Properly enqueue block style_handles in pattern editor

[bor0]

The function _wp_get_iframed_editor_assets enqueues editor_style_handles, but not style_handles.

In the admin editor, frontend styles are already on the page, so this works. In a frontend block editor (e.g. Pattern Creator), they are never loaded, breaking blocks like Cover that rely on them for canvas layout/background CSS.

Additionally, __unstableIframe dropped its head prop in Gutenberg 22.x, preventing EditorStyles from injecting theme styles into the canvas (fonts, CSS custom properties, etc.).

The proposed patch here carries workarounds for both:

• PHP: hooks into enqueue_block_assets to enqueue style_handles inside the capture window
• JS: moves EditorStyles from the head prop to a child of Iframe

[bor0]

Frontend with patch applied (no visual changes)

[bor0]

Backend with patch applied (styles are properly attached, visually looks like frontend)

[bor0]

cc @dd32 for another one, would appreciate your input

[dd32]

@bor0 Please check you select the correct component, as in this case it'd redirect you to the Github source where you can PR it :) ​https://github.com/WordPress/pattern-directory/pulls

[bor0]

Thanks! Created ​https://github.com/WordPress/pattern-directory/pull/740