I believe you fix the issue of the links opening inside the menu box is by adding this to your mainmenu.html file: https://pastebin.com/raw/CF9RwjLK (I had to put it in a pastebin because neocities was fighting me). I tested this on windows 10 on a chromium browser and it works for me. It sets the base point for opening the pages in index.html instead of mainmenu.html so they don't load inside the menu box.