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Eww, Copilot AI might auto-launch with Windows 11 soon
(www.pcworld.com)
This is a most excellent place for technology news and articles.
Go to the compatibility menu in settings and set a default version of proton, and it will let you install whatever (though there's still the possibility of it not working).
You're right that it's a bad experience, and I'm not sure when it changed to not have a default or if it's a bug, but that's the solution for that issue.