Merge Subtitles:

The manually added subtitles are retrieved and stored in $_tracks.
Dynamically detected subtitles are then added to $_tracks only if the language is not already present (i.e., not manually added).

#Toan Code ____ to move edits to childtheme on class-streamtube-core-post.php and Vido.min.js for TextTracks and Subtitile preload.
add_filter('streamtube/player/file/setup', function ($setup, $source) {

    if (!array_key_exists('mediaid', $setup)) {
        return $setup;
    }

    $post_id = $setup['mediaid'];

    if (!get_post_status($post_id)) {
        return $setup;
    }

    $default_subtitle = strtolower(get_option('player_default_subtitle'));

    $_tracks = array();

    // Retrieve manually added subtitles
    if (array_key_exists('tracks', $setup)) {
        foreach ($setup['tracks'] as $manual_track) {
            $_tracks[$manual_track['srclang']] = $manual_track;
        }
    }

    // Get the video URL meta value
    $video_url = get_post_meta($post_id, 'video_url', true);

    if (!$video_url) {
        return $setup;
    }

    // Extract the YouTube ID from the video URL
    preg_match('/(?:youtube\.com\/(?:[^\/\n\s]+\/\S+\/|(?:v|e(?:mbed)?)\/|.*[?&]v=)|youtu\.be\/)([a-zA-Z0-9_-]{11})/', $video_url, $matches);

    if (!$matches || !isset($matches[1])) {
        return $setup;
    }

    $youtube_id = $matches[1];

    // Directory where subtitle files are stored
    $subtitle_dir = WP_CONTENT_DIR . '/uploads/subtitles/' . $youtube_id;

    // Check if directory exists
    $subtitle_files = array();
    if (is_dir($subtitle_dir)) {
        // Scan the directory for .vtt files
        $subtitle_files = glob($subtitle_dir . '/*.vtt');
    }

    // Add local subtitle files if not already added
    if ($subtitle_files) {
        foreach ($subtitle_files as $subtitle_file) {
            $file_name = basename($subtitle_file, '.vtt');
            $file_parts = explode('.', $file_name);

            if (count($file_parts) >= 2 && $file_parts[0] === $youtube_id) {
                // Extract language code before the hyphen if present
                $language = explode('-', $file_parts[1])[0];
                $source = content_url('/uploads/subtitles/' . $youtube_id . '/' . basename($subtitle_file));

                // Check if this language is already added from meta
                if (!isset($_tracks[$language])) {
                    $_tracks[$language] = array(
                        'kind'      => 'captions',
                        'srclang'   => $language,
                        'label'     => streamtube_core_get_language_by_code(strtolower($language))['name'] ?? '',
                        'src'       => $source,
                    );
                }
            }
        }
    }

    // Prepare the tracks for the player setup
    $setup['tracks'] = array_values($_tracks);

    // Set the default subtitle
    if ($_tracks) {
        foreach ($setup['tracks'] as &$track) {
            $language_code = strtolower($track['srclang']);
            if (($default_subtitle == 'first' && $track === reset($setup['tracks'])) || ($default_subtitle == $language_code)) {
                $track['default'] = true;
            }
        }
    }

    return $setup;
}, 30, 2);