Drupal 8 Book module tree add region name to FILE NAME SUGGESTIONS

region-suggestion.md

<!-- FILE NAME SUGGESTIONS:
   * book-tree--book-toc-180.html.twig
   * book-tree--book-toc-180--sidebar-first.html.twig
   x book-tree.html.twig
-->

• The second suggestion is the new suggestion based on what region the block is placed in
• This allows for targeted templates based on block placement

yourtheme.theme

// Add to your subtheme .theme file
// Replace 'THEMENAME'

// Add a region variable to a block.
function THEMENAME_preprocess_block(&$variables) {
  if (isset($variables["elements"]["#id"])) {
    $block_id = $variables["elements"]["#id"];
    $block = \Drupal\block\Entity\Block::load($block_id);

    if ($block) {
      $variables["content"]["#attributes"]["region"] = $block->getRegion();
    }
  }
}

// add a template suggestion based on region name
function THEMENAME_theme_suggestions_alter(array &$suggestions, array $variables, $hook) {
  if (isset($variables["attributes"]["region"])) {
    $suggestions[] = $variables["theme_hook_original"] . "__" . $variables["attributes"]["region"];
  }
}