Hooking to page_attributes_dropdown_pages_args filter adds a dropdown to the Page Attributes metabox

[nfmohit]

Hi team!
I hope you're all doing well in these challenging times.

WordPress: 5.4-RC2-47447
Classic Editor: 1.5

This only happens with the classic editor. I'm seeing an unusual behaviour with page_attributes_dropdown_pages_args hook. Basically, I'm using it to limit the depth of a hierarchical post type's to 1 using the following code:

<?php
function set_bp_custom_menu_depth( $a ) {
        global $post;
        if( $post->post_type == 'bp_custom_menu_page' ) {
                $a['depth'] = 1;
                return $a;
        }
}
add_action( 'page_attributes_dropdown_pages_args', 'set_bp_custom_menu_depth' );

But whenever I use that hook, it adds a dropdown to the "Page Attributes" metabox in the page edit screens. ​Here's a screencast for reference. It also removes the "(no parent)" option from the "Parent" dropdown. ​Here's a screencast for reference.

This also happens even if you just hook an empty function to it, for example:

<?php
add_filter( 'page_attributes_dropdown_pages_args', function() {} );

I want to confirm if this is expected behaviour or a bug. Please advise.

Kind regards,
Nahid

[johnbillion]

@nfmohit This is more of a support question which you'll need to post on wordpress.org/support, but the problem seems to be that a) you're using add_action instead of add_filter, and b) you're missing a return statement in your callback(s). This combined causes the default arguments to be passed to wp_dropdown_pages() which causes it to be unexpectedly output instead of assigned to a variable and used later.

[nfmohit]

Replying to johnbillion:

@nfmohit This is more of a support question which you'll need to post on wordpress.org/support, but the problem seems to be that a) you're using add_action instead of add_filter, and b) you're missing a return statement in your callback(s). This combined causes the default arguments to be passed to wp_dropdown_pages() which causes it to be unexpectedly output instead of assigned to a variable and used later.

Hi, @johnbillion !
I hope you're doing well!

Thank you so much for getting back to me!

Ugh! I feel so silly for posting this here. I think personally, I came to a conclusion assuming it as a bug too soon. I'm very sorry about that.

Thank you for pointing the issues out with the attached code. The add_action was actually a typo while I added the code there, it had the same effect when using add_filter, but the issue was that I wasn't returning a statement. I appear to have resolved the issue with the following code:

<?php
function set_bp_custom_menu_depth( $a ) {
        global $post;
        if( $post->post_type == 'bp_custom_menu_page' ) {
                $a['depth'] = 1;
                return $a;
        }
        return $a;
}
add_filter( 'page_attributes_dropdown_pages_args', 'set_bp_custom_menu_depth' );

Again, thank you very much for your help and sincere apologies again for posting this in the wrong place.

Kind regards,
Nahid

[SergeyBiryukov]

No worries, thanks for the follow-up!