Allow core widgets to be used in the legacy widgets block

[jorgefilipecosta]

The legacy widget block should not be able to allow users to use core legacy widgets for which we already have an equivalent block, but it should still load these widgets.

For example, if the user uses a calendar widget on the blocks widget screen we want to show the calendar widget inside the legacy widget, and then allow the user to convert it to the calendar block.

Currently, the legacy widget did not load the core blocks, this PR fixes this problem and makes sure the core blocks are correctly loaded, although the user can not select them when inserting a new legacy widget.

How has this been tested?

I created a new post.
I pasted the following code on the code editor:

<!-- wp:legacy-widget {"identifier":"WP_Widget_Calendar","instance":{"title":"My calendar"},"isCallbackWidget":false} /-->

<!-- wp:legacy-widget {"identifier":"WP_Widget_Calendar","instance":{"title":"My calendar"},"isCallbackWidget":false} /-->

<!-- wp:legacy-widget {"identifier":"WP_Widget_Tag_Cloud","instance":{"title":"Tags","count":0,"taxonomy":"post_tag"},"isCallbackWidget":false} /-->

<!-- wp:legacy-widget {"identifier":"WP_Widget_Archives","instance":{"title":"My archives","count":0,"dropdown":0},"isCallbackWidget":false} /-->

I switched to the visual editor and verified the legacy widgets loaded correctly and I was able to preview them.

[noisysocks]

I confirmed that following your testing steps works! Aside from my comment about visibleLegacyWidgets being an object, this is good to go.

Why is it that when I visit the Widgets (beta) page the legacy widgets don't render?

[jorgefilipecosta]

I confirmed that following your testing steps works! Aside from my comment about visibleLegacyWidgets being an object, this is good to go.

Why is it that when I visit the Widgets (beta) page the legacy widgets don't render?

Thank you for the review @noisysocks 👍 The legacy widget does not work on the widgets page because we decided to use a reference system for the existing widgets, legacy widgets still don't support that system.
That system is implemented at #15801.

[noisysocks]

This looks good! 👍

One minor thing I'm noticing is that when you're viewing a built-in widget, clicking the 'Change widget' button takes you to the placeholder where you can't go back. I think it makes sense to remove the 'Change widget' button for built-in widgets.

[noisysocks]

Should we move this component to its own file ./placeholder.js?

[noisysocks]

Hope you don't mind I pushed up cf265ce which removes the 'Change widget' button for built-in widgets and 7f37bbe which moves LegacyWidgetPlaceholder to its own file. Revert these commits or merge the PR—up to you! 😀

[wixaw]

Hello
I have this error now with my older block legacy-widget :

<p class="editor-warning__message block-editor-warning__message">Votre site n’inclut pas le support pour le bloc «core/legacy-widget». Vous pouvez laisser ce bloc intact ou le retirer complètement.</p>
Why did this block disappear?
Thanks

[noisysocks]

@wixaw: This feature is now behind an experiment setting. Have you enabled the experiment? You can do this in WP Admin under Gutenberg > Experiments.

[wixaw]

Hello
After update 5.11 WP and Gutenberg 9.1.1 it no longer appears in Gutenberg > Experiments

I see the release note to 9.1.0 but i not understand where is now
I rollback to 9.0.0

[wixaw]

@youknowriad @noisysocks i need help please

[noisysocks]

@wixaw: The setting was removed from Gutenberg → Experiments in #24685 meaning that the widgets block editor is now loaded for all users of the Gutenberg plugin. The Legacy Widget block was removed from the post editor in #24900 meaning that it is only available for use in the widgets block editor.

[wixaw]

ok thank's, it's a shame not to be able to have our homemade plugins in our pages anymore. we are going to use siteorigin as before to integrate them. :/