Core block editor styles registered with frontend stylesheet URL instead of editor stylesheet URL

[flixos90]

While taking a closer look at the register_block_style_handle() function for #58528, I noticed that all core block editor style handles (i.e. wp-block-{$block_name}-editor) are registered pointing to the frontend stylesheet (style.css) rather than the editor stylesheet (editor.css). This happens because the style.css file name is hard-coded in the function regardless of whether the style or editorStyle is being registered.

Effectively, right now every pair of wp-block-{$block_name} and wp-block-{$block_name}-editor handles uses the same stylesheet file, which looks wrong given that several of them have dedicated editor stylesheets.

Looking at the Gutenberg plugin, it appears to work correctly there since Gutenberg has a dedicated logic path for editor styles, see ​https://github.com/WordPress/gutenberg/blob/3a419b840da0117ea05925c994861919acbe0c3d/lib/blocks.php#L277. So this is specifically a WordPress core bug that we should fix.

Adding this to the 6.3 milestone for consideration.

Trac ticket: https://core.trac.wordpress.org/ticket/58605

This PR also includes test coverage for the core-specific code paths of register_block_style_handle() which was missing before.

[flixos90]

PR ​https://github.com/WordPress/wordpress-develop/pull/4679 is ready for review.

It fixes the bug and adds test coverage for the core block-specific behavior, which was missing before. The tests ensure that the bug is fixed as intended.

[flixos90]

In 56005:

Editor: Fix block editor styles being registered with frontend stylesheets.

This changeset fixes a bug where WordPress core's own block editor styles (wp-block-{$block_name}-editor) were referencing the same CSS files as their frontend equivalents (wp-block-{$block_name}). This would result in incorrect frontend styles potentially being loaded in the block editor.

Tests for the related logic have been added.

Props flixos90, joemcgill, mukesh27, spacedmonkey.
Fixes #58605.

Committed in https://core.trac.wordpress.org/changeset/56005.