silverstripe-blog/code/forms/BlogAdminSidebar.php

<?php

class BlogAdminSidebar extends FieldGroup
{
    /**
     * @return bool
     */
    public function isOpen()
    {
        $sidebar = Cookie::get('blog-admin-sidebar');

        if ($sidebar == 1 || is_null($sidebar)) {
            return true;
        }

        return false;
    }
}