Remember microprint state on reload

app/test/firehose_web/live/microprints_live_test.exs

@@ -139,4 +139,42 @@ defmodule FirehoseWeb.MicroprintsLiveTest do
              "file B should show Collapse button when expanded"
     end
   end
+
+  describe "URL state persistence" do
+    test "expanded path is restored from query param on mount", %{conn: conn} do
+      files = MicroprintsLive.scan_source_files()
+      file_a = List.first(files)
+
+      {:ok, view, _html} = live(conn, ~p"/microprints?expanded=#{file_a}")
+
+      html = render(view)
+      assert html =~ "Collapse",
+             "Expanded file should show Collapse button when restored from URL param"
+      assert html =~ ~s(phx-value-path="#{file_a}")
+    end
+
+    test "unknown expanded path in URL is ignored gracefully", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/microprints?expanded=nonexistent.ex")
+
+      html = render(view)
+      refute html =~ "Collapse",
+             "Nonexistent expanded path should not show Collapse"
+    end
+
+    test "expand updates URL with expanded param", %{conn: conn} do
+      files = MicroprintsLive.scan_source_files()
+      file_a = List.first(files)
+
+      {:ok, view, _html} = live(conn, ~p"/microprints")
+
+      view
+      |> element("button[phx-value-path=\"#{file_a}\"]", "Expand")
+      |> render_click()
+
+      # URL should contain expanded param after clicking Expand
+      html = render(view)
+      assert html =~ "Collapse",
+             "File should remain expanded after push_patch updates URL"
+    end
+  end
 end