If you try to start Streamlit in a GitHub Codespace with

streamlit run YourFile.py

and open the provided URL, you'll be stuck at

Start Streamlit with

streamlit run YourFile.py --enableCORS=false --enableXsrfProtection=false

or add the following lines to your .streamlit\config.toml

[server]
enableCORS = false
enableXsrfProtection = false

After this, you can access your streamlit app in the browser from inside the codespace without a problem.

WARNING: You might want to set these back to their default values true or remove the lines before publishing.