Fix incorrect values for __back_compat_meta_box in docs

[mkaz]

Description

The documentation has an example of the parameter to pass in to add_meta_box to disable once the meta box has been converted to a block. This value should be true, not false.

__back_compat_meta_box => true

You can confirm this by looking at the code which is looking for the value to be true, and if so will unset from the list of meta boxes to be shown.

if ( isset( $data['args']['__back_compat_meta_box'] ) && $data['args']['__back_compat_meta_box'] ) {
    unset( $meta_boxes[ $page ][ $context ][ $priority ][ $name ] );
}

From: https://github.com/WordPress/gutenberg/blob/master/lib/meta-box-partial-page.php#L149

How has this been tested?

Use an older WordPress plugin that has a metabox, and update with the parameter __back_compat_meta_box set to false and true to the add_meta_box()

It should not show in Gutenberg when set to true.

Types of changes

Documentation update.

[chrisvanpatten]

Here's an excerpt of @pento's original post on the flag:

__back_compat_meta_box

This flag lets you set whether the meta box is intended to be used in the block editor, or if it has been replaced by block-based functionality. Setting it to true signifies that this meta box should only be loaded in the classic editor interface, and the block editor should not display it. Setting it to false signifies that this meta box is intended to be loaded in the block editor. If the flag isn’t set, then WordPress 5.0 will assume that the meta box is intended to be loaded in the block editor.

To be honest I'm not sure I understand it better after reading this but maybe the extra context will be helpful 😂

[mkaz]

Thanks @chrisvanpatten that does help clarify for me, and reassures me the PR is needed.

From @pento's explanation:

Setting it to true signifies that this meta box should only be loaded in the classic editor interface, and the block editor should not display it.

And the docs say below the example code:

When Gutenberg is used, this meta box will no longer be displayed in the meta box area, as it now only exists for backward compatibility purposes. It will continue to display correctly in the Classic editor, should some other meta box cause a fallback.

So, it seems like it true is the value the example should be in the docs.

[jorgefilipecosta]

This looks a "bug" in the documentation, and the value should be true.