diff --git a/core/modules/link/src/Plugin/DataType/LinkUrlComputed.php b/core/modules/link/src/Plugin/DataType/LinkUrlComputed.php
index 13d571a68f..59bf96a9f0 100644
--- a/core/modules/link/src/Plugin/DataType/LinkUrlComputed.php
+++ b/core/modules/link/src/Plugin/DataType/LinkUrlComputed.php
@@ -15,7 +15,7 @@
  *   label = @Translation("Link URL")
  * )
  */
-class LinkUrlComputed extends Uri implements CacheableDependencyInterface  {
+class LinkUrlComputed extends Uri implements CacheableDependencyInterface {
 
   /**
    * The generated URL.
@@ -44,6 +44,8 @@ public function getValue() {
   public function setValue($value, $notify = TRUE) {
     if (!empty($value)) {
       $parsed = UrlHelper::parse($value);
+      // If the path is not an external URL then add 'internal:' prefix to make
+      // it a valid uri.
       if (strpos($parsed['path'], ':') === FALSE) {
         $parsed['path'] = 'internal:' . $parsed['path'];
       }
diff --git a/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php b/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php
index afd83b3e59..8902180896 100644
--- a/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php
+++ b/core/modules/link/src/Plugin/Field/FieldType/LinkItem.php
@@ -190,10 +190,13 @@ public function onChange($property_name, $notify = TRUE) {
       $property = $this->get('url');
       if ($url = $property->getValue()) {
         $parsed = UrlHelper::parse($url);
+        // If the path is not an external URL then add 'internal:' prefix to
+        // make it a valid uri.
         if (strpos($parsed['path'], ':') === FALSE) {
           $parsed['path'] = 'internal:' . $parsed['path'];
         }
         $this->writePropertyValue('uri', $parsed['path']);
+        // Only set the options if we have query parameters or a fragment.
         if (!empty($parsed['query']) || !empty($parsed['fragment'])) {
           $this->writePropertyValue('options', ['query' => $parsed['query'], 'fragment' => $parsed['fragment']]);
         }
@@ -223,6 +226,9 @@ public function setValue($values, $notify = TRUE) {
       }
     }
     parent::setValue($values, $notify);
+    // Support setting the field item with only url property, but make sure
+    // values stay in sync if only url property is passed.
+    // NULL is a valid value, so we use array_key_exists().
     if (is_array($values) && array_key_exists('url', $values) && !array_key_exists('uri', $values)) {
       $this->onChange('url', FALSE);
     }
diff --git a/core/modules/menu_link_content/tests/src/Functional/Rest/MenuLinkContentResourceTestBase.php b/core/modules/menu_link_content/tests/src/Functional/Rest/MenuLinkContentResourceTestBase.php
index 039198c845..af3ded6c13 100644
--- a/core/modules/menu_link_content/tests/src/Functional/Rest/MenuLinkContentResourceTestBase.php
+++ b/core/modules/menu_link_content/tests/src/Functional/Rest/MenuLinkContentResourceTestBase.php
@@ -90,7 +90,7 @@ protected function getNormalizedPostEntity() {
       'link' => [
         [
           'uri' => 'http://www.urbandictionary.com/define.php?term=drama%20llama',
-          'url' => 'http://www.urbandictionary.com/define.php?term=drama%20llama',
+          'url' => 'http://www.urbandictionary.com/define.php?term=drama%20llama#a-fragment',
           'options' => [
             'fragment' => 'a-fragment',
             'attributes' => [