Sage 10 with Gutenberg Full Site Editing (FSE)

Requirements

A patched roots/acorn package is used by the theme as theme runtime dependency (in theme composer.json) which loader preserves the block template paths.
remove_theme_support('block-templates') must be absent, as FSE will not work otherwise (important).
templates/ directory, index.html and theme.json (see the FSE-specific theme files below).

Optional

The Gutenberg Plugin is not required, but adds extra Gutenberg-specific functionality. With earlier WordPress core versions, FSE was not supported and the Gutenberg plugin was required for FSE features, now FSE support is part of WordPress core. Using the Gutenberg plugin allows to lock-in a specific Gutenberg version, ensuring base styles and DOM staying the same for the theme. On the other hand, an additional plugin has to be used and maintained in relation to theme adjustments. On a roots.io Bedrock site you usually would add the plugin to the Bedrock site (not the theme!) composer.json as a WPackagist dependency.

Please note

Double-check whether remove_theme_support('block-templates') is absent from your theme setup, as otherwise FSE will not work.

Theme installation

Clone this repository.
Invoke composer install.
(Optional) Ensure suitable node version, e.g. using nvm from the .nvmrc by invoking nvm install && nvm use.
Invoke either npm install or yarn install (npm install -g yarn for installing/updating yarn classic (1.x)).
(Remove either yarn.lock or package-lock.json, depending on the package manager you are using, as bud will not build otherwise in order to prevent potential issues.)
Invoke npm run build.
(Mount the theme into a WordPress site and activate it).
Open the Site Editor:

Admin area → Appearance → Editor (not beta anymore).
(Logged in as editor+) → Front end → Admin bar → Edit site

Theme structure

themes/your-theme-name/          # → Root of your Sage based theme
├── templates/                   # → Block templates directory (❗required for a FSE theme (for the `index.php` inside)) (formerly named `block-templates`)
│   ├── index.html               # → Block template for the posts (fallback) (❗required for a FSE theme)
│   ├── page.html                # → Block template for singular page, default
│   ├── page-without-header.html # → Block template for singular page, custom template (without header block part)
│   ├── home.html                # → Block template for posts page (specific page selected as blog page)
│   └── front-page.html          # → Block template for front page (specific page selected as front page)
├── parts/                       # → Block parts directory (can be used in block templates, among others) (formerly named `block-parts`)
│   ├── header.html              # → Block part for a header (there can be more headers, if needed)
│   ├── footer.html              # → Block part for a footer (there can be more footers, if needed)
│   └── example.html             # → Block part for a generic block part (uncategorized)
├── patterns/                    # → Block patterns directory
│   └── some-pattern.php         # → Block example pattern
├── theme.json                   # → Generated by Sage theme build process (`bud`) or directly edited (❗required for a FSE theme)

Notes

You can register the frontend styles as editor styles, then Gutenberg Editor will automatically post-process those styles and wrap all selectors in .editor-styles-wrapper for proper styles isolation (which prevents those frontend styles leaking into the editor UI itself). The editor styles are added as they are/were added to the TinyMCE editor (WordPress Classic Editor). This allows the theme to be agnostic towards the technique with which the Gutenberg editor isolates the frontend styles (from its UI). This becomes more important now as in the near future Gutenberg will use iframes. Therefore hardcoded style isolation (by prefixing a editor-styles-wrapper CSS class selector) would then either be unnecessary or even require additional post-processing which itself was intended to be avoided in the first place.
You can also add editor-specific styles for adjusting the DOM elements added by the editor or the editor UI itself (for specific fixes). Those editor styles, even if they may be named "editor styles", would not be added as editor style (as it is called in WordPress) in a technical sense, but rather just enqueued on the admin/backend editor pages as normal styles (which would not be post-processed and hence isolated by Gutenberg editor either) (as by using enqueue_block_editor_assets).
add_theme_support('block-templates') is not needed when theme is already autodetected to be a FSE theme (by presence of theme.json and templates/).
Disabling FSE using remove_theme_support('block-templates') is ignored when block templates are in place, Design → Editor is still offered to the user. – But this will currently result in an unexpected, non-JSON-, HTML-response for _wp-find-template, preventing the Gutenberg Editor from initializing, hence the Gutenberg Editor page stays blank.
Block styles currently can not be registered by convention (as by adding them as files into a specific folder, using meta data as comments/JSON, as this is already possible with block patterns). – Currently block styles can only be registered using register_block_style (in server-side PHP; recommended init action hook) or registerBlockStyle (in editor JavaScript). This can change with future releases of Gutenberg and WordPress core.
Get the translation source strings using the WP CLI i18n, as this tool is able to parse not only the normal core PHP WordPress translation functions, but also all the metadata in theme.json; Block patterns and Block parts for color, font-size, block part names, etc. – And recently it can also parse Blade-PHP template files. Sage 10 already uses these for its translation-related npm scripts.
In case you are wondering why no page header/footer is visible, this depends on the WordPress Reading settings (Your latest post or A static page under Settings → Reading → Your homepage displays).
You can easily add SCSS/SASS (+ PostCSS) support, just apply the adjustments made to the Sage 10 SCSS theme.
The great bud-wp-editor-query plugin allows to add editor-specific styles directly in the frontend styles, using a custom and valid CSS media query syntax. An example is included in the app.css.

Known issues and fixes

Template part blocks can not render; JavaScript error `Cannot read properties of undefined (reading 'tinymce' [...]` in Gutenberg Editor

Block template or block parts contain plain, "naked" HTML without block-specific comments.

Related issues

Customization by user

By default, the user can override the theme-provided block templates and block parts, those modifications are stored as special posts in database. The site editor sidebar can be opened by clicking on the logo/icon on the upper left corner in the side editor. From that sidebar the block templates and template parts lists can be viewed and the user customizations reset.

Home versus Front page templates

This was a gotcha for me, as I first had not completely understood the exact difference between those two. This very well made article explains the differences: https://davidsutoyo.com/articles/difference-front-page-php-home-php/

Gutenberg Editor/JavaScript errors like `Block "core/post-comments" is not registered` (for comment-specific core blocks)

The Disable Comments Plugin may cause this as it can also remove the comment-specific core blocks. Note: In block templates those blocks have markup as <!-- wp:post-comment.

Gutenberg Editor/JavaScript errors like `Block "core/post-title" is not registered` (for blog-/post-specific core blocks)

The Disable Blog Plugin may cause this as it can also remove the blog-/post-specific core blocks. Note: In block templates those blocks have markup as <!-- wp:post-title.

Template part suddenly not found after switching theme

Template parts can be locked to a specific theme. This is useful when non-generic parts are used that are specific to a theme. Generic parts (header; footer), with no theme enforced, would automatically remap when switching themes. But when generic parts are locked to a specific theme, they will not be found when switching to another theme.

PHP error `Target class [sage.view] does not exist.`

This can occur when the patched acorn library is installed, FSE is enabled, but no FSE-specific files being available in the theme.

The Gutenberg Editor page loads, but stays blank! 😮

This happens when the backend sends an unexpected response (invalid JSON (so also just frontend HTML)) or no response. The Gutenberg Editor currently catches any JSON parse errors and silently stops initializing (staying blank). Adding or changing the order of template loaders can cause this, hence this example Sage 10 FSE theme uses a patched version of the Sage theme acorn runtime that does not respond with a matching Blade-PHP non-block template. Also the aforementioned disabling FSE using remove_theme_support('block-templates') in a FSE theme causes this.

dsturm/sage10-fse