WordPress 6.9.1 is screwing up our front end styles, had to revert

[webmaster in texas]

When updating to WordPress 6.9.1, our front page columns/blocks are all messed up. I'm not sure how this is happening, we've never had an update affect our front end this way.

[pbiron]

Welcome to Trac.

What theme are you using? Is it a block-based theme or a classic theme?

[westonruter]

@webmaster-in-texas Hello. If you're using a classic theme (i.e. you don't have the Site Editor available), this may be a duplicate of #64389, or at least related. Please confirm.

Also, if that is the case, there is a hotfix plugin to undo the performance enhancement in 6.9 that caused the issue. Please try ​Load Combined Core Block Assets (as referenced in ​WordPress 6.9 Hotfixes). It provides a way to opt in to loading separate block styles via a query parameter to see if a core update fixes the issue. Sharing your site URL with that plugin active would be helpful for us to debug as well.

If you can test the ​pull request for the aforementioned ticket to see if it fixes the issue for you, that would also be very helpful.

[westonruter]

Re-posting my comment from #64389:

I've just finished another pass at improving the preservation of classic theme CSS cascade when loading block styles on demand.

The PR description has been fleshed out with all the details, but in short:

1. Determining where to insert the styles hoisted from the footer has been made more robust by inserting placeholder inline styles. This simplifies the insertion logic since it doesn't have to hunt around for references for where styles should be inserted. It merely needs to replace the placeholders with the captured styles.
2. The CSS cascade is now improved for inline styles which had been added to wp-block-library. Now when loading separate block styles, any such inline styles get added after any of the separate core block styles.

Note: Do not have Gutenberg active when testing this, as the patch has not yet been applied to Gutenberg's forked version of wp_enqueue_global_styles().

Review and testing much appreciated: ​https://github.com/WordPress/wordpress-develop/pull/10875

I really hope to get this into the next beta.

[westonruter]

In [61945] I've committed significant improvements to the CSS cascade in classic themes which will be part of 7.0-beta5 due out tomorrow.

I do not know for sure whether this ticket is a duplicate of #64389 but as far as I know it is. So I'm closing this as a duplicate.

If it isn't, we can re-open.