Rewriting request.url for loadSpec requests does not update URL selector box

Q&A (please complete the following information)

• OS: Windows
• Browser: Chrome
• Version: 69
• Method of installation: swagger-ui-dist
• Swagger-UI version: 3.19.3
• Swagger/OpenAPI version: Swagger 2.0

Content & configuration

Example Swagger/OpenAPI definition:
N/A

Swagger-UI configuration options:

function _usp(url, fn) {
    url = url.split('?');
    var usp = new URLSearchParams(url[1]);
    fn(usp);
    url[1] = usp.toString();
    return url[1] ? url.join('?') : url[0];
}

function setQueryParam(url, key, value) {
    return _usp(url, function (usp) {
        usp.set(key, value);
    });
}

SwaggerUI({
        requestInterceptor: function (request) {
            if (request.loadSpec) {
                // pass query params from the swagger-ui page to the spec request
                var query = new URLSearchParams(window.location.search).entries();
                for (var it = query.next(); !it.done; it = query.next()) {
                    request.url = setQueryParam(request.url, it.value[0], it.value[1]);
                }
            }
            return request;
        };
        // ... other stuff
})

Describe the bug you're encountering

To reproduce...

Steps to reproduce the behavior:

1. Write a request interceptor that modifies the url of loadSpec requests
2. Load the page
3. Wait for spec to load
4. See that the spec selector box has the old URL

Expected behavior

Spec selextor textbox updates with changes made by interceptor.

Additional context or thoughts

https://github.com/swagger-api/swagger-ui/blob/a86fcf312a0d9b04d99982bf88a3d95884eddb9a/src/core/plugins/download-url.js#L38-L40 seems to be the problem, setting the URL back to the old value

Start with src/core/plugins/download-url.js around lines 38-40 and trace how a requestInterceptor changes request.url for loadSpec requests. Reproduce the issue with the provided interceptor, then verify that the spec selector textbox reflects the rewritten URL instead of the original URL.