Callback panel is not expanded on page load

[clns]

The issue can be seen at https://clns.github.io/swagger-ui-bug/.

Q	A
Bug or feature request?	Bug
Which Swagger/OpenAPI version?	3.0.1
Which Swagger-UI version?	3.13.2
How did you install Swagger-UI?	CDN
Which browser & version?	Google Chrome Version 65.0.3325.181 (Official Build) (64-bit)
Which operating system?	Microsoft Windows 10

Demonstration API definition

It can also be viewed at https://clns.github.io/swagger-ui-bug/spec.json.

{
  "openapi": "3.0.1",
  "info": {
    "title": "Showcase UI Bugs",
    "version": "1.0.0"
  },
  "paths": {
    "/url": {
      "post": {
        "tags": [
          "Tag"
        ],
        "requestBody": {
          "description": "Json-encoded object.",
          "required": true,
          "content": {
            "application\/json": {
              "schema": {
                "type": "string"
              }
            }
          }
        },
        "responses": {
          "200": {
            "description": "OK"
          }
        },
        "callbacks": {
          "myWebhook": {
            "$request.body#/url": {
              "post": {
                "requestBody": {
                  "description": "Json-encoded object.",
                  "required": true,
                  "content": {
                    "application\/json": {
                      "schema": {
                        "type": "string"
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  }
}

Configuration (browser query string, constructor, config.yaml)

window.onload = function () {

    const ui = SwaggerUIBundle({
        url: 'spec.json',
        dom_id: '#swagger-ui',
        deepLinking: true,
        presets: [
            SwaggerUIBundle.presets.apis,
            SwaggerUIStandalonePreset
        ],
        layout: "StandaloneLayout",
        docExpansion: 'none',
        defaultModelsExpandDepth: 0,
        defaultModelRendering: 'example'
    })

    window.ui = ui
}

Expected Behavior

The callback panel should be expanded on page load, similar to how the endpoint is.

Current Behavior

Nothing gets expanded on page load.