Fix default sorting of threaded list when THREAD=REFS isn't supported.

The result need to be re-sorted even if sort column is not set. This fixes
the issue when new messages aren't displayed on top of the list.

CHANGELOG

@@ -1,6 +1,7 @@
 CHANGELOG Roundcube Webmail
 ===========================
 
+- Fix default sorting of threaded list when THREAD=REFS isn't supported
 - Fix list mode switch to 'List' after saving list settings in Larry skin (#1489164)
 - Fix error when there's no writeable addressbook source (#1489162)
 - Fix zipdownload plugin issue with filenames charset (#1489156)

program/lib/Roundcube/rcube_imap.php

@@ -1336,17 +1336,16 @@ class rcube_imap extends rcube_storage
         // THREAD=REFERENCES:     sorting by sent date of root message
         // THREAD=REFS:           sorting by the most recent date in each thread
 
-        if ($this->sort_field && ($this->sort_field != 'date' || $this->get_capability('THREAD') != 'REFS')) {
-            $index = $this->index_direct($this->folder, $this->sort_field, $this->sort_order, false);
+        if ($this->get_capability('THREAD') != 'REFS') {
+            $sortby = $this->sort_field ? $this->sort_field : 'date';
+            $index  = $this->index_direct($this->folder, $sortby, $this->sort_order, false);
 
             if (!$index->is_empty()) {
                 $threads->sort($index);
             }
         }
-        else {
-            if ($this->sort_order != $threads->get_parameters('ORDER')) {
-                $threads->revert();
-            }
+        else if ($this->sort_order != $threads->get_parameters('ORDER')) {
+            $threads->revert();
         }
     }