A meta box can be prevented from being returned to its original position

[johnbillion]

Given a site with an advanced meta box and a side meta box on the post editing screen, if the side meta box is moved into the advanced position it's not possible to move it back to the side. Tested both with drag and drop and by clicking the up/down position arrows introduced in 5.5.

Steps to reproduce:

1. Add the following to a plugin:

add_action( 'add_meta_boxes', function() : void {
	\add_meta_box(
		'test-advanced',
		'Advanced',
		function() {
			echo 'Advanced';
		},
		'post',
		'advanced'
	);
	\add_meta_box(
		'test-side',
		'Side',
		function() {
			echo 'Side';
		},
		'post',
		'side'
	);
} );

2. Load the post editing screen and confirm one advanced and one side meta box are present.
3. Move the side meta box into the advanced section, either by clicking its "up" arrow or by dragging and dropping it.
4. Reload the post editing screen (required step).
5. Observe that it's not possible to move the side meta box back to the sidebar. It can't be dragged into the sidebar, and its down button is disabled.

This does not affect the classic editor, even if you're silly enough to move all the side meta boxes over into the advanced section.

[johnbillion]

Update: The opposite is true too. If you move the advanced meta box into the side and reload the page, it cannot be moved back.

[Frykky]

I think i found the same bug.

I have 3 metaboxes under the content in page editor:
1)Yoast
2)WPML
3)ACF group field for post_type=page

I move 3 metaboxes to right pane and now it seems impossible to move back under content.

[lopo]

Looking at it during the Contributor Day at WordCamp Europe 2023

I cannot reproduce anymore on 6.2.2. @johnbillion do you think this is still relevant?

[johnbillion]

👍