This is something I also found annoying… TextFugu actually already marks your pages by using standard browser functionality that uses your history to mark links.

The reason it doesn’t work on the lesson page is because the next/previous buttons inside the lessons have an extra “/#top” parameter at the end of the link, and therefore the browser sees it as 2 different locations.

Personally I just wrote a quick user-script to remove the “/#top” part on all the links. You can install a browser extension called Greasemonkey and then add my script if you want to use it.

// ==UserScript==
// @name        TextFugu - LinkFixer
// @namespace   Thierdox
// @include     http://www.textfugu.com/*
// @version     1
// @grant       none
// ==/UserScript==

(function() {
	var links = document.querySelectorAll('a.btn');

	var i = links.length;
	while (i--) {
		links.href = links.href.replace('/#top', '');
	}
})();

I think if you add “/#top” to all the links on the lesson page it would also fix it. So that would be a real quick way to fix it if the site owner wants to do it.