Block editor: render a real default block in place of the default appender

[ellatrix]

What?

Renders a ghost of the container's default block in place of the default block appender while the block list is empty: a real block object, rendered before it exists in the store, and inserted unchanged when the user enters it. The client ID, the DOM element, and the caret survive materialization. The default block resolves from the block list settings, then the editor default: the empty canvas ghosts a paragraph, an empty buttons block ghosts a button.

Why?

The default appender is a lookalike <p role="button">. Rendering the actual block instead means:

• The empty state looks like the block the user gets: real styles, real placeholder, real markup.
• The CSS styles and the DOM structure are the same before and after entry, so there is no appender-to-block swap and no layout shift.
• Entering it replaces and refocuses nothing: the element, the caret and the client ID carry over.
• It is announced as a block, with the appender's accessible name kept until entry.
• Blocks can now add attributes to the default block (via the block list settings' defaultBlock), and the empty state renders them: the appender could only ever look like a plain paragraph. This makes it possible to replace the cover block's scaffolded title paragraph with a declared default block, in a follow-up.

How?

1. Items derives the ghost while the list is empty: createBlock of the resolved default, memoized so the object is stable per empty period, rendered through the ordinary BlockListBlock with the ghost object as a fallback data source.
2. BlockListBlockProvider falls back to the ghost object when the store has no entry; the ghost is never selected before entry, so the selection dependent selectors are moot.
3. Entering the ghost (pointerdown or focusin) inserts that same object. Same client ID, same key, same element; the provider's data source flips from the prop to the store.

Nothing is registered in the store before entry: no state, no actions, no lifecycle. On keyboard entry there is a theoretical one-tick window where a focus-time selection dispatch could precede the insert; in practice the selection sync rides the asynchronous selectionchange event and lands after the insert, and React consumers render once per task, so no inconsistent state has been observable.

Testing Instructions

1. Create a new post. The body shows a real empty paragraph with the usual placeholder instead of the appender. The post has no content until you enter it.
2. Click it and type. The caret stays put; the paragraph becomes the first block.
3. Press Escape or click outside. A new trailing paragraph renders after the content.
4. Insert a Group block and select it while empty. The paragraph renders inside it.
5. Insert an empty Buttons block: a ghost button renders. Note: selecting the container moves focus into the ghost, which materialises it; that is the focus rule working, but it means selecting an empty restricted container creates its first child.

🤖 Generated with Claude Code

[github-actions]

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[github-actions]

Size Change: +290 B (0%)

Total Size: 7.65 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-editor/index.min.js	435 kB	+326 B (+0.08%)
build/scripts/block-library/index.min.js	357 kB	-36 B (-0.01%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in bf567e9.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/31141552953
📝 Reported tests:

overrides block style variation styles in the editor and on the front end in /test/e2e/specs/editor/various/block-custom-css.spec.js, passed after 1 failed attempt.

Error: apiRequestContext.fetch: socket hang up
Call log:
  - → GET http://localhost:8889/wp-json/wp/v2/posts?per_page=100&status=publish%2Cfuture%2Cdraft%2Cpending%2Cprivate%2Ctrash
    - user-agent: Playwright/1.62.1 (x64; ubuntu 24.04) node/20.20 CI/1
    - accept: */*
    - accept-encoding: gzip,deflate,br
    - X-WP-Nonce: 94c1c26d8c
    - cookie: wordpress_test_cookie=WP%20Cookie%20check; wordpress_logged_in_23778236db82f19306f247e20a353a99=admin%7C1786243072%7CICgdCM8nDbe5sD89nborLAueAAZcNbYLKgV4DLBOPRz%7Cdb66b215c95619fba41204b3808f154d7861da5bf4f271b77d1caf41ded32fb9; wp-settings-time-1=1786070801

    at RequestUtils.rest (/home/runner/work/gutenberg/gutenberg/packages/e2e-test-utils-playwright/src/request-utils/rest.ts:119:39)
    at RequestUtils.deleteAllPosts (/home/runner/work/gutenberg/gutenberg/packages/e2e-test-utils-playwright/src/request-utils/posts.ts:33:27)
    at /home/runner/work/gutenberg/gutenberg/test/e2e/specs/editor/various/block-custom-css.spec.js:20:17

uses the anchor chosen for a heading after publish in /test/e2e/specs/editor/blocks/table-of-contents.spec.js, passed after 1 failed attempt.

Error: apiRequestContext.fetch: socket hang up
Call log:
  - → GET http://localhost:8889/wp-json/wp/v2/templates
    - user-agent: Playwright/1.62.1 (x64; ubuntu 24.04) node/20.20 CI/1
    - accept: */*
    - accept-encoding: gzip,deflate,br
    - X-WP-Nonce: 384be40361
    - cookie: wordpress_test_cookie=WP%20Cookie%20check; wordpress_logged_in_23778236db82f19306f247e20a353a99=admin%7C1786243080%7CDGFWmsG76nu5t0J3xS1Vj5y4P2FLCVNEwlM9jsgMwhd%7C2f92dd07b160cbef9969c461a890554f107fe162c84bf23372726de3e4f2d90b; wp-settings-time-1=1786070461

    at RequestUtils.rest (/home/runner/work/gutenberg/gutenberg/packages/e2e-test-utils-playwright/src/request-utils/rest.ts:119:39)
    at RequestUtils.deleteAllTemplates (/home/runner/work/gutenberg/gutenberg/packages/e2e-test-utils-playwright/src/request-utils/templates.ts:38:31)
    at /home/runner/work/gutenberg/gutenberg/test/e2e/specs/editor/blocks/table-of-contents.spec.js:112:17