Cover: Create the initial paragraph when the block gains a background

[ellatrix]

What?

Creates the Cover block's initial inner paragraph in the actions that give the block a background (selecting media, choosing an overlay color, enabling the featured image), instead of declaring it as an inner blocks template.

Why?

1. The template only ever applied on placeholder exit. Cover's placeholder branch returns early without rendering the inner blocks container, so the template hook never ran until a background was chosen. Creating the paragraph in those actions encodes the actual behavior directly.
2. Collaboration correctness. Template insertion runs independently on every peer after mount; a user action runs only on the acting client, so peers receive the background and the paragraph together (see RTC: Fix inner template multiple insertion #79021 for the general problem).
3. Removes the Cover: Avoid content loss when the templateLock value is all or contentOnly #45632 guard. The conditional template prop existed to avoid re-synchronizing locked content; with no template there is nothing to guard.

How?

A scaffoldInnerBlocks() helper inserts the paragraph (with the existing fontSize handling from #34993) when the inner blocks are empty, called from onSelectMedia, onSetOverlayColor, and toggleUseFeaturedImage. The block transforms already create the paragraph explicitly.

One behavior change: a saved empty Cover that already has a background no longer gains a paragraph when the post is opened; it shows the block appender instead.

Testing Instructions

1. Insert a Cover block; the media/color placeholder appears as before.
2. Pick an overlay color or upload media; the title paragraph appears focused, type into it.
3. Transform an image into a Cover; the caption paragraph is present.
4. Undo after picking a color; the paragraph and background revert together.

Testing Instructions for Keyboard

Insert a Cover, Tab to a color swatch in the placeholder, press Enter, and type the title.

🤖 Generated with Claude Code

[github-actions]

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Co-authored-by: ellatrix <ellatrix@git.wordpress.org>
Co-authored-by: jsnajdr <jsnajdr@git.wordpress.org>
Co-authored-by: youknowriad <youknowriad@git.wordpress.org>

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[github-actions]

Size Change: +134 B (0%)

Total Size: 7.67 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-library/index.min.js	333 kB	+134 B (+0.04%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in 320bd1b.
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/28982004957
📝 Reported issues:

• [Flaky Test] two users concurrently move list items (RTC WebSocket) #79951 in /test/e2e/specs/editor/collaboration/collaboration-stress.spec.ts

[youknowriad]

I feel like this makes the code of the block more fragile and prone to error as we iterate on the block. That said, I'm wondering, is there a way to use a static template here and still have the block show a placeholder in some occasions?

[ellatrix]

True, let me try it

[ellatrix]

Ok if we set the template and show a placeholder anyway, the only issue is that when you set a background etc., we'll have to set selection manually afterwards to the paragraph.

[youknowriad]

Ok if we set the template and show a placeholder anyway, the only issue is that when you set a background etc., we'll have to set selection manually afterwards to the paragraph.

I feel like this behavior is the right one for a cover, I could be wrong though.

[ellatrix]

@youknowriad It would look something like this (based on top of the main PR): #80120

[ellatrix]

I'm honestly also wondering if the empty paragraph shouldn't just be an appender. Tbh, it would be good to redesign the "default" appender to actually render a real (unmaterialised) paragraph block instead of a fake paragraph. That way the default appender could also be configured to take any default attributes such as font size and we wouldn't have to do this whole template thing.

[youknowriad]

I'm honestly also wondering if the empty paragraph shouldn't just be an appender.

I kind of agree, can we try it and see how it feels?

[jsnajdr]

Looks good 👍 It's also nice that this PR replaces the old templateLock workaround from #45632.

[ellatrix]

Ok, let's do this for now, I'll try the appender approach also but at least it's no longer a blocker for #80027.