What happened?

In the post editor (not site editor), if you try to enable the custom fields display panel (via [Kebab Menu] > Preferences > Panels > Additional), the editor doesn't reload and appears as if the setting did not persist. Manually reloading the page or navigating away and back shows that the setting was retained.

What was expected?

Applying the setting should reload the editor automatically, and then the custom fields area should be visible.

Steps to Reproduce

1. Open a new or existing post.
2. Open the "Kebab Menu" (three vertical dots) in the upper-right corner of the page, and select Preferences.
3. Click the Panels tab and locate the "Custom fields" toggle.
4. Toggle it on and click the "Show & Reload Page" button.
5. Observe that the button shows progress, but appears to hang.
6. Navigate away and back to the page, OR click in the Playground address bar (not the browser address bar!) and press Return to reload the editor.
7. Observe that the custom fields panel is now visible in the editor.
8. Unapplying the setting also requires manually reloading as indicated in Step 6.

Additional Details

• The "Show & Reload Page" button changes to display progress, but then simply hangs.

  

• DevTools: When the button is clicked, a beacon request is made to /wp-admin/admin-ajax.php, but does not succeed, and a corresponding error appears in the JS console: [Error] Failed to load resource: The URL can’t be shown (post.php, line 0).

  

• The same error appears whether on /wp-admin/post.php or /wp-admin/post-new.php.