// previously (i.e. global to the chat module)
    var KEY_TAB = 9;
    // ...

    function textEntryKeydown(event) {
        if (event.which == KEY_TAB) {
            var userlist = $('#userlist tr').map(function(){return this.id.substr(5)});
            autocompleteUsername($(event.target), userlist);
            return false;
        }
    }