fix scrolling through the player list in the mp lobby:

...trying to preserve the selected player the viewport of the list
would always be reset to show the selected player

src/multiplayer_ui.cpp

@@ -730,8 +730,7 @@ void ui::set_user_menu_items(const std::vector<std::string>& list)
 	std::vector<std::string>::const_iterator i =
 			std::find(user_list_.begin(), user_list_.end(), selected_user_);
 	if(i != user_list_.end()) {
-		users_menu_.reset_selection();
-		users_menu_.move_selection(i - user_list_.begin());
+		users_menu_.move_selection_keeping_viewport(i - user_list_.begin());
 	}
 }
 

src/widgets/menu.cpp

@@ -517,6 +517,14 @@ void menu::move_selection(size_t id)
 	}
 }
 
+// public function
+void menu::move_selection_keeping_viewport(size_t id)
+{
+	if(id < item_pos_.size()) {
+		set_selection_pos(item_pos_[id], true, NO_MOVE_VIEWPORT);
+	}
+}
+
 void menu::reset_selection()
 {
 	set_selection_pos(0);

src/widgets/menu.hpp

@@ -160,6 +160,7 @@ public:
 	int selection() const;
 
 	void move_selection(size_t id);
+	void move_selection_keeping_viewport(size_t id);
 	void reset_selection();
 
 	// allows user to change_item while running (dangerous)