[wp-trac] [WordPress Trac] #45250: Backward compatibility: Impossible to prefill a new post in WP 5.0

[WordPress Trac]

#45250: Backward compatibility: Impossible to prefill a new post in WP 5.0
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 Reporter:  Chouby        |       Owner:  (none)
     Type:  defect (bug)  |      Status:  new
 Priority:  normal        |   Milestone:  5.0
Component:  Editor        |     Version:  5.0
 Severity:  normal        |  Resolution:
 Keywords:  has-patch     |     Focuses:
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Comment (by Chouby):

 I see now that `edit-form-advanced.php` is also enqueuing scripts outside
 the `admin_enqueue_scripts` action. I never paid attention to that.

 To understand my issue to make Polylang work correctly with WP 5.0, you
 should know that I often hook to the action `add_meta_boxes` to do various
 things such as prefilling the content, create new content (taxonomy terms,
 media), remove the editor for the translations of the posts page, or
 manage compatibility with some plugins. I understand that all these things
 were not the pimary goal of the action `add_meta_boxes` but with the
 classic editor, `add_meta_boxes` is the first non-deprecated action after
 the current `$post_ID` is known and before anything is loaded in the
 editor.

 With the current status of WP 5.0, the action `add_meta_boxes` is fired
 after the content is preloaded via the REST API which breaks all the
 features I hooked to this action. So I propose 2 alternatives to the
 proposed patch.

 1. Move the call to `register_and_do_post_meta_boxes( $post );` at the
 beginning of `edit-form-advanced.php` still to restore the current
 behavior of WP < 5.0 (even with Gutenberg) where the action is fired
 before the content is loaded.

 2. Add a new action after the `$post_ID` is known and before the content
 is loaded. Ideally this new action should be avalaible in the 2 editors
 for new and edited posts.

 What do you think?

 NB: I just created a related issue #45263 as the block editor now allows
 to edit the posts page which is not the case with the classic editor. It's
 related because I currently use the `add_meta_boxes` action to reproduce
 the same behavior for the translations of the posts page.

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Ticket URL: <https://core.trac.wordpress.org/ticket/45250#comment:7>
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