Allow child themes to use template parts from parent themes in Full Screen Editor

[catacaustic]

Creating template parts in a theme for the Full Site Editor does not allow template parts from a parent theme to be used by a child theme. These will show when editing a template, and allow you to insert/update/delete them when editing templates, but an error message shows on the public-facing page - "Template part has been deleted or is unavailable: *template-name*".

All that is needed is an additional check if a child theme is active and if so check against the parent theme as well as the child theme.

[catacaustic]

Diff file

[catacaustic]

Sorry, missed which file this applies to -

wp-includes\blocks\template-part.php

[ironprogrammer]

Welcome back to Trac, @catacaustic, and thanks for the report!

This issue looks similar to ​https://github.com/WordPress/gutenberg/issues/44243, where a workaround was to remove the theme name from <!-- wp:template-part -->. For instance, change:

<!-- wp:template-part {"slug":"header","tagName":"header","theme":"my-theme"} /-->

to

<!-- wp:template-part {"slug":"header","tagName":"header"} /-->

Would you mind testing to see if this also fixes the errors you're seeing? Your proposed patch may offer a solution there as well. Thank you!