Widgets: Add support for the Widgets Editor on `after_setup_theme` instead of `widgets_init`.

This better aligns with developer expectations, as `add_theme_support()` and `remove_theme_support()` are meant to be called within functions attached to the `after_setup_theme` hook.

This also adds the `widgets-block-editor` feature to the docblock for `add_theme_support()`. 

Props kevin940726, caseymilne, jamesros161, noisysocks, Mamaduka, audrasjb, zieladam, hellofromTonya, desrosj.
Fixes #53424.
Built from https://develop.svn.wordpress.org/trunk@51214


git-svn-id: http://core.svn.wordpress.org/trunk@50823 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/default-filters.php

@@ -597,6 +597,7 @@ add_filter( 'plupload_default_settings', 'wp_show_heic_upload_error' );
 add_filter( 'nav_menu_item_id', '_nav_menu_item_id_use_once', 10, 2 );
 
 // Widgets.
+add_action( 'after_setup_theme', 'wp_setup_widgets_block_editor', 1 );
 add_action( 'init', 'wp_widgets_init', 1 );
 
 // Admin Bar.

wp-includes/theme.php

@@ -2479,6 +2479,7 @@ function get_theme_starter_content() {
  * @since 5.5.0 The `core-block-patterns` feature was added and is enabled by default.
  * @since 5.5.0 The `custom-logo` feature now also accepts 'unlink-homepage-logo'.
  * @since 5.6.0 The `post-formats` feature warns if no array is passed.
+ * @since 5.8.0 The `widgets-block-editor` enables the Widgets block editor.
  *
  * @global array $_wp_theme_features
  *
@@ -2511,6 +2512,7 @@ function get_theme_starter_content() {
  *                          - 'title-tag'
  *                          - 'wp-block-styles'
  *                          - 'widgets'
+ *                          - 'widgets-block-editor'
  * @param mixed  ...$args Optional extra arguments to pass along with certain features.
  * @return void|false Void on success, false on failure.
  */

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.8-beta2-51213';
+$wp_version = '5.8-beta2-51214';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

wp-includes/widgets.php

@@ -1801,8 +1801,6 @@ function wp_widgets_init() {
 
 	register_widget( 'WP_Widget_Block' );
 
-	add_theme_support( 'widgets-block-editor' );
-
 	/**
 	 * Fires after all default WordPress widgets have been registered.
 	 *
@@ -1811,6 +1809,18 @@ function wp_widgets_init() {
 	do_action( 'widgets_init' );
 }
 
+/**
+ * Enables the widgets block editor. This is hooked into 'after_setup_theme' so
+ * that the block editor is enabled by default but can be disabled by themes.
+ *
+ * @since 5.8.0
+ *
+ * @access private
+ */
+function wp_setup_widgets_block_editor() {
+	add_theme_support( 'widgets-block-editor' );
+}
+
 /**
  * Whether or not to use the block editor to manage widgets. Defaults to true
  * unless a theme has removed support for widgets-block-editor or a plugin has