What?

Closes #79019

Introduce a font-library theme support that provides a single mechanism for enabling or disabling the Font Library across both the Block Editor and the WP Admin interface.

When the Font Library is disabled using remove_theme_support( 'font-library' ), the Font Library UI is no longer available in the editor and the Fonts admin screen is removed and cannot be accessed directly.

Why?

Prior to this change, developers could disable the Font Library in the editor by setting fontLibraryEnabled to false via the block_editor_settings_all filter.

However, after the introduction of the dedicated Fonts admin screen, this approach no longer fully disabled the feature. The editor UI could be hidden while the Fonts screen under Appearance remained accessible through both the menu and direct URLs.

This creates an inconsistent experience and requires developers to implement multiple custom solutions to fully disable the feature.

Providing a dedicated font-library theme support aligns with existing WordPress patterns and offers a single, centralized way to control Font Library availability.

How?

This PR introduces a new font-library theme support and uses it as the source of truth for Font Library availability.

Font Library Admin Screen

• Core's Fonts submenu registration is removed to avoid duplicate menu items.

• The Gutenberg Fonts screen is only registered when the active theme supports font-library.

• When the feature is disabled, direct access to:

  • themes.php?page=font-library.php
  • themes.php?page=font-library-wp-admin

  is blocked and returns an error response.

Block Editor Settings

• gutenberg_get_block_editor_settings() now derives fontLibraryEnabled from current_theme_supports( 'font-library' ).
• This ensures the editor UI automatically reflects the enabled/disabled state of the feature.
• Existing customizations using block_editor_settings_all continue to work because they can still override the setting through the existing filter.

Testing Instructions

Default Behavior

1. Activate Gutenberg.
2. Open the Site Editor.
3. Verify the Font Library is available from the Typography tools.
4. Navigate to Appearance → Fonts.
5. Verify the Fonts screen is accessible.

Disable the Font Library

1. Add the following to the active theme:

add_action(
	'after_setup_theme',
	function() {
		remove_theme_support( 'font-library' );
	}
);

2. Refresh the WP Admin.
3. Verify the Fonts submenu is no longer visible under Appearance.
4. Open the Site Editor.
5. Verify Font Library controls are no longer available.

Direct URL Access

1. With the theme support removed, visit:

/wp-admin/themes.php?page=font-library.php

2. Verify access is denied.

3. Visit:

/wp-admin/themes.php?page=font-library-wp-admin

4. Verify access is denied.

Backward Compatibility

1. Add the following filter:

add_filter(
	'block_editor_settings_all',
	function( $settings ) {
		$settings['fontLibraryEnabled'] = false;
		return $settings;
	}
);

2. Verify the Font Library is still disabled in the editor.
3. Verify no regressions are introduced to existing customization workflows.

Screenshots or screencast