Layout: Ensure defaultEditorStyles are still output in Classic themes

[andrewserong]

What?

Fixes #42902

When outputting base layout styles in a theme that otherwise does not output theme global styles, ensure the type of the base layout styles is not set to theme so that it isn't considered as theme styles by the check in

gutenberg/packages/edit-post/src/editor.js

Line 178 in 9f0e6be

: defaultEditorStyles;

Why?

As of #40875, base layout styles are output as part of the Theme JSON class — in order to support Classic themes using the default editor styles, we need to ensure that the base layout styles are not considered "theme" styles by the above check. Otherwise themes that do not provide their own editor styles will not have default editor styles to ensure elements in the editor look pleasing.

How?

When outputting base layout styles for Classic themes, use a different __unstableType so that it isn't treated as theme styles. These styles get appended into the presets styles as part of the following line, which is the desired behaviour:

gutenberg/packages/edit-post/src/editor.js

Line 174 in 9f0e6be

...presetStyles,

Testing Instructions

1. In an older Classic theme like Dara (https://wordpress.org/themes/dara/), test that the editor styles now look correct
2. Test that Social Icons and Columns blocks render correctly
3. In a Blocks-based theme, smoke test that Layout styles for Columns, Group, and Buttons still work as expected

Screenshots or screencast

With Dara theme applied

Before	After

[andrewserong]

@Mamaduka I know we're very close to the 13.8 release time, but I was wondering if it's worth trying to get this PR cherry picked in, too, in order to fix this editor styling regression for some Classic themes? (If the approach here looks okay)

[Mamaduka]

@andrewserong, thanks for the ping.

I've one more item to cherry-pick for today's release and this seems essential enough 👍

[andrewserong]

Excellent, thank you again! 🙇

[glendaviesnz]

Tested well for me on Dara, and also tested a few different block and non-block themes and the editor fonts were the same between 13.6 and this branch.

Would probably be good to get some more testing from someone else as well though if we have time before getting it picked into 13.8

[ramonjd]

I checked on Dara:

Trunk	This PR

I also tested a few other classic themes (2011, 2013, 2015 and 2021) but couldn't see a difference between trunk and this PR. Things appears as expected.

LGTM!

[andrewserong]

Thanks for reviewing and testing, folks! I'll merge this in now.