Attempting to return region in theme suggestions for the book module book tree block

Editing question based on information from @NoSssweat

The book module uses the same hook for the book tree block and the rendered index view in the book page.

I have used the below code to return file name suggestions, so that I can override the html output for the book tree block in a specific region - "sidebar-first". I expected the below code would spit out something like book-tree--book-toc-180--sidebar-first.html.twig. Instead, it just spits out a file name suggestion that is already being suggested:

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

This file name suggestion is the same for the main content on the book page. How can I target any book tree blocks within the sidebar-first region?

// Add a region variable to a block.
// http://kristiankaa.dk/article/drupal8-region-specific-menu-theme-hook-suggestion
function uswds_subtheme_preprocess_book_tree__book_toc_180(&$variables) {
    if (isset($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
// http://kristiankaa.dk/article/drupal8-region-specific-menu-theme-hook-suggestion
function uswds_subtheme_theme_suggestions_book_tree__book_toc_180_alter(array &$suggestions, array $variables) {
    if (isset($variables["attributes"]["region"])) {
        $suggestions[] = $variables["theme_hook_original"] . "__" . $variables["attributes"]["region"];
    }
}

2nd attempt I did the below, and it works but it only works for the book module default block, although any other book module block plugin all mostly use book-tree.html.twig

// Add a region variable to a block.
function uswds_subtheme_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 uswds_subtheme_theme_suggestions_alter(array &$suggestions, array $variables, $hook) {
  if (isset($variables["attributes"]["region"])) {
    $suggestions[] = $variables["theme_hook_original"] . "__" . $variables["attributes"]["region"];
  }
}

or how can I do something in twig, like {% if region == 'sidebar-first' %} and just override book-tree.html.twig

// Adds a region attribute to a block.
function uswds_subtheme_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();
    }
  }
}

/**
 * Implements hook_preprocess_HOOK().
 *
 * Creates region variable
 *
 * Gets rid of the region attribute added in uswds_subtheme_preprocess_block() as it's
 * not a valid HTML attribute.
 */
function uswds_subtheme_preprocess_book_tree(&$variables) {
  if (isset($variables['attributes']['region'])) {
    // creates region variable for book-tree.html.twig
    $variables['region'] = $variables['attributes']['region'];
    // unset invalid html attribute.
    unset($variables['attributes']['region']);
  }
}

Nice try, since this is a book and not menu, the process is similar, but not exactly the same. I was hoping you would figure it out.

But anyways, here you go:

/**
 * Implements hook_preprocess_HOOK().
 *
 * Pass block region value to content so this can be used in
 * uswds_subtheme_theme_suggestions_menu_alter() since $variables['elements']
 * is not available there.
 */
function uswds_subtheme_preprocess_block(&$variables) {
  if (isset($variables['elements']['#id']) && $variables['base_plugin_id'] === 'book_navigation') {
    $region = \Drupal\block\Entity\Block::load($variables['elements']['#id'])->getRegion();
    $content = $variables['content'];
    foreach ($content as $content_key => $content_info) {
      if (is_numeric($content_key)) {
        $variables['content'][$content_key]['#attributes']['region'] = $region;
      }
    }
  }
}

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 *
 * Provide region based book suggestions.
 */
function uswds_subtheme_theme_suggestions_book_tree_alter(&$suggestions, array $variables) {
  if (isset($variables['attributes']['region'])) {
    $suggestion = 'book__' . $variables['theme_hook_original'] . '__' . $variables['attributes']['region'];
    $suggestion = str_replace('-', '_', $suggestion);
    $suggestions[] = $suggestion;
  }
}

/**
 * Implements hook_preprocess_HOOK().
 *
 * Get rid of the region attribute added in uswds_subtheme_preprocess_block() as it's
 * not a valid HTML attribute.
 */
function uswds_subtheme_preprocess_book_tree(&$variables) {
  if (isset($variables['attributes']['region'])) {
    unset($variables['attributes']['region']);
  }
}

Using all of the help from @NoSssweat and I was able get this working:

// Add a region variable to a block.
function uswds_subtheme_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 uswds_subtheme_theme_suggestions_alter(array &$suggestions, array $variables, $hook) {
  if (isset($variables["attributes"]["region"])) {
    $suggestions[] = $variables["theme_hook_original"] . "__" . $variables["attributes"]["region"];
  }
}

Which outputs (the second suggestion including the region):

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

Update to working version:

<?php
// Add a region variable to a block. Only applies to some versions of plugins that use book-tree.html.twig 
function uswds_subtheme_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();
      }
    }
  }

// Adds template suggestion to blocks. Seems to work everywhere, but doesn't target the book tree
function uswds_subtheme_theme_suggestions_block_alter(array &$suggestions, array $variables){
    if (!empty($variables['elements']['#id'])) {
        $block = \Drupal\block\Entity\Block::load($variables['elements']['#id']);
        $region = $block->getRegion();
        // adds suggestion with region and block id
        $suggestions[] = 'block__' . $region . '__' . $variables['elements']['#id'];
        // adds suggestion with region id
        $suggestions[] = 'block__' . $region;
    }
}


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