[Add region-specific Drupal 8 template suggestions for blocks]
https://gist.github.com/mogtofu33/fdb587329a13eedc854ef264abdd56a9
// this is needed for region-based block template suggestions
use Drupal\block\Entity\Block;
function radicati_d8_theme_suggestions_block_alter(array &$suggestions, array $variables) {
if (!empty($variables['elements']['#id'])) {
if ($block = Block::load($variables["elements"]["#id"])) {
$region = $block->getRegion();
$suggestions[] = 'block__' . $region;
$suggestions[] = 'block__' . $region . '__' . $variables['elements']['#base_plugin_id'];
$suggestions[] = 'block__' . $region . '__' . $variables['elements']['#id'];
$suggestions[] = 'block__' . $region . '__' . $variables['elements']['#base_plugin_id'] . '__' . $variables['elements']['#id'];
}
}
}