How to avoid issue “Unable to find a physical ancestor for a theme”

Question

I have already checked this topic and this one

But both them not actually solve 100% my real problem. My theme is installed via composer and it will placed in vendor folder. It's not have any parent for inheritance. Sometimes magento recognize my theme but other try it'doesn't find it

My registration.php for theme

\Magento\Framework\Component\ComponentRegistrar::register(
\Magento\Framework\Component\ComponentRegistrar::THEME,
'vendors/VendorName/theme-name',
__DIR__
);

As you can see theme define above not like any other theme of magento will start with frontend folder

Is this is cause of my problem ? Can anyone in core magento team help me solve this issue! This problem maybe not bug of magento iam not sure
Expect results: I want 100% work theme when i run setup:upgrade . Not ramdomly fail

Answer 1

I would assume that at least the area (frontend or adminhtml) must be defined in the first part of the theme path like in the blank theme, which would be a good example for a new theme without a parent:

https://github.com/magento/magento2/blob/2.3-develop/app/design/frontend/Magento/blank/registration.php#L9

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
use \Magento\Framework\Component\ComponentRegistrar;
ComponentRegistrar::register(ComponentRegistrar::THEME, 'frontend/Magento/blank', __DIR__);

Although they are not getting deeper into it, it's also described that way in the devdocs for new frontend themes: https://devdocs.magento.com/guides/v2.3/frontend-dev-guide/themes/theme-create.html