Help:Update to MediaWiki 1.39

MediaWiki™ is the software which runs all of the wiki components on Fandom communities. Respecting our commitment to keep MediaWiki up to date on a regular basis after the implementation of the Unified Community Platform, we are running at least one major update every year. Those major updates are updating not only the MediaWiki version, also the extensions and functionalities used in the whole platform. The previous major update done in Fandom was the update to MediaWiki 1.37.

This upgrade won't change the design, so our wikis will keep the same look as they had previously. Our engineers will make the needed changes to be sure that our admins won't need to do design adjustments. Most of the changes will happen under the hood, meaning they're not user-facing, but they will be important to ensure that everything is still working correctly.

During the migration to 1.39, the wikis will be set in read-only mode. It shouldn't take more than 30 minutes (major wikis could need more hours). One week before migration, a banner will appear on communities to let them know the migration is scheduled. For one week after the migration, a similar banner to let users know the community is now on the new version.

If your wiki is experiencing problems after the migration, please contact to your Wiki Representative or Fandom Staff. There is also a specific channel in our Discord server created for conversations about the upgrade, go to  and feel free to ask your questions or concerns.

See more information about the upgrade process in this blog post.

Timeline
In order to ensure a sweet upgrade, the migrations are done in different batches, starting with 3-4 wikis without complex extensions, and adding more traffic, content, and extensions progressively in the subsequent batches. Depending on the number of wikis and extensions included in every batch, it can take from 1 day to 2 weeks to complete.

NOTE: This section will be updated with the estimated dates after publishing the blog post announcing them.

Changes
The following list contains a summary of the most relevant changes for regular users and visitors. For a detailed list, see the official release notes.

Important notes for admins

 * CSS customizations should work as previously in 1.37. This upgrade hasn't introduced relevant changes in this area and our engineers have applied changes on their own to avoid moving this work to the admins.

Security

 * SECURITY: HTMLUserTextField exposes existence of hidden users.
 * SECURITY: On  the message "alreadyrolled" can leak revision deleted user name.
 * SECURITY:  update title after redirects.

Articles and editing

 * Validate length of user email on /.
 * Repair language selector for SVGs.
 * The  feature was removed following a consultation due to performace reasons.
 * New  parameter on action=delete that allows you to delete the associated talk page of a subject page.
 * Similarly, new  parameter on   that allows you to restore all revisions of the associated talk page.
 * The character  is included in the list of invalid username characters. It's a reserved delimiter for external user names.
 * Optional automatic user creation on page save.
 * Administrators now have the option to delete/undelete the associated  page when they are (un)deleting a given page.   and   options were added to the   and   actions APIs.
 * is now a wikitext built-in magic word, expanding to . This is conventionally used as an escape mechanism to allow the use of   in unnamed template arguments. Defining   to expand to something other than   has been deprecated since 1.36, with affected pages put into a special tracking category for migration.
 * Bot passwords are now supported when using the REST API.

External libraries

 * Upgrade  from 3.6.0 to 3.6.1.

Official release notes
For more information about the changes, check the official release notes:
 * 1.38 Release Notes
 * 1.39 Release Notes