Local font asset files not loading in post Editor when metaboxes are enabled (i.e. Editor is not iframed)

[ndiego]

Description

#51178 fixed a bug in 15.9 that prevented fonts included in themes to be loaded in the Editor, both post and Site Editor. This was due to a change in #50875.

While #51178 fixed the issue when the Editor is iframed, if a plugin registers metaboxes, the Editor is no longer iframed. As a result, the fonts are no longer loaded.

cc @hellofromtonya @ellatrix

Step-by-step reproduction instructions

1. Using TT3, switch to the Block out style variation, which uses a custom heading font making it easy to see the difference.
2. Install and activate Yoast, or another plugin that registers a metabox in the post Editor.
3. Create a new page/post and add a few headings. See that the custom IBM Plex Mono font is not actually getting applied.
4. Deactivate Yoast and see that the font is now applied.

Screenshots, screen recording, code snippet

With Yoast Active	Without Yoast Active

Environment info

• WordPress 6.2
• Gutenberg 15.9.1

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[hellofromtonya]

This issue will be resolved with Font Face, which is replacing the Fonts API.

[hellofromtonya]

I'm closing this issue.

Why? No further work will happen on the Fonts API.

Why? It is being replaced by Font Face and will be removed when Font Library is merged.

[hellofromtonya]

I'm not seeing where this issue was directly introduced within the Fonts API itself. @ndiego reported the issue still exists with the new Font Face generator and printer. Since it exists in both the Fonts API and new Font Face, I suspect the root cause is not within either.

I'm reopening this issue to capture the discussion, investigation, and separate resolving PR.