Description

This is in regard to the core/cover block's isDark check here:

When set inside a template part as part of a core/template-part block, which itself is used in a core/query loop, it can cause endless looping of isDark being set between false and true. This happens when there are at least two different images where one evaluates to true and the other false. Since the origin template is the same, this causes the isDark attribute to bounce between the two values.

Commenting out the setAttributes() call in the above function stops the endless looping.

Step-by-step reproduction instructions

1. Install WordPress.
2. Set Twenty Twenty-Five as the theme.
3. Edit the default Hello world! post.
4. Upload and set the following image as the featured image:
   
5. Create a new post, with whatever title and content. Set the following as its featured image:
   
6. Create a new file /wp-content/themes/twentytwentyfive/parts/teaser.html:

   <!-- wp:cover {"useFeaturedImage":true,"dimRatio":30,"isUserOverlayColor":true,"isDark":false,"sizeSlug":"large","layout":{"type":"default"}} -->
   <div class="wp-block-cover is-light"><span aria-hidden="true" class="wp-block-cover__background has-background-dim-30 has-background-dim"></span><div class="wp-block-cover__inner-container">
   </div></div>
   <!-- /wp:cover -->

7. Edit Blog Index template (go to /wp-admin/site-editor.php?canvas=edit&p=%2Fwp_template%2Ftwentytwentyfive%2F%2Fhome) and paste the following into the template in Code Editor mode:

   <!-- wp:query {"queryId":1,"query":{"perPage":10,"pages":0,"offset":0,"postType":"post","order":"desc","orderBy":"date","author":"","search":"","exclude":[],"sticky":"","inherit":true,"taxQuery":null,"parents":[]},"align":"full","layout":{"type":"default"}} -->
   <div class="wp-block-query alignfull">
     <!-- wp:post-template {"align":"full","layout":{"type":"default"}} -->
       <!-- wp:template-part {"slug":"teaser","theme":"twentytwentyfive"} /-->
     <!-- /wp:post-template -->
   </div>
   <!-- /wp:query -->

8. Exit Code Editor and see the constant replacement behavior of the blocks.

Screenshots, screen recording, code snippet

Environment info

• WordPress 6.9, Twenty Twenty-Five
• Google Chrome 143.0.7499.193 (64 bit)
• Desktop Windows 11

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure