Code added by WP_List_Table::_js_vars is duplicated

[sorich87]

When a list table is 'ajax enabled' (e.g. comments list table on edit-comments.php), javascript code is added by WP_List_Table::_js_vars to the footer. If you look at the source code in your browser, you will see that code is duplicated.

[solarissmoke]

The problem is that two instances of the List_Table class are created while loading the page, and each time the constructor hooks $this->_js_vars to admin_footer. In the case of edit-comments.php:

wp-admin/edit-comments.php:14

$wp_list_table = _get_list_table('WP_Comments_List_Table');

and then again in wp-admin/includes/template.php:337

$wp_list_table = _get_list_table('WP_Comments_List_Table');

I wonder if there isn't a fundamental problem here in that we are creating multiple list tables instances when we should be using only one. Certainly the current js stuff assumes there is only one list table.

[sorich87]

Attached patch fixes the issue but I am not sure if it breaks something else.

[sorich87]

Forgot global in previous patch

[scribu]

The idea was that at most one list table was ajax-enabled at a time.

[chriscct7]

It seems this patch would be a good candidate for inclusion in core, and the patch doesn't need to be refreshed.

[wonderboymusic]

In 31019:

WP_List_Table's constructor registers an 'admin_footer' callback when AJAX is enabled. As such, if you create multiple instances of the class, you get multiple JS blobs on the page when the callbacks fire. Extra instances get made when wp_comment_reply() invokes the class constructor without checking for the existing instance via the $wp_list_table global.

We shall check the $wp_list_table global in wp_comment_reply().

Props sorich87.
Fixes #17413.