hook_preprocess_views_view_grouping() is now invoked for all grouping levels, including single-level grouping

hook_preprocess_views_view_grouping() was only invoked for nested (multi-level) grouping. For single-level grouping, StylePluginBase::renderGroupingSets() bypassed the views_view_grouping theme hook and delegated directly to renderRowGroup(), which uses the style's own theme (e.g. views_view_unformatted). The preprocess hook was therefore never triggered.

Two changes have been made to fix this:

1. StylePluginBase::renderGroupingSets()

   At the leaf level (rows are individual result rows, not nested groups), when grouping is configured, the method now builds a #theme => views_view_grouping render array instead of calling renderRowGroup() directly. This ensures hook_preprocess_views_view_grouping() is invoked at every grouping level.

   Before:

   // Leaf rows always went through renderRowGroup(), skipping views_view_grouping.
   $single_output = $this->renderRowGroup($set['rows']);
   

   After:

   if (!empty($this->options['grouping'])) {
     $single_output = [
       '#theme' => $theme_functions,
       '#view' => $this->view,
       '#grouping' => $this->options['grouping'][$level] ?? [],
       '#rows' => $set['rows'],
     ];
   }
   else {
     $single_output = $this->renderRowGroup($set['rows']);
   }
   

2. ViewsThemeHooks::preprocessViewsViewGrouping()

   The preprocess function now detects whether $variables['rows'] contains nested grouping sets (arrays with a 'group' key) or already-rendered rows (leaf level), and handles each case:

   • Non-leaf level: calls renderGroupingSets() recursively, as before.
   • Leaf level (including single-level grouping): wraps the rows in the style's own theme render array.

   $variables['content'] is now always an indexed array of render arrays at every level.

   Additionally, $variables['title'] is now wrapped in a Markup object to prevent Twig from double-escaping the already-HTML-escaped rendered field value used as the group title.

Impact on site builders and module developers

This change affects you if you have:

• A custom views-view-grouping.html.twig template — it will now also be invoked for single-level grouping, where it was previously skipped. Review your template to ensure it handles both leaf and non-leaf levels correctly.
• A hook_preprocess_views_view_grouping() implementation — it will now fire for all grouping levels. The $variables['content'] variable is always an indexed array of render arrays; if your hook assumed a different structure for single-level grouping, update it accordingly.
• A hook_preprocess_views_view_grouping() implementation that outputs $variables['title'] — it is now a Markup object; do not re-escape it.