FSE: Toggle editor post title visibility with a filter / Set block template editor setting

[mmtr]

Changes proposed in this Pull Request

Toggle editor post title visibility with a filter

This PRs implements a more robust approach than #34214 as suggested by @gwwar in order to avoid potential rendering flashes (#34214 (comment)).

Now, the a8c/post-content block always renders a post title before the content, but is hidden by default. It will show up when the admin_body_class filter adds the show-post-title-before-content class, which will also hide the regular editor post title. This makes unnecessary the selector isFullSitePage added in #34214, so it has been removed on this PR.

Set block template editor setting

The above caused a side effect. Since the FSE plugin creates a default template that is assigned to all pages without a specific template, we would get the show-post-title-before-content class when creating a new page. This was making the post title to be missing because at the moment we are not applying the template parts when we start a new post.

This has been fixed on this PR by defining a block template on the template editor setting

Follow-up

This PR will introduce a couple of known issues:

• Seems that the page templates (Portfolio, Services, etc) assume the page will be empty. So now that we apply the FSE template when starting a new page, selecting any other template than Blank will result in the content of the selected page template to be appended after the footer template part.
• The a8c/post-content block can be removed by the user, resulting in a missing post title since the show-post-title-before-content would keep the regular editor post title hidden.

Should we fix them before landing this PR?

Testing instructions

• Apply this branch to your FSE testing environment.
• Trash all templates and template parts.
• Deactivate and then reactivate the FSE plugin.
• Create a new page.
• Select the Blank template.
• Make sure the default FSE template is applied.
• Make sure the title of the page is displayed below the header template part and at the beginning of the post content block.
• Try inserting a post content blog a not full site page (i.e. in a regular post or in template).
• Verify the post title is visible in the editor and the default FSE template is not applied.

[matticbot]

Test live: https://calypso.live/?branch=update/fse-set-template-editor-setting

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[kwight]

Everything works as described (including the issues mentioned). I'd say we merge, then address the issues as individual followups.

[vindl]

It looks like this PR broke the New Page functionality - when trying to insert a new page after it I get a blank screen with the following error:

Uncaught (in promise) TypeError: Cannot read property 'attributes' of undefined

Created an issue to track it here #34481.