Customizer: Remove Menus panel when a theme does not support menus

By overriding check_capabilities(), we can ensure that the Menus panel
is removed if a theme does not have support for 'menus' nor 'widgets'.

This ensures that the Menus panel does not appear when using a block
theme, which is confusing to users.

See #54888.
Props hellofromTonya, costdev, peterwilsoncc.

Built from https://develop.svn.wordpress.org/branches/5.9@52623


git-svn-id: http://core.svn.wordpress.org/branches/5.9@52211 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-customize-panel.php

@@ -232,10 +232,11 @@ class WP_Customize_Panel {
 	 * feature support required by the panel.
 	 *
 	 * @since 4.0.0
+	 * @since 5.9.0 Method was marked non-final.
 	 *
 	 * @return bool False if theme doesn't support the panel or the user doesn't have the capability.
 	 */
-	final public function check_capabilities() {
+	public function check_capabilities() {
 		if ( $this->capability && ! current_user_can( $this->capability ) ) {
 			return false;
 		}

wp-includes/customize/class-wp-customize-nav-menus-panel.php

@@ -98,4 +98,27 @@ class WP_Customize_Nav_Menus_Panel extends WP_Customize_Panel {
 		<li class="customize-control-title customize-section-title-nav_menus-heading"><?php _e( 'Menus' ); ?></li>
 		<?php
 	}
+
+	/**
+	 * Checks required user capabilities and whether the theme has the
+	 * feature support required by the panel.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @return bool False if theme doesn't support the panel or the user doesn't have the capability.
+	 */
+	public function check_capabilities() {
+		/*
+		 * WP_Customize_Panel::$theme_supports only supports checking one
+		 * theme_supports, so instead we override check_capabilities().
+		 */
+		if (
+			! current_theme_supports( 'menus' ) &&
+			! current_theme_supports( 'widgets' )
+		) {
+			return false;
+		}
+
+		return parent::check_capabilities();
+	}
 }

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-RC3-52619';
+$wp_version = '5.9-RC3-52623';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.