swagger-ui-bundle.js GET request hangs while trying to load swagger-ui page served from flask/waitress on an IP other than localhost

Q&A

• OS: Windows 10
• Browser: Chrome 80.0.3987.122, Firefox 73.0.1
• Method of installation: through pip install flask-swagger-ui and updating swagger-ui /dist to the latest release obtained from https://github.com/swagger-api/swagger-ui/releases/tag/v3.25.0
• Swagger-UI version: 3.25.0
• Swagger/OpenAPI version: Swagger 3.0, OpenAPI 3.0

Describe the bug you're encountering

When using Swagger-UI (through the flask-swagger-ui python package) and served with the flask/waitress web server, Swagger-UI works fine when using localhost but fails to load the page when using any IP/URL other than localhost. Specifically, the page hangs while trying to GET the static swagger-ui-bundle.js file (see screenshot).
The workaround was to load swagger-ui-bundle.js using unpkg instead of loading the local version. It's interesting to notice that other static files (swagger-ui-standalone-preset.js, swagger-ui.css, etc.) are loading fine.

To reproduce...

1. Create a hello world flask project using the flask-swagger-ui python package (https://github.com/sveint/flask-swagger-ui#usage)
2. Update the flask-swagger-ui py package /dist directory to contain the latest swagger-ui release
3. Run the server with the real IP address of the system (not using localhost)
4. Attempt to load the swagger-ui page using Chrome or Firefox

Expected behavior

The swagger-ui page should load just fine, like when using localhost.

Screenshots

Additional context or thoughts

Since this problem is specifically related to the swagger-ui-bundle.js file, and because other static files are served correctly without problems, I assume this is not an issue related to python/flask/waitress.

Start by reproducing the Swagger UI 3.25.0 page through Flask/Waitress on a non-localhost IP and observe the GET for swagger-ui-bundle.js alongside the other static assets. Compare how swagger-ui-bundle.js is loaded and served with swagger-ui-standalone-preset.js and swagger-ui.css. Done means the page loads successfully through an IP or URL in Chrome and Firefox, as it does through localhost.