Only display featured image UI when theme supports it too

editor/components/post-type-support-check/index.js

@@ -1,29 +1,37 @@
 /**
  * External dependencies
  */
-import { get, some, castArray } from 'lodash';
+import { get, includes, some, castArray } from 'lodash';
 
 /**
  * WordPress dependencies
  */
 import { withSelect } from '@wordpress/data';
 
-function PostTypeSupportCheck( { postType, children, supportKeys } ) {
+function PostTypeSupportCheck( { postType, children, supportKeys, themeSupports } ) {
+	supportKeys = castArray( supportKeys );
 	const isSupported = some(
-		castArray( supportKeys ), ( key ) => get( postType, [ 'supports', key ], false )
+		supportKeys, ( key ) => get( postType, [ 'supports', key ], false )
 	);
 
 	if ( ! isSupported ) {
 		return null;
 	}
 
+	// 'thumbnail' and 'post-thumbnails' are intentionally different.
+	if ( includes( supportKeys, 'thumbnail' )
+		&& ! get( themeSupports, 'post-thumbnails', false ) ) {
+		return null;
+	}
+
 	return children;
 }
 
 export default withSelect( ( select ) => {
 	const { getEditedPostAttribute } = select( 'core/editor' );
-	const { getPostType } = select( 'core' );
+	const { getPostType, getThemeSupports } = select( 'core' );
 	return {
+		themeSupports: getThemeSupports(),
 		postType: getPostType( getEditedPostAttribute( 'type' ) ),
 	};
 } )( PostTypeSupportCheck );

lib/rest-api.php

@@ -370,6 +370,11 @@ function gutenberg_ensure_wp_json_has_theme_supports( $response ) {
 
 		$site_info['theme_supports']['formats'] = $formats;
 	}
+	if ( ! array_key_exists( 'post-thumbnails', $site_info['theme_supports'] ) ) {
+		if ( get_theme_support( 'post-thumbnails' ) ) {
+			$site_info['theme_supports']['post-thumbnails'] = true;
+		}
+	}
 	$response->set_data( $site_info );
 	return $response;
 }