Allow disabling AMP Customizer later

Previously, you had to hook very early on `plugins_loaded` to be able to
disable the Customizer. This was not possible for sites that only have
access to the theme. This switches around the load order of the
Customizer so that the filter to disable the AMP Customizer can run any
time before `after_setup_theme`.

amp.php

@@ -18,6 +18,7 @@
 require_once( AMP__DIR__ . '/back-compat/back-compat.php' );
 require_once( AMP__DIR__ . '/includes/amp-helper-functions.php' );
 require_once( AMP__DIR__ . '/includes/admin/functions.php' );
+require_once( AMP__DIR__ . '/includes/admin/class-amp-customizer.php' );
 require_once( AMP__DIR__ . '/includes/settings/class-amp-customizer-settings.php' );
 require_once( AMP__DIR__ . '/includes/settings/class-amp-customizer-design-settings.php' );
 
@@ -152,24 +153,16 @@ function amp_render_post( $post_id ) {
  * preview page isn't flagged as an AMP template, the core panels will be re-added and the AMP panel
  * hidden.
  *
- * @internal This callback must be hooked before priority 10 on 'plugins_loaded' to properly unhook
- *           the core panels.
- *
  * @since 0.4
  */
 function _amp_bootstrap_customizer() {
-	/**
-	 * Filter whether to enable the AMP template customizer functionality.
-	 *
-	 * @param bool $enable Whether to enable the AMP customizer. Default true.
-	 */
-	$amp_customizer_enabled = apply_filters( 'amp_customizer_is_enabled', true );
-
-	if ( true === $amp_customizer_enabled ) {
-		amp_init_customizer();
-	}
+	// Drop core panels (menus, widgets) from the AMP customizer
+	// `customize_loaded_components` runs super early so we need to call this regardless of whether the AMP customizer is enabled or not
+	add_filter( 'customize_loaded_components', array( 'AMP_Template_Customizer', '_unregister_core_panels' ) );
+
+	add_action( 'after_setup_theme', 'amp_maybe_init_customizer' );
 }
-add_action( 'plugins_loaded', '_amp_bootstrap_customizer', 9 );
+add_action( 'plugins_loaded', '_amp_bootstrap_customizer', 9 ); // Should be hooked before priority 10 on 'plugins_loaded' to properly unhook core panels.
 
 /**
  * Redirects the old AMP URL to the new AMP URL.

includes/admin/functions.php

@@ -9,11 +9,17 @@
 /**
  * Sets up the AMP template editor for the Customizer.
  */
-function amp_init_customizer() {
-	require_once( AMP__DIR__ . '/includes/admin/class-amp-customizer.php' );
+function amp_maybe_init_customizer() {
+	/**
+	 * Filter whether to enable the AMP template customizer functionality.
+	 *
+	 * @param bool $enable Whether to enable the AMP customizer. Default true.
+	 */
+	$amp_customizer_enabled = apply_filters( 'amp_customizer_is_enabled', true );
 
-	// Drop core panels (menus, widgets) from the AMP customizer
-	add_filter( 'customize_loaded_components', array( 'AMP_Template_Customizer', '_unregister_core_panels' ) );
+	if ( true !== $amp_customizer_enabled ) {
+		return;
+	}
 
 	// Fire up the AMP Customizer
 	add_action( 'customize_register', array( 'AMP_Template_Customizer', 'init' ), 500 );