smart_scroll_chatbox.js

// Figure out heights and things
var chat = document.getElementById('chat_space');
var currentScrollPos = chat.scrollTop;
var maxScrollPos = chat.scrollHeight - chat.clientHeight;
var scrolled = (currentScrollPos == maxScrollPos) ? true : false

// Clear out the box and replace with data from ajax

// Should we scroll?
if (scrolled) chat.scrollTop = chat.scrollHeight;
else chat.scrollTop = currentScrollPos;