[5.3] Show correct custom fields when creating article after filtering by multiple categories (joomla#45353)

Pull Request for Issue joomla#45303 

This PR fixes a bug where creating a new article after filtering by multiple categories (e.g. "News" and "Events") incorrectly displays all custom fields from all those categories on the "Fields" tab.

Author: AdarshSantoria

administrator/components/com_content/src/Model/ArticleModel.php

@@ -587,7 +587,18 @@ protected function loadFormData()
                         ((isset($filters['published']) && $filters['published'] !== '') ? $filters['published'] : null)
                     )
                 );
-                $data->set('catid', $app->getInput()->getInt('catid', (!empty($filters['category_id']) ? $filters['category_id'] : null)));
+
+                // If multiple categories are filtered, pick the first one to avoid loading all fields
+                $filteredCategories = $filters['category_id'] ?? null;
+                $selectedCatId      = null;
+
+                if (\is_array($filteredCategories)) {
+                    $selectedCatId = (int) reset($filteredCategories);
+                } elseif (!empty($filteredCategories)) {
+                    $selectedCatId = (int) $filteredCategories;
+                }
+
+                $data->set('catid', $app->getInput()->getInt('catid', $selectedCatId));
 
                 if ($app->isClient('administrator')) {
                     $data->set('language', $app->getInput()->getString('language', (!empty($filters['language']) ? $filters['language'] : null)));