Make the selected item visible in listbox::layout_children()

The biggest change from this is that list boxes now keep the selected item
visible when you change the sorting option.

This change also keeps an add-on visible in the new add-on manager after
installing it, which is why I made this change in the first place.

changelog

@@ -28,6 +28,9 @@ Version 1.13.6+dev:
  * Lua API:
    * New attributes in side proxy:
      num_units, num_villages, total_upkeep, expenses, net_income
+ * User Interface:
+   * List boxes now keep the selected item visible when you change the sorting
+     option.
  * Wesnoth Formula Engine:
    * New str_upper and str_lower functions for case transformations
  * WML Engine:

players_changelog

@@ -24,6 +24,10 @@ Version 1.13.6+dev:
  * Language and i18n:
    * Updated translations: British English, Czech, German, Portuguese (Brazil).
 
+ * User Interface:
+   * List boxes now keep the selected item visible when you change the sorting
+     option.
+
  * Miscellaneous and bug fixes:
    * Fixed severe lag/freeze on slow PCs (bug #25356)
 

src/gui/widgets/listbox.cpp

@@ -690,10 +690,23 @@ void listbox::layout_children(const bool force)
 	assert(content_grid());
 
 	if(need_layout_ || force) {
+		const int selected_item = generator_->get_selected_item();
+
 		content_grid()->place(content_grid()->get_origin(),
 							  content_grid()->get_size());
 
-		content_grid()->set_visible_rectangle(content_visible_area_);
+		const SDL_Rect& visible = content_visible_area_;
+
+		content_grid()->set_visible_rectangle(visible);
+
+		if(selected_item != -1) {
+			SDL_Rect rect = generator_->item(selected_item).get_rectangle();
+
+			rect.x = visible.x;
+			rect.w = visible.w;
+
+			show_content_rect(rect);
+		}
 
 		need_layout_ = false;
 		set_is_dirty(true);