The problem, introduced with Safari 18

The very recently released Safari 18 on macOS Sequoia seems to have introduced a bug that makes the columns in the classic editor interface of WordPress overlap. Metaboxes that should actually be below the main content field are placed under the second sidebar column.

Easy Fix: add CSS to WordPress admin

You can very easily fix this by loading the following CSS in your admin area:

/* Temporary Fix for classic editor bug in Safari 18 */
#postbox-container-2 { clear: left; }
.index-php #postbox-container-2 { clear: none; }

Even easier: install a tiny plugin that loads the fix

To make things even easier I’ve written a tiny WordPress plugin that does that and nothing more. Download and activate the plugin – and you are good to go.

Meanwhile let’s hope there will be an official fix for the issue.