diff --git a/core/lib/Drupal/Core/Entity/EntityDisplayBase.php b/core/lib/Drupal/Core/Entity/EntityDisplayBase.php
index a138c3b..cfb0ac2 100644
--- a/core/lib/Drupal/Core/Entity/EntityDisplayBase.php
+++ b/core/lib/Drupal/Core/Entity/EntityDisplayBase.php
@@ -22,6 +22,10 @@
 
   use ThirdPartySettingsTrait;
 
+  /**
+   * The 'mode' for runtime EntityDisplay objects used to render entities with
+   * arbitrary display options rather than a configured view mode or form mode.
+   */
   const CUSTOM_MODE = '_custom';
 
   /**
@@ -215,39 +219,42 @@ public function toArray() {
    * - or that are not supposed to be configurable.
    */
   protected function init() {
-    // Fill in defaults for extra fields.
-    $context = $this->displayContext == 'view' ? 'display' : $this->displayContext;
-    $extra_fields = \Drupal::entityManager()->getExtraFields($this->targetEntityType, $this->bundle);
-    $extra_fields = isset($extra_fields[$context]) ? $extra_fields[$context] : array();
-    foreach ($extra_fields as $name => $definition) {
-      if (!isset($this->content[$name]) && !isset($this->hidden[$name])) {
-        // Extra fields are visible by default unless they explicitly say so.
-        if (!isset($definition['visible']) || $definition['visible'] == TRUE) {
-          $this->content[$name] = array(
-            'weight' => $definition['weight']
-          );
-        }
-        else {
-          $this->hidden[$name] = TRUE;
+    // Only populate defaults for "official" view modes and form modes.
+    if ($this->mode !== static::CUSTOM_MODE) {
+      // Fill in defaults for extra fields.
+      $context = $this->displayContext == 'view' ? 'display' : $this->displayContext;
+      $extra_fields = \Drupal::entityManager()->getExtraFields($this->targetEntityType, $this->bundle);
+      $extra_fields = isset($extra_fields[$context]) ? $extra_fields[$context] : array();
+      foreach ($extra_fields as $name => $definition) {
+        if (!isset($this->content[$name]) && !isset($this->hidden[$name])) {
+          // Extra fields are visible by default unless they explicitly say so.
+          if (!isset($definition['visible']) || $definition['visible'] == TRUE) {
+            $this->content[$name] = array(
+              'weight' => $definition['weight']
+            );
+          }
+          else {
+            $this->hidden[$name] = TRUE;
+          }
         }
       }
-    }
 
-    // Fill in defaults for fields.
-    $fields = $this->getFieldDefinitions();
-    foreach ($fields as $name => $definition) {
-      if (!$definition->isDisplayConfigurable($this->displayContext) || (!isset($this->content[$name]) && !isset($this->hidden[$name]))) {
-        $options = $definition->getDisplayOptions($this->displayContext);
+      // Fill in defaults for fields.
+      $fields = $this->getFieldDefinitions();
+      foreach ($fields as $name => $definition) {
+        if (!$definition->isDisplayConfigurable($this->displayContext) || (!isset($this->content[$name]) && !isset($this->hidden[$name]))) {
+          $options = $definition->getDisplayOptions($this->displayContext);
 
-        if (!empty($options['type']) && $options['type'] == 'hidden') {
-          $this->hidden[$name] = TRUE;
+          if (!empty($options['type']) && $options['type'] == 'hidden') {
+            $this->hidden[$name] = TRUE;
+          }
+          elseif ($options) {
+            $this->content[$name] = $this->pluginManager->prepareConfiguration($definition->getType(), $options);
+          }
+          // Note: (base) fields that do not specify display options are not
+          // tracked in the display at all, in order to avoid cluttering the
+          // configuration that gets saved back.
         }
-        elseif ($options) {
-          $this->content[$name] = $this->pluginManager->prepareConfiguration($definition->getType(), $options);
-        }
-        // Note: (base) fields that do not specify display options are not
-        // tracked in the display at all, in order to avoid cluttering the
-        // configuration that gets saved back.
       }
     }
   }
@@ -347,6 +354,8 @@ protected function getFieldDefinitions() {
 
     if (!isset($this->fieldDefinitions)) {
       $definitions = \Drupal::entityManager()->getFieldDefinitions($this->targetEntityType, $this->bundle);
+      // For "official" view modes and form modes, ignore fields whose
+      // definition states they should not be displayed.
       if ($this->mode !== static::CUSTOM_MODE) {
         $definitions = array_filter($definitions, array($this, 'fieldHasDisplayOptions'));
       }