Inconsitency to core: theme support feature 'editor-styles' <-> 'editor-style'

[j-hoffmann]

Describe the bug
When Gutenberg is collecting the styles to be loaded into the editor, theme support for the feature editor-styles determines whether styles from the global variable $editor_styles are loaded in lib/client-assets.php:

if ( $editor_styles && current_theme_supports( 'editor-styles' ) ) ...

When using the core function add_editor_style() to add stylesheets to the $editor_styles global, the function sets theme support for the feature editor-style (note, this time without "s" at the end):

function add_editor_style( $stylesheet = 'editor-style.css' ) {
add_theme_support( 'editor-style' );
[...]
}

The problem is, stylesheets added via add_editor_style() will not get loaded in Gutenberg unless separately setting support for editor-styles (with "s" at the end):

add_theme_support( 'editor-styles' );

To Reproduce
Steps to reproduce the behavior:

1. Include a stylesheet via add_editor_style()
2. It will be loaded in classic editor but not in Gutenberg
3. Apply add_theme_support( 'editor-styles' );
4. Now the stylesheet also applies to Gutenberg

Expected behavior
add_editor_style() should be enough to apply the stylesheet in both editors, so Gutenberg should be checking current_theme_supports( 'editor-style' ).

Desktop

• OS: Windows 10
• Browser Firefox 63

Additional context

• WordPress 4.9.8
• Gutenberg 4.5.1

[mkaz]

From @joyously in Slack

I'm not sure when this change crept into the Gutenberg doc (Theme section), but it is incorrect. This part https://wordpress.org/gutenberg/handbook/designers-developers/developers/themes/theme-support/#editor-styles acknowledges that the new way is a little different, and has add_theme_support('editor-styles');. Then this part https://wordpress.org/gutenberg/handbook/designers-developers/developers/themes/theme-support/#enqueuing-the-editor-style incorrectly associates the new loading and parsing with the old add_editor_style( 'style-editor.css' );. The old one uses add_theme_support('editor-style');. Note that the old one is still available for the old editor and it is singular. The new one for the new editor is plural.

Comments added to commit here: e4fd51c#diff-c14ad3a4c41ef0d7bf709f5ebe52ba64

[mkaz]

Closed in: #14917