Metaboxes are not shown when editor rendering mode is set to template-locked

[fabiankaegy]

The editor usually shows the traditional metaboxes at the bottom of the screen. Big players such as Yoast or Multilingualpress are still using this metabox location so we even made the effort to now support having it visible when the post editor is rendered in an iframe.

However in testing I just noticed that we are only showing the metaboxes when the rendering mode of the editor is set to post-only. To me this doesn't really make sense. The template-locked rendering mode to me also deserves to get the metabox area displayed. Because at that point in time you are still editing a post and therefore metadata of said post.

Only when you get to the template-only mode does it make sense to hide that metabox area as we are no longer in the context of a single post.

gutenberg/packages/edit-post/src/components/layout/index.js

Lines 427 to 429 in bf84ac6

	showMetaBoxes:
	! DESIGN_POST_TYPES.includes( currentPostType ) &&
	isRenderingPostOnly,