How do I add Category Body Classes to Admin?

Question

So why are category classes not added to the admin body class with this function?

I see the current post ID in page source as part of the complete body class, so the basic function of adding body classes to admin with admin_body_class does work.

Any ideas?

function base_admin_body_class( $classes )
{
    // Current action
    if ( is_admin() && isset($_GET['action']) ) {
        $classes .= 'action-'.$_GET['action'];
    }
    // Current post ID
    if ( is_admin() && isset($_GET['post']) ) {
        $classes .= ' ';
        $classes .= 'post-'.$_GET['post'];
    }

   // Get Categories
    if ( is_admin() && isset($_GET['post']) ) {

    $cats = wp_get_post_categories( $post->ID, array( 'fields' => 'all' ) );
    $cats = wp_list_pluck( $cats, 'slug' );
    foreach ( $cats as $cat )
    {
        $classes .= ' category-' . $cat;
    }
}

    return $classes;
}
add_filter('admin_body_class', 'base_admin_body_class');

Answer 1

You're using $post->ID which is undefined. Try $_GET['post'] instead:

// Get Categories
if ( is_admin() && isset($_GET['post']) ) {
    $cats = wp_get_post_categories( (int)$_GET['post'], array( 'fields' => 'all' ) );
    $cats = wp_list_pluck( $cats, 'slug' );
    foreach ( $cats as $cat ) {
        $classes .= ' category-' . $cat;
    }
}