Regression: Cover Block style changes in 5.9 break opacity for older cover blocks

[mrwweb]

Description

I have a site using the standard cover block with no customizations. When updating to WordPress 5.9, the block's opacity setting does not work until resaving the page which forces the cover block's markup and classes to change.

Prior to WordPress 5.9, the has-background-dim-XX class was on the cover block container. That class was moved to a span that's a direct descendant of the parent div. These markup changes won't be applied to legacy content until a post is resaved, but it appears the styles that make old cover block opacity work were removed.

Solution: The has-background-dim-XX classes that work when applied to the Cover block's parent container should be left in core indefinitely for the purposes of backward compatibility.

Step-by-step reproduction instructions

1. In WordPress 5.8.3 or earlier, create a cover block with an opacity other than 50% (so that one of the has-background-dim-XX classes are applied)
2. Update to WordPress 5.9
3. Cover block has 50% opacity
4. Save the page
5. Cover block markup is changed and custom opacity is reapplied

Screenshots, screen recording, code snippet

WordPress 5.8.3 (90% opacity, classes: wp-block-cover alignfull has-background-dim-90 has-palette-2-background-color has-background-dim has-parallax is-style-no-padding):

WordPress 5.9:

Environment info

• WordPress 5.9
• Themes Tested: Twenty Twenty One and theme.json child theme of Michelle

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[mrwweb]

A quick note that I've updated the issue a few times and have replicated the issue on a simpler install using the Twenty Twenty One theme.

[annezazu]

Adding to the 5.9.x project board so it can be reviewed. Thank you for flagging!

[glendaviesnz]

This is fixed by #38362, which is merged and flagged for inclusion in 5.9.1

[annezazu]

Ah ha! Just was about to come back through here after noticing your lovely PR. Thank you!

[glendaviesnz]

An additional note for anyone waiting for this to go out in 5.9.1 - adding this to site custom CSS should fix the issue in the meantime.