After upgrading from 2.2.7 -> 2.3.1, we have successfully switched from deprecated authorize.net MD5 based hash direct post to SHA-512 signature key method.
The issue with new authorizenet module is now we'r not able to create partial refunds.
Looking and comparing with old module, I found vendor/magento/module-authorizenet/Model/Directpost.php:68 has
protected $_canRefundInvoicePartial = true;
Question, what would be a proper way to extend new authorizenet module, to support partial refunds.