head/title tag for posts without titles?

(To clarify: it’s not so much the trimming words from the content that I worry about; it’s the conditional testing for the right things, and what it should be hooking into in order to work.)

FWIW, I don’t remember how I found this, but this is what worked.

// Fake a title-tag for untitled posts
add_filter( 'document_title_parts', function( $title_parts_array ) {
        if ( is_single() && empty($title_parts_array['title']) ) {
                global $post;
                $content = $post->post_content;
                $trimmed = wp_trim_words( $content, 8, ' … ' );
                $trimmed = esc_html( $trimmed );
                $title_parts_array['title'] = $trimmed;
        }
        return $title_parts_array;
} );