Fix setting default values for repeater in update context (#4098)

Credit to @CptMeatball

modules/backend/formwidgets/Repeater.php

@@ -268,6 +268,7 @@ class Repeater extends FormWidgetBase
         $config->alias = $this->alias . 'Form'.$index;
         $config->arrayName = $this->getFieldName().'['.$index.']';
         $config->isNested = true;
+        $config->enableDefaults = self::$onAddItemCalled;
 
         $widget = $this->makeWidget('Backend\Widgets\Form', $config);
         $widget->bindToController();

modules/backend/widgets/Form.php

@@ -1071,7 +1071,7 @@ class Form extends WidgetBase
             $field = $this->allFields[$field];
         }
 
-        $defaultValue = !$this->model->exists
+        $defaultValue = $this->shouldFetchDefaultValues()
             ? $field->getDefaultFromData($this->data)
             : null;
 
@@ -1081,6 +1081,18 @@ class Form extends WidgetBase
         );
     }
 
+    /**
+     * Checks if default values should be taken from data.
+     * This should be done when model exists or when explicitly configured
+     */
+    protected function shouldFetchDefaultValues() {
+        $enableDefaults = object_get($this->config, 'enableDefaults');
+        if ($enableDefaults === false) {
+            return false;
+        }
+        return !$this->model->exists || $enableDefaults;
+    }
+
     /**
      * Returns a HTML encoded value containing the other fields this
      * field depends on