Execute button non-responsive when using form data input if read only field exists in schema

Q&A (please complete the following information)

• OS: macOS 11.3.1
• Browser: Tested with Brave 1.24.82 and Firefox 88.0
• Swagger-UI version: 3.44.0 (and whatever runs on editor.swagger.io)
• Swagger/OpenAPI version: OpenAPI 3.0.3

Content & configuration

Steps to re-produce:

1. Visit https://editor.swagger.io.
2. Paste the schema provided below.
3. Expand POST /sample/ endpoint
4. Set Request body: multipart/form-data
5. Enter any text in description
6. Click Execute

You should see that nothing happens.

Example Swagger/OpenAPI definition:

openapi: 3.0.3
info:
  title: 'Sample issue'
  version: 0.0.0
paths:
  /sample/:
    post:
      operationId: sample_create
      description: Sample viewset.
      tags:
      - sample
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Sample'
          application/x-www-form-urlencoded:
            schema:
              $ref: '#/components/schemas/Sample'
          multipart/form-data:
            schema:
              $ref: '#/components/schemas/Sample'
        required: true
      responses:
        '201':
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Sample'
          description: ''
components:
  schemas:
    PatchedSample:
      type: object
      description: Sample serializer to demonstrate issue.
      properties:
        id:
          type: integer
          readOnly: true
        description:
          type: string
    Sample:
      type: object
      description: Sample serializer to demonstrate issue.
      properties:
        id:
          type: integer
          readOnly: true
        description:
          type: string
      required:
      - description
      - id

If you remove the readOnly: true line from the id field on the Sample schema, the problem goes away.

Describe the bug you're encountering

The execute button is non-responsive when trying to make a POST request to an API with a schema that contains a read only field.

Additional context or thoughts

I suspect that the validation done when hitting Execute is including the id read only field in the backround?

Reproduce the issue in editor.swagger.io with the provided OpenAPI 3.0.3 schema, using multipart/form-data and a readOnly id field. Start by tracing what happens when Execute is clicked and compare it with the schema after removing readOnly; done means the request executes normally while preserving read-only field behavior.