diff --git a/core/lib/Drupal/Core/Field/WidgetBase.php b/core/lib/Drupal/Core/Field/WidgetBase.php
index 4624ab0..0e57903 100644
--- a/core/lib/Drupal/Core/Field/WidgetBase.php
+++ b/core/lib/Drupal/Core/Field/WidgetBase.php
@@ -71,8 +71,7 @@ public function form(FieldItemListInterface $items, array &$form, array &$form_s
     // If the widget is handling multiple values (e.g Options), or if we are
     // displaying an individual element, just get a single form element and make
     // it the $delta value.
-    $definition = $this->getPluginDefinition();
-    if (isset($get_delta) || $definition['multiple_values']) {
+    if ($this->handlesMultipleValues() || isset($get_delta)) {
       $delta = isset($get_delta) ? $get_delta : 0;
       $element = array(
         '#title' => check_plain($this->fieldDefinition->getLabel()),
@@ -279,20 +278,22 @@ public function extractFormValues(FieldItemListInterface $items, array $form, ar
     $values = NestedArray::getValue($form_state['values'], $path, $key_exists);
 
     if ($key_exists) {
-      // Remove the 'value' of the 'add more' button.
-      unset($values['add_more']);
+      // Account for drag-and-drop reordering if needed.
+      if (!$this->handlesMultipleValues()) {
+        // Remove the 'value' of the 'add more' button.
+        unset($values['add_more']);
 
-      // The original delta, before drag-and-drop reordering, is needed to
-      // route errors to the corect form element.
-      foreach ($values as $delta => &$value) {
-        $value['_original_delta'] = $delta;
+        // The original delta, before drag-and-drop reordering, is needed to
+        // route errors to the corect form element.
+        foreach ($values as $delta => &$value) {
+          $value['_original_delta'] = $delta;
+        }
+
+        usort($values, function ($a, $b) {
+          return SortArray::sortByKeyInt($a, $b, '_weight');
+        });
       }
 
-      // Account for drag-and-drop reordering.
-      usort($values, function ($a, $b) {
-        return SortArray::sortByKeyInt($a, $b, '_weight');
-      });
-
       // Let the widget massage the submitted values.
       $values = $this->massageFormValues($values, $form, $form_state);
 
@@ -303,7 +304,7 @@ public function extractFormValues(FieldItemListInterface $items, array $form, ar
       // Put delta mapping in $form_state, so that flagErrors() can use it.
       $field_state = field_form_get_state($form['#parents'], $field_name, $form_state);
       foreach ($items as $delta => $item) {
-        $field_state['original_deltas'][$delta] = $item->_original_delta;
+        $field_state['original_deltas'][$delta] = isset($item->_original_delta) ? $item->_original_delta : $delta;
         unset($item->_original_delta, $item->_weight);
       }
       field_form_set_state($form['#parents'], $field_name, $form_state, $field_state);
@@ -341,8 +342,7 @@ public function flagErrors(FieldItemListInterface $items, array $form, array &$f
 
       // Only set errors if the element is accessible.
       if (!isset($element['#access']) || $element['#access']) {
-        $definition = $this->getPluginDefinition();
-        $is_multiple = $definition['multiple_values'];
+        $handles_multiple = $this->handlesMultipleValues();
 
         $violations_by_delta = array();
         foreach ($field_state['constraint_violations'] as $violation) {
@@ -361,7 +361,7 @@ public function flagErrors(FieldItemListInterface $items, array $form, array &$f
           // Pass violations to the main element:
           // - if this is a multiple-value widget,
           // - or if the violations are at the ItemList level.
-          if ($is_multiple || $delta === NULL) {
+          if ($handles_multiple || $delta === NULL) {
             $delta_element = $element;
           }
           // Otherwise, pass errors by delta to the corresponding sub-element.
@@ -435,4 +435,16 @@ protected function getFieldSetting($setting_name) {
     return $this->fieldDefinition->getSetting($setting_name);
   }
 
+  /**
+   * Returns whether the widget handles multiple values.
+   *
+   * @return bool
+   *   TRUE if a single copy of formElement() can handle multiple field values,
+   *   FALSE if multiple values require separate copies of formElement().
+   */
+  protected function handlesMultipleValues() {
+    $definition = $this->getPluginDefinition();
+    return $definition['multiple_values'];
+  }
+
 }