Layouts: Update editor layout definitions to better select the first block

[yogeshbhutkar]

What?

Closes #69989

This PR fixes the unintentional top margin (margin-block-start) on the post content block when placed within the template query. (Likely introduced in #66003)

Why?

This happens because the following line tries to apply a top margin of 0 px to the first child.

gutenberg/packages/block-editor/src/layouts/definitions.js

Line 103 in 551c62f

selector: ' > :first-child',

The problem occurs when there are multiple <style> and <SVG> elements before the first element within the editor. Refer to the following DOM structure:

These elements hold the preview styles related to the content block.

Ref.

gutenberg/packages/block-library/src/post-content/edit.js

Line 59 in 05f769e

return <div { ...blockPreviewProps }></div>;

How?

Since all the blocks within the editor have a .wp-block class, we can update the selection logic to be:
' > :nth-child(1 of .wp-block)'

Because the .wp-block class is not present in the front-end and since the problem doesn't occur in there (as the <style> and <svg> tags are not rendered within the parent block), I didn't feel the need to update the rules for the corresponding PHP file.

Testing Instructions

1. Create a new post, paste the following markup in the code editor, and save it.

Click to toggle the code

<!-- wp:group {"className":"is-style-section-4","style":{"spacing":{"padding":{"right":"var:preset|spacing|40","left":"var:preset|spacing|40","top":"var:preset|spacing|40","bottom":"var:preset|spacing|40"}}},"layout":{"type":"flex","flexWrap":"nowrap"}} -->
<div class="wp-block-group is-style-section-4" style="padding-top:var(--wp--preset--spacing--40);padding-right:var(--wp--preset--spacing--40);padding-bottom:var(--wp--preset--spacing--40);padding-left:var(--wp--preset--spacing--40)"><!-- wp:group {"style":{"layout":{"columnSpan":1,"rowSpan":1}},"layout":{"type":"constrained"}} -->
<div class="wp-block-group"><!-- wp:group {"layout":{"type":"flex","flexWrap":"nowrap"}} -->
<div class="wp-block-group"><!-- wp:paragraph {"style":{"elements":{"link":{"color":{"text":"#29c8f8"}}}}} -->
<p class="has-link-color">Welcome to <a href="http://www.wordpress.org">WordPress</a>. This is your first post. Edit or delete it, then start writing! Welcome to WordPress. </p>
<!-- /wp:paragraph -->

<!-- wp:paragraph {"style":{"elements":{"link":{"color":{"text":"#3ee732"}}}}} -->
<p class="has-link-color">Welcome to WordPress. This is your first post. Edit or delete it, then start writing! Welcome to <a href="http://wordpress.org">WordPress</a>. </p>
<!-- /wp:paragraph --></div>
<!-- /wp:group -->

<!-- wp:buttons {"style":{"layout":{"columnSpan":2,"rowSpan":1}},"layout":{"type":"flex","justifyContent":"center"}} -->
<div class="wp-block-buttons"><!-- wp:button -->
<div class="wp-block-button"><a class="wp-block-button__link wp-element-button">Button</a></div>
<!-- /wp:button --></div>
<!-- /wp:buttons --></div>
<!-- /wp:group --></div>
<!-- /wp:group -->

2. From wp-admin, navigate to Appearance > Editor > Templates > Blog Home.

3. Confirm that the Post Content block has no extra margin on top and looks exactly like the front-end.

Testing Instructions for Keyboard

Same.

Screenshots

Before	After

[github-actions]

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Co-authored-by: yogeshbhutkar <[email protected]>
Co-authored-by: biplob-g <[email protected]>
Co-authored-by: iamtakashi <[email protected]>

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