--- a/program/lib/Roundcube/rcube_message_header.php	Sat Sep 20 12:16:50 2025 -0400
+++ b/program/lib/Roundcube/rcube_message_header.php	Mon Oct 06 10:34:51 2025 -0400
@@ -23,8 +23,11 @@
  * @package    Framework
  * @subpackage Storage
  * @author     Aleksander Machniak <alec@alec.pl>
+ * HST added the abstraction level below because of plugins/thunderbird_labels
+ * use of a new property 'list_flags'
  */
-class rcube_message_header
+
+abstract class r_m_h_base
 {
     /**
      * Message sequence number
@@ -187,6 +190,27 @@
      */
     public $flags = array();
 
+    /**
+     * Threaded message property, HST added
+     *
+     * @var int
+     */
+    public $parent_uid;
+
+    /**
+     * Threaded message property, HST added
+     *
+     * @var int
+     */
+    public $depth;
+
+    /**
+     * Threaded message property, HST added
+     *
+     * @var array
+     */
+    public $has_children;
+
     // map header to rcube_message_header object property
     private $obj_headers = array(
         'date'      => 'date',
@@ -272,6 +296,15 @@
     }
 }
 
+if (in_array('thunderbird_labels',
+	     (rcube::get_instance()->config->get('plugins'))))
+{
+  class rcube_message_header extends r_m_h_base {
+    public $list_flags = array();
+  }
+} else {
+  class rcube_message_header extends r_m_h_base { }
+ }
 
 /**
  * Class for sorting an array of rcube_message_header objects in a predetermined order.