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;
	}
}