• Flushing any caching plugins you might be running, as well as server and/or?browser caches. Not just your browser, but any op cache or content network cache as well?such as Cloudflare. That will solve many weird JavaScript issues.
• Flushing Managed host caches. Managed WP hosting often has special caches. If your host has a “Purge Varnish” or “Flush Memcache” tool, try that. You can ask your provider to flush memcache and Varnish for you if necessary.
• Troubleshooting with your browser. Your browser can help you identify JavaScript issues or conflicts and?this article can assist you in doing that diagnosis. This could help identify Visual Editor issues as well.
• Deactivating?all plugins?(yes, all) to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s). If you can’t get into your admin dashboard, try resetting the plugins folder by?SFTP/FTP?or PhpMyAdmin (read “How to deactivate all plugins when you can’t log in to wp-admin” if you need help). Sometimes, an apparently inactive plugin can still cause problems. Also remember to deactivate any plugins in the mu-plugins folder. The easiest way is to rename that folder to?mu-plugins-old
• Switching to the Twenty Sixteen theme to rule out any theme-specific problems. If you can’t log in to change themes, you can remove the theme folders via?SFTP/FTP?so the only one is?twentysixteen. That will force your site to use it.
• Manually upgrading. When all else fails, download a fresh copy of the latest.zip file of 4.9 (top right on this page) to your computer, and use that to copy up. You may need to delete the wp-admin and wp-includes folders on your server (NOTE: do not delete the?wp-content?directory or your?wp-config.php?file) Read the?Manual Update directions first.

If you need to create a support topic, consider getting the Health Check plugin to provide debug data for the support volunteers.

• PressThis has been moved into a?separate plugin, you will be prompted to install it if you try using this feature.
• MediaElements.js, the library used to play media files, such as audio or video,?has been updated, removing support for older browsers.
• Changing emails now require?email validation?before the change takes effect.
• User roles and capabilities (access rules) have been?improved and extended.
• The?themes and plugins editors have been overhauled, now utilizing Code Mirror, and better sandboxing to avoid breaking sites.
• (For developers) the screen-reader-text class?has been modernized?for the admin interface.
• The new media widgets from WordPress 4.8, and improved text widget, have?received a handful of improvements.

• If you add or remove new page templates or other files to a theme without changing the version number, it may take up to an hour for the changes to take effect. This is due to caching, and is being discussed in #42573 – Reverted in 4.9.1
• I don’t see the new comment form checkbox. This may be due to your plugin or theme changing the comment form from the default provided by WordPress. It’s also worth noting that the box is only available to anonymous, non-logged in users.

• Saving posts or pages is not possible in all browsers when using Visual Composer version 5.4.2 or older.

Plugins with known issues

• Contact Form by BestWebSoft will cause the plugin/theme editors to time out and not save your changes. Workaround: Disable this plugin to use the plugin or theme editors.

mySQLi now default

• As of 4.9.2, WordPress now uses the mySQLi extension by default. If you have plugins or older code that uses the mysql_* functions directly, these may cease to function. Update your plugins to the latest version, or find alternative plugins.

• JavaScript heavy pages, such as the Customizer or post editors, break when using languages with formal/informal variations. This is being tracked and has a workaround in #42574 – Fixed in 4.9.1.
• Theme and plugin editors are unable to save files when hosted on Windows servers. See #42609 – Fixed in 4.9.1.
• Images with captions in my post are bigger than they should and the background fills up larger parts of the content area. See #42604.