Fix bug where Next/Prev button in mail view didn't work with multi-folder search result (#6793)

CHANGELOG

@@ -10,6 +10,7 @@ CHANGELOG Roundcube Webmail
 - Fix bug in parsing vCard data using PHP 7.3 due to an invalid regexp (#6744)
 - Fix bug where bold/strong text was converted to upper-case on html-to-text conversion (6758)
 - Fix bug in rcube_utils::parse_hosts() where %t, %d, %z could return only tld (#6746)
+- Fix bug where Next/Prev button in mail view didn't work with multi-folder search result (#6793)
 
 RELEASE 1.3.9
 -------------

program/js/app.js

@@ -9061,8 +9061,15 @@ function rcube_webmail()
   // get the IMP mailbox of the message with the given UID
   this.get_message_mailbox = function(uid)
   {
-    var msg = (this.env.messages && uid ? this.env.messages[uid] : null) || {};
-    return msg.mbox || this.env.mailbox;
+    var msg;
+
+    if (this.env.messages && uid && (msg = this.env.messages[uid]) && msg.mbox)
+      return msg.mbox;
+
+    if (/^[0-9]+-(.*)$/.test(uid))
+      return RegExp.$1;
+
+    return this.env.mailbox;
   };
 
   // build request parameters from single message id (maybe with mailbox name)