Kino.JS.Live (#453)

lib/kino/debug/pipeline.ex

@@ -31,6 +31,8 @@ defmodule Kino.Debug.Pipeline do
 
     %{id: last_id} = List.last(items)
 
+    send(self(), :update_result_frame)
+
     {:ok,
      ctx
      |> assign(
@@ -44,8 +46,7 @@ defmodule Kino.Debug.Pipeline do
        error: nil,
        call_count: 1,
        changed?: false
-     )
-     |> update_result_frame()}
+     )}
   end
 
   @impl true
@@ -127,6 +128,10 @@ defmodule Kino.Debug.Pipeline do
     {:noreply, ctx}
   end
 
+  def handle_info(:update_result_frame, ctx) do
+    {:noreply, update_result_frame(ctx)}
+  end
+
   defp handle_items_change(ctx, items) do
     id =
       ctx.assigns.items

test/kino/debug_test.exs

@@ -54,6 +54,23 @@ defmodule Kino.Debug.Test do
       assert dbg_line == __ENV__.line - 28
     end
 
+    test "initial render with result requiring Kino.start_child/1" do
+      # Rendering %UserAsTable{} creates a Kino.JS.Live
+      call_dbg(
+        %UserAsTable{id: 1}
+        |> Map.replace!(:id, 2)
+        |> Map.replace!(:id, 3)
+      )
+
+      {_kino, _output, frame_ref} = assert_dbg_pipeline_render()
+
+      assert_output(%{
+        type: :frame_update,
+        ref: ^frame_ref,
+        update: {:replace, [%{type: :js}]}
+      })
+    end
+
     test "updates result when a pipeline step is disabled" do
       call_dbg(
         1..5

test/support/test_modules/user_as_table.ex

@@ -0,0 +1,9 @@
+defmodule UserAsTable do
+  defstruct [:id]
+
+  defimpl Kino.Render do
+    def to_livebook(user) do
+      Kino.DataTable.new([%{id: user.id}]) |> Kino.Render.to_livebook()
+    end
+  end
+end