CSS files not loaded on AJAX calls

A `git bisect` is probably needed if we are not sure.

drupal_set_message set a flash message and displayed after a page reload, right? the CSS doesn't load from AJAX.

No errors on my testing (8.4.x)

EDIT:
and Bartik should always load the messages.css. @see bartik.libraries.yml

Found the problem:
#2853509: Don't render status messages if there are no messages but also include their assets if there might be

Here's a fix. Basically prior to #2853509: Don't render status messages if there are no messages but also include their assets if there might be all classy themes were guaranteed to have classy's messages.css loaded - we can easily do just that.

In fact here's a better fix that preserves the inheritance of libraries so overrides etc work as expected.

@xjm pointed out that we'd need a change clear to pick up the libraries change. Patch attached also fixes the tests. Big Pipe's life becomes a bit simpler because it no longer needs to add the messages library for status messages.

Here's specific tests to ensure that classy's messages.css is loaded for both classy and themes that extend classy like Bartik and Seven.

this might be completely related, or completely unrelated, depending on the approach:
#2876314: Allow a view to load style libraries

+++ b/core/modules/system/system.install
@@ -1794,6 +1794,12 @@ function system_update_8301() {
 
 /**
+ * Clear caches to ensure Classy's message library is always added.
+ */
+function system_update_8302() {
+}
+
+/**

Let's add an empty post update rather than an empty hook_update_N() to minimize numbering conflicts and similar with other patches.

Also: shouldn't CSS added via AJAX callbacks be added to the page? We have a whole set of functionality around that in the AJAX system (ajax page state etc.) so I'm surprised it's not just working to be honest.

Nice idea @catch - now one patch for 8.3.x and 8.4.x. Also @Graber emailed me and mentioned that the attach in classy's status-messages.html.twig is no longer necessary. Which is true.

@catch I don;t think the Ajax page state thing is working because the JS libraries for tabledrag don't depend on the message library - this is because the message library is only a thing in Classy and not core.

One less new line in D8 output...

Can't classy add a hook_libraries_info_alter() to add itself as a dependency?

@catch but we can't know all the libraries in contrib and custom that would need this. It was added to every page.

Rather than having a regression, a critical, a major, a BC break, and disruption to ongoing work, I've reverted the original issue and so closing this as duplicate. If someone can recapture the work here over there I'd appreciate it. Thanks!

@Graber #2853509: Don't render status messages if there are no messages but also include their assets if there might be