Issue #3053803: Fix extra fields to be rendered via layout builder.

custom_elements.install

+<?php
+
+/**
+ * @file
+ * Module install file.
+ */
+
+/**
+ * Implements hook_install().
+ */
+function custom_elements_install() {
+  // Make sure our module comes after layout_builder.
+  // @see custom_elements_module_implements_alter().
+  module_set_weight('custom_elements', 10);
+}
+
+/**
+ * Set module weight.
+ */
+function custom_elements_update_8201() {
+  // Make sure our module comes after layout_builder.
+  // @see custom_elements_module_implements_alter().
+  module_set_weight('custom_elements', 10);
+}

custom_elements.module

@@ -76,6 +76,14 @@ function custom_elements_module_implements_alter(&$implementations, $hook) {
     unset($implementations['custom_elements']);
     $implementations['custom_elements'] = $group;
   }
+  // Be sure our entity_view_alter hook comes last, even after layout builder
+  // which also tries to make it come last.
+  // @see custom_elements_install().
+  if ($hook === 'entity_view_alter') {
+    $group = $implementations['custom_elements'];
+    unset($implementations['custom_elements']);
+    $implementations['custom_elements'] = $group;
+  }
 }
 
 /**
@@ -95,5 +103,13 @@ function custom_elements_entity_view_alter(array &$build, Drupal\Core\Entity\Ent
   // Alter the build array to apply custom elements rendering.
   if (!empty($build['#custom_elements_enabled'])) {
     $build['#theme'] = 'custom_element';
+
+    // Add layout builder sections, but be sure to do so once extra fields
+    // were added by layout_builder_entity_view_alter()
+    // @see custom_elements_module_implements_alter().
+    // @see \Drupal\custom_elements\CustomElementsEntityViewDisplayTrait::buildMultipleViaCustomElements()
+    if (!empty($build['#custom_elements_add_layout_builder_section'])) {
+      $build['#custom_element']->setSlot('sections', $build['_layout_builder']);
+    }
   }
 }

src/CustomElementsEntityViewDisplayTrait.php

@@ -31,9 +31,10 @@ trait CustomElementsEntityViewDisplayTrait {
       $build['#view_mode'] = $this->originalMode;
       $build['#custom_element'] = $this->getCustomElementGenerator()->generate($entity, $build['#view_mode']);
 
-      // Add layout build sections unless already done so.
-      if (isset($build['_layout_builder']) && empty($build['#custom_element']->getSlot('sections'))) {
-        $build['#custom_element']->setSlot('sections', $build['_layout_builder']);
+      // Add layout build sections later during entity_view_alter().
+      // @see custom_elements_entity_view_alter()
+      if (isset($build['_layout_builder']) && !isset($build['#custom_elements_add_layout_builder_section'])) {
+        $build['#custom_elements_add_layout_builder_section'] = TRUE;
       }
     }
   }