fix(microprints): collapse expanded file when highlighting a different file

- Add test in microprints_live_test.exs that documents the uncoupled state bug
- Fix handle_event("highlight_line") to collapse expanded_path when
  highlighting a line in a different file
- All 159 tests pass

app/lib/firehose_web/live/microprints_live.ex

@@ -105,10 +105,18 @@ defmodule FirehoseWeb.MicroprintsLive do
         _ -> String.to_integer(line)
       end
 
+    # Collapse any currently expanded file when highlighting a different file
+    expanded =
+      case socket.assigns.expanded_path do
+        ^path -> socket.assigns.expanded_path
+        _ -> nil
+      end
+
     {:noreply,
      socket
      |> assign(:highlighted_path, path)
-     |> assign(:highlighted_line, highlighted)}
+     |> assign(:highlighted_line, highlighted)
+     |> assign(:expanded_path, expanded)}
   end
 
   @impl true