inline styles ( wp_add_inline_style ) not working in iframed Full Site Editor

[leeshadle]

Description

Inline styles ( wp_add_inline_style / $wp_styles->add_inline_style ) aren't working in the iframed Full Site Editor.

@ellatrix is working on a fix for this but I wanted to create this issue in case anyone else ran into the same bug.

Step-by-step reproduction instructions

1. Enqueue stylesheet for a plugin in the Full Site Editor and try adding inline styles using wp_add_inline_style:

function enqueue_editor_styles() {
  // Enqueue editor only styles.
  wp_enqueue_style(
    'my-plugin-block-editor',
    plugin_dir_url( __FILE__ ) . 'build/index.css',
    array(),
    '1.0.0'
  );
  // Add inline styles
  wp_add_inline_style( 'my-plugin-block-editor', 'p { font-size: 100px; }' );
}
add_action(  'enqueue_block_editor_assets', 'enqueue_editor_styles'  );

Expected behaviour

Here's what it looks like when this is working:

Actual behaviour

Here's what's happening in the iframed Full Site Editor:

WordPress information

• WordPress version: 5.7.2
• Gutenberg version: 10.9.1
• Are all plugins except Gutenberg deactivated? No
• Are you using a default theme (e.g. Twenty Twenty-One)? No

Device information

• Device: Desktop
• Operating system: macOS Catalina
• Browser: Chrome 91.0.4472.114