Fix: Bootstrap theme loading

src/Select2.php

@@ -4,6 +4,7 @@
 
 namespace Yii2\Extensions\Select2;
 
+use Yii2\Extensions\Select2\Themes\Select2BS5ThemeAsset;
 use yii\helpers\Html;
 use yii\helpers\Json;
 use yii\widgets\InputWidget;
@@ -44,7 +45,7 @@ protected function registerPlugin($name)
             if (!isset($options['theme'])) {
                 //Do we have BS5?
                 if (class_exists('Yii2\Asset\BootstrapAsset')) {
-                    $view->registerCssFile('@npm/select2-bootstrap-5-theme/dist/select2-bootstrap-5-theme.min.css');
+                    Select2BS5ThemeAsset::register($view);
                     $options['theme'] = 'bootstrap-5';
                 } else {
                     $options['theme'] = 'classic';

src/Themes/Select2BS5ThemeAsset.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Yii2\Extensions\Select2\Themes;
+
+use yii\web\AssetBundle;
+
+
+class Select2BS5ThemeAsset extends AssetBundle
+{
+    public $sourcePath = '@npm/select2-bootstrap-5-theme/dist';
+
+    public $js = [];
+
+    public $css = [
+        'select2-bootstrap-5-theme.min.css',
+    ];
+
+    public function init()
+    {
+        parent::init();
+
+        //which BS version is installed (from highest to lowest)
+        if (class_exists('yii\bootstrap5\BootstrapAsset')) { //BS5 official
+            $this->depends[] = 'yii\bootstrap5\BootstrapAsset';
+        } else if (class_exists('Yii2\Asset\BootstrapAsset')) { //BS5 Yii2-Ext
+            $this->depends[] = 'Yii2\Asset\BootstrapAsset';
+        }
+    }
+}