readOnly on an object property

I want to put a readOnly to an object property but I can't find a proper way to do it.

If I have:

def1:
    type: "object"
    properties:
      prop:
        $ref: "#/definitions/def2"

def2:
    type: "object"

A) I can put a readOnly in the ref property, but I think that this is incorrect in the swagger defintion. Also the parser ignores here the readOnly property. Eg:

def1:
    type: "object"
    properties:
      prop:
        readOnly: true
        $ref: "#/definitions/def2"

def2:
    type: "object"

B) I can put a readOnly in the def2 definition, but readOnly is only define for properties and not for definitions. This doesn't work either. Eg:

def1:
    type: "object"
    properties:
      prop:
        $ref: "#/definitions/def2"

def2:
    readOnly: true
    type: "object"

C) Also I noticed that if I expand the property, the InlineFlattenerResolver removes the readOnly.
Before flatten:

def1:
    type: "object"
    properties:
      prop:
         type: "object"
         readOnly: true

After flatten:

def1:
    type: "object"
    properties:
      prop:
        $ref: "#/definitions/def2"

def2:
    type: "object"

So, how can I do it right? is there a way to define readOnly on an object property?

Start by reviewing the Swagger/OpenAPI rules for readOnly on object properties and the parser behavior shown in the examples. Then inspect InlineFlattenerResolver, especially how it handles an inline object property that becomes a definition; done should establish the supported placement and whether readOnly is preserved during flattening.