The address default value isn't always shown when ajax is used

This might be a core issue, not sure yet how to properly fix this.

We used to implement the event and now the widget alter hook to set address default values based on fields on the logged in user. That worked fine for the first payment method. However, when you already have a stored payment method, for example with stripe, and then want to add a new one, the default values aren't there.

Took me quite some debugging but I tracked it down to some form ajax trickery that initializes the user input of all fields to NULL and then somehow that doesn't get overwritten with the actual fields.

It could be related to how the fields are created during process callbacks, so maybe for example date fields are affected in a similar way.

I found some existing issues which might the same issue, e.g. #3037289: On duplicated entities, field address is always empty or #2914208: Address field not keeping it's value when being edited inside an ief, which was closed as duplicate of the default value issue, but might actually have been this. I have been testing with the most recent version. Was actually hoping that would fix my problem, but it didn't :)

As a workaround, I did now this in my widget alter hook:

// Workaround for initializing an address field on ajax, which might have
  // its input values set to null.
  $input = $form_state->getUserInput();
  $address_input = &NestedArray::getValue($input, array_merge($element["#field_parents"], ['address', 0, 'address']));
  if (is_array($address_input) && array_key_exists('country_code', $address_input) && $address_input['country_code'] === NULL) {
    NestedArray::unsetValue($input, array_merge($element["#field_parents"], ['address', 0, 'address']));
    $form_state->setUserInput($input);
  }

Obviously not very pretty.