Fixed undo manager to properly handle the undo stack (SuperTux#1504)

* Fixed undo manager to properly handle the undo stack (will no longer ask to save a freshly opened, unmodified level)

* Fixed issue with the preceeding correction of the Undo manager

Co-authored-by: Semphris <invalid@example.com>

src/editor/editor.cpp

@@ -98,7 +98,8 @@ Editor::Editor() :
   m_enabled(false),
   m_bgr_surface(Surface::from_file("images/background/antarctic/arctis2.png")),
   m_undo_manager(new UndoManager),
-  m_ignore_sector_change(false)
+  m_ignore_sector_change(false),
+  m_level_first_loaded(false)
 {
   auto toolbox_widget = std::make_unique<EditorToolboxWidget>(*this);
   auto layers_widget = std::make_unique<EditorLayersWidget>(*this);
@@ -395,7 +396,6 @@ Editor::delete_current_sector()
 void
 Editor::set_level(std::unique_ptr<Level> level, bool reset)
 {
-  m_undo_manager->reset_index();
   std::string sector_name = "main";
   Vector translation;
 
@@ -436,6 +436,13 @@ Editor::set_level(std::unique_ptr<Level> level, bool reset)
   m_layers_widget->refresh_sector_text();
   m_toolbox_widget->update_mouse_icon();
   m_overlay_widget->on_level_change();
+
+  if (!m_level_first_loaded)
+  {
+    m_undo_manager->try_snapshot(*m_level);
+    m_undo_manager->reset_index();
+    m_level_first_loaded = true;
+  }
 }
 
 void

src/editor/editor.hpp

@@ -177,6 +177,8 @@ class Editor final : public Screen,
 
   std::unique_ptr<UndoManager> m_undo_manager;
   bool m_ignore_sector_change;
+
+  bool m_level_first_loaded;
 
 private:
   Editor(const Editor&) = delete;