How to view the APIs in SwaggerEditor / SwaggerHub and even to generate server code while it needs several files?

For example, I define the Category in root yaml file as:

(openapi: 3.0.0)
components:
  schemas:
    Category:
      $ref: 'openapi-petstore-category.yml#/components/schemas/Category'

and the specific content of Category is defined in another file: openapi-petstore-category.yml.

It works well when I use the swagger-codegen-cli to generate spring server, just like
java -jar swagger-codegen-cli-3.0.11.jar generate -i ./oas3/root-petstore.yml -l spring -o petstore/test1/

However, if I want to see the APIs information in SwaggerEditor / SwaggerHub, it is allowed to just upload one file. And the root yaml file in SwaggerEditor / SwaggerHub appears the error "Could not resolve reference: Tried to resolve a relative URL , without having a basePath. path 'openapi-petstore-category.yml' basePath: 'undefined'"

Then I attempt to add the file's path in my computer as 'basePath', but it doesn't work.

Start with the root-petstore.yml example and its referenced openapi-petstore-category.yml file, then reproduce the relative-reference error in SwaggerEditor or SwaggerHub. Compare that behavior with the swagger-codegen-cli command shown; done would require a documented, supported way to view or process this multi-file definition, if the project owns that workflow.