Steps to replicate the issue:
Select "Always enable safe mode" at https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering
Save preferences and view the same preference tab again.
What happens?:
The message at https://en.wikipedia.org/wiki/MediaWiki:Prefs-custom-cssjs-safemode is now displayed:
"Custom CSS/JavaScript are currently unavailable as safe mode has been enabled. Please disable safe mode to use custom CSS/JavaScript."
The problem is that the message is in a very narrow box with a single character per line. The vertical length of the box is more than three screens for me. The screenshot is from Vector 2022 and Firefox. The same happens in two other tested browsers, skins, and wikis.
What should have happened instead?:
A wide box with the same message should have been displayed.