I encountered an odd issue where on my production site (not my local site) a certain page would result in the aggregated CSS/JS files failing to load with a 418 response (yes, the teapot one).
As explained I could only reproduce this on my production site, and not my local machine so I wouldn't know how reproducible this could be, but...
The problem page is the revision diff page, provided by the diff module →
/admin/appearance/admin/config/development/performance/admin/config/content/diff/general/admin/appearance/admin/appearanceI haven't had this issue on any other pages with the Adminimal theme.
The reason I figure this is an Adminimal issue (and not Diff) is it only occurs with the Adminimal theme on that certain page.
While I could never figure out why this was occurring, what I did notice is when this failure occurs, the aggregated files never get created, yet they're being referenced to in the resulting page, deleting the js/css folders in /sites/default/files would get re-generated on all other pages, but would not get regenerated for this problem page when using Adminimal.
For reasons beyond my understanding I was able to fix this by modifying adminimal_theme.libraries.yml to add adminimal_theme/global-styling dependencies to the other two libraries.
I figure this change shouldn't break anything regardless and is general good practice for library entries as the google-fonts and custom-styling libraries are kind of useless loaded without the main theme's library.
Needs review
1.7
Code
It turns out that the 418 error was caused by dream host's firewall for "malicious HTTP requests"
https://help.dreamhost.com/hc/en-us/articles/215947927-Enabling-the-Web-Application-Firewall
Regardless for some reason the patch to add the theme dependency still resolves this issue.