[wp-trac] [WordPress Trac] #63589: Unable to live preview changes to Additional CSS in the Customizer when using a Block Theme

[WordPress Trac]

#63589: Unable to live preview changes to Additional CSS in the Customizer when
using a Block Theme
-------------------------------------+--------------------------
 Reporter:  westonruter              |       Owner:  westonruter
     Type:  defect (bug)             |      Status:  accepted
 Priority:  normal                   |   Milestone:  6.8.2
Component:  Customize                |     Version:  6.2
 Severity:  normal                   |  Resolution:
 Keywords:  has-patch needs-testing  |     Focuses:
-------------------------------------+--------------------------

Comment (by rollybueno):

 == Reproduction Report
 === Description
 This report confirms whether the issue outlined here, where live previews
 of Additional CSS fail to render in the Customizer when using a block
 theme, can be consistently reproduced under a specific environment.

 === Environment
 - WordPress: 6.9-alpha-60093-src
 - PHP: 8.2.28
 - Server: nginx/1.27.5
 - Database: mysqli (Server: 8.4.5 / Client: mysqlnd 8.2.28)
 - Browser: Chrome 137.0.0.0
 - OS: Linux
 - Theme: Twenty Twenty-Five 1.2 (Block Theme)
 - MU Plugins: None
 - Plugins:
   * Query Monitor 3.18.0
   * Test Reports 1.2.0

 === Steps to Reproduce
 1. Activate a block-based theme (e.g. Twenty Twenty-Five).
 2. Enable the Customizer manually if it's not available: Visit: `/wp-
 admin/customize.php`
 3. In the Customizer, go to the **Additional CSS** section.
 4. Add a CSS rule, such as: `p { color: red; }`
 5. Observe the preview panel.


 === Actual Results

 ✅ Error condition occurs: The CSS rule is not reflected in the live
 preview.

 The change only appears on the frontend after clicking Publish and page
 refresh.

 === Additional Notes
 - The Customizer is not enabled by default in block themes, but can still
 be accessed directly or re-enabled programmatically for testing purposes.

 === Supplemental Artifacts
 **Customiser state after adding basic CSS** `p { color:red; }`, before
 clicking on **Publish**
 [[Image(https://i.imgur.com/T6JMlLw.png)]]

 **After clicking Publish** - the button turns diabled but the text color
 still not updated.

 [[Image(https://i.imgur.com/ovq2CGb.png)]]

 **After page refresh** - only after doing this, the text color from
 ''Additional CSS'' applied.
 [[Image(https://i.imgur.com/acBmGoL.png)]]

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Ticket URL: <https://core.trac.wordpress.org/ticket/63589#comment:6>
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