Confirmed

The easy way to see this error is to add a readmore link in an article.

IF the editor.css was being loaded then that would be displayed as a dotted red line inside tinymce. But as it is not being loaded the readmore line is just displayed as a solid grey line

Exactly, that's what I noticed too.

annoyingly this error is not present when you install joomla manually including composer install and npm install which is why no one noticed it before

Thats crazy @brianteeman. Do you have the option of doing a diff or something similar of the installations of both variants?

I just did that and I dont see anything useful

I just updated Joomla! to 6.0.1 and TinyMCE seems to work fine with Edge, but not with Chrome, even after clearing browser data. But in Chrome incognito window, the editor works! Strange...

Edit: Tested backend.

I just tested this: My various test pages in Chrome, Edge, Edge incognito, Firefox: In no case was editor.css loaded.
All tests in the backend

Confirmed. Works on sites with 5.4.1 but not with 6.0.

Confirmed on an update from 6.0.0 to 6.0.1 (worked on 6.0.0)

Does anyone have a 6.0.0 and a 6.0.1 to verify that it works in 6.0.0? If so, we would need to examine the difference between 6.0.0 and 6.0.1 in this regard.

As I am currently traveling, I am unable to perform the comparison at this time.

So, I made a diff between a Joomla! 6.0.0 and 6.0.1 installation.

Among other things, the differences were in

/media/plg_editors_tinymce/js/tinymce.min.js

and, of course, their minimized and compressed versions.

A diff showed only this change:

25024# diff ./k0jdemoe6/htdocs/media/plg_editors_tinymce/js/tinymce.js  ./k0jdemoe5/htdocs/media/plg_editors_tinymce/js/tinymce.js
162a163
>       options.content_css = mql.matches ? 'dark' : 'default';
164a166
>       options.content_css = 'dark';

where k0jdemoe6 is 6.0.0 and k0jdemoe5 is 6.0.1.

After copying the files:

cp ./k0jdemoe6/htdocs/media/plg_editors_tinymce/js/tinymce.min.js.gz ./k0jdemoe5/htdocs/media/plg_editors_tinymce/js/tinymce.min.js.gz

and clearing the browser cache, the problem is solved.

indicated by the red line, as mentioned by @brianteeman

I tested it with the standard template and with my own template “ahsimple”; in both cases, editor.min.css was loaded in the backend.

Can someone repeat this test to validate the whole thing?

That would be my fault then

You probably wanted to test how careful we are when editing texts :-)

What are the next steps? Can I just submit a PR? It's been a long time since my last PR, so it would be a good opportunity to practice.

Or will you do it, Brian?

I would still like to see why even with that change it works in a dev environment. That would suggest reverting that bugfix is not the solution

You're right. I hope it doesn't take too long. From the user's point of view, I would appreciate it if the next update included a fix, even if it's only a temporary rollback solution until the final fix is available.

I have built up my 6.0-dev environment from a fresh pull and there can confirm the issue. Changing the file as suggested by @ahotzler does nor make a difference.

Hm, crazy, i repaired some sites with the copy method. Maybe you should also copy the uncompressed versions?

Strange, I just had a case where a page on which I copied the file as described did not load the editor.css classes when editing a post.

I closed the editor, turned on the inspector, created a new post: editor.css was loaded.

Then I reopened the previous article, and now it was loaded there too—but I didn't change anything except opening the inspector.

Yes there a are changes in the TinyMCE.js that seems to prevent the loading
Around line 163 - two new lines: ( // NEW !!!)

// For templates with OS preferred color scheme
if ('colorSchemeOs' in document.documentElement.dataset) {
  const mql = window.matchMedia('(prefers-color-scheme: dark)');
  options.skin = mql.matches ? skinDark : skinLight;
  options.content_css = mql.matches ? 'dark' : 'default'; // NEW!!!!
} else if (document.documentElement.dataset.colorScheme === 'dark') {
  options.skin = skinDark;
  options.content_css = 'dark'; // NEW !!!
}

without the two lines - the js loads just fine.

weird that those lines prevent editor.css from loading
and weird that even with those lines there is no problem with a git checkout - only with a download

Well guys, #46377 need to be reverted OR revorked.

content_css IS the editor.css
https://www.tiny.cloud/docs/tinymce/latest/add-css-options/

So why does it work correctly if you do a git checkout and then composer and npm install but doesn't work when you download a release

It really doesn’t matter from my perspective.

What does matter is that users currently can’t work with it.

That’s why I fully agree with your view and suggested the following approach:

For now, revert to the previously working state and include the fix in the next update.

We can still investigate the unusual behavior afterwards — the main goal should be to ensure that regular users can work without issues.

So why does it work correctly if you do a git checkout and then composer and npm install but doesn't work when you download a release

I did not checked that deep, maybe something wrong with build script (again).

Or you have something with colorScheme in cookies for Dev installation.
It willl work in checkout when colorScheme is not dark and not "OS", because in tinymce.js there:
document.documentElement.dataset.colorScheme === 'dark'

The fix is pretty simple.
The code from #46377 should check if (!options.content_css) before setting the value.

Please test #46543

closing as we have a pr for testing #46543