[Fonts API] All registered fonts are being rendered outside of the Site Editor

[hellofromtonya]

Description

In lib/compat/wordpress-6.2/gutenberg_resolve_assets_override(), all of the registered fonts are being rendered when not in the Site Editor. This can cause a performance issue when a plugin(s) registers additional fonts for user selection.

The way it's supposed to work is this:

• When in the Site Editor, all registered fonts are inserted into the iframe, allowing users to preview typography selections in the Styles UI for possible selection.
• When not in the Site Editor, only enqueued fonts should be inserted into the iframe. These enqueued fonts are all of the theme defined fonts and any user-selected fonts from plugins.

Step-by-step reproduction instructions

1. On your test site, activate the TT3 theme and Gutenberg plugin.
2. Install and activate this test plugin.
3. Open a post by going to Posts > and selecting or adding a post.
4. In your browser, open Dev Tools to the Inspector tab.
5. Search for wp-fonts-jetpack-google-fonts.

The expected behavior:

• <style id="wp-fonts-jetpack-google-fonts"> should not exist.
• <style id="wp-fonts-local"> should exist (these are the theme's defined fonts).

Screenshots, screen recording, code snippet

With no user selected fonts, <style id="wp-fonts-jetpack-google-fonts"> should not exist in a Post's iframed editor HTML.

Notice in the following screenshot, it does exist in the Post's iframed editor HTML 🐞 ❌

Environment info

• WordPress: 6.2
• Gutenberg: 15.5 and trunk

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]

Reopening as #49646 was reverted for now, i.e. until other dependencies are committed.

[hellofromtonya]

I've marked this issue as blocked. It's currently blocked by #40363. Why?

See this explanation: #49646 (comment)

It needs to be bundled with the #40363 feature enhancement which (when implemented) will automatically enqueue all user selected fonts, selected from the Site Editor's Global Typography UI. Why? One of the most popular Google Fonts implementation has a bug in it where its fonts will not enqueue. The bug was hidden until this PR fixed the Fonts API for the iframe assets. Delaying the shipment of this PR's fix avoids breaking sites.

[hellofromtonya]

A fix is ready for merging. However, IMO the bugfix should wait until #40362 is resolved. I explain the reasoning here.

[hellofromtonya]

Update:

This issue is no longer valid for the Fonts API once the Font Library is merged.

Why?

As shared in Ongoing Roadmap update #41479 (comment), the Fonts API will no longer be expose "register" or "enqueue" functionality. Instead, it will pull the fonts from theme.json merged data.

I'll close this issue.