Issue #2175517 by chx: Entity displays are themselves config entities.

core/lib/Drupal/Core/Entity/Display/EntityDisplayInterface.php

@@ -7,10 +7,12 @@
 
 namespace Drupal\Core\Entity\Display;
 
+use Drupal\Core\Config\Entity\ConfigEntityInterface;
+
 /**
  * Provides a common interface for entity displays.
  */
-interface EntityDisplayInterface {
+interface EntityDisplayInterface extends ConfigEntityInterface {
 
   /**
    * Creates a duplicate of the EntityDisplay object on a different view mode.