Unregistered thumbnail sizes still listed.

I just noticed when you regenerate thumbnails for a single image it shows check marks next to the registered sizes and crosses next to the unregistered ones. This feature seems to be missing from the page to process all images.

The list of thumbnail sizes is fetched using the get_intermediate_image_sizes() which doesn’t use the “intermediate_image_sizes_advanced” hook. You can filter the thumbnail sizes using the “intermediate_image_sizes” filter though with something like this code:

add_filter( 'intermediate_image_sizes', 'rr_remove_image_sizes' );
function rr_remove_image_sizes( $sizes ) {
    if ( ( $key = array_search( 'medium', $sizes ) ) !== false ) {
        unset( $sizes[ $key ] );
    }
    return $sizes;
}