Show all authors in drop down panel while choosing author for a post

The author dropdown in the post editor is generated by the function post_author_meta_box()

That particular function uses the wp_dropdown_users() function to display the dropdown. The problem is that the arguments specified for the instance do not include the “show_option_all” argument. That argument should default to “all” but if you’ve got a filter on wp_dropdown_users_args somewhere else (maybe a plugin or in the theme) that may be why your list is truncated.

To add a little trickiness to the problem, post_author_meta_box() does not have a direct filter to change the dropdown. So my approach would be to hook into filtering the dropdown. You can run a filter on wp_dropdown_users_args to add the show_option_all:

function my_wp_dropdown_users_args( $query_args, $r ) {
    $query_args['show_option_all'] = 'all';
    return $query_args;
}

Note my use of priority “50”. That is because of what I mentioned in the beginning. The use of the dropdown ordinarily defaults to “all” but the fact that your list is truncated may indicate the presence of another filter on this same hook. So just in case, I would specify the priority of this filter to run a little late so that hopefully it overrides any other earlier filters on this same hook.