Confused page ordering if filter drops parent pages

[erdnah]

Problem

In Wordpress's page management section, the page ordering is confused if a page filter drops a page parent.

Originally, I experienced this with the User Access Manager Plugin:
​http://wordpress.org/extend/plugins/user-access-manager/
This plugin only shows allowed pages to the user.

Example

Page A
- Page A1
-- Page A1.1
-- Page A1.2

By dropping page A, the ordering becomes for example:

-- Page A1.1: Parent Page is Page A1
-- Page A1.2: Parent Page is Page A1
- Page A1: Parent Page is A

But should become:

Page A1: Parent Page is A
-- Page A1.1
-- Page A1.2

Solution

The confusion results from the function _display_rows_hierarchical
in wp-admin/includes/class-wp-posts-list-table.php which only adds pages to $top_level_pages whose parent is 0 – the wrong assumption here.

By adding pages to $top_level_pages whose parent is either 0 or doesn't exist in $pages we get a usable order:
So I added to the above-mentioned file:

function is_parent_in_pages( $parent, $pages ) {    
    foreach ( $pages as $page ) {
        if ( $page->ID == $parent ) return true;
    }
    return false;
}  

and changed _display_rows_hierarchical
from

if ( 0 == $page->post_parent )
    $top_level_pages[] = $page;
else 
    $children_pages[ $page->post_parent ][] = $page;

to

if ( 0 == $page->post_parent || !$this->is_parent_in_pages( $page->post_parent, $pages ))
    $top_level_pages[] = $page;
else
    $children_pages[ $page->post_parent ][] = $page;

And finally - in order to remove the leading dash of $top_level_pages - I removed
the $level++ in function single_row (same file) below "case 'title':"

Small change, better user experience :)

Correction of my Example section:

It should become:

Page A1: Parent Page is A
- Page A1.1
- Page A1.2

(one dash only of course, not two)

Display of proposed changes and patched changes

Make changes recommended in report; also remove "Page Parent" text when parent is trashed.

Attached a patch for the proposed changes, as well as a screenshot. It seemed awkward to me to display "Post Parent: X" when "X" had been trashed, so I also removed that text for that case. See the attached screenshot, 21272-trashing-parents.jpg.

Tested on 3.5-alpha-21751 and it works as described. My only question would be should "grandchild" then show "child" as the parent? ex (Grandchild | Parent Page : Child)

Replying to MikeHansenMe:

Tested on 3.5-alpha-21751 and it works as described. My only question would be should "grandchild" then show "child" as the parent? ex (Grandchild | Parent Page : Child)

The ​code that adds the filter for "Parent Page:" to display is commented:

//sent level 0 by accident, by default, or because we don't know the actual level

Because this patch causes the child and grandchild page's hierarchical level to be set correctly (not sent in error or unknown), "Parent Page:" doesn't display for either the child or the grandchild. See the furthest-right window in this screenshot

Who should make the final call on this?

As noted in the ​IRC discussion, the patch would make the output more consistent with the default behavior of wp_list_pages().

18402.diff​ adds an @since to the method docblock and completes the @return tag.