Error in redirect when clicking Follow button from With Replies page (Mastodon v4)

Bug description

When clicking the Follow button from the "Posts and replies" page, the username seems to be incorrectly parsed from the URL. The result is a redirect to "https:///authorize_interaction?uri=with_replies@". The "with_replies" is where their actual username should appear.

Steps to reproduce

Go to a Mastodon user you don't follow.
Click Posts and Replies or Media
Click the Follow button

Actual behavior

The URL opened does not include the user's username, but instead the last part of the URL. ('with_replies" if on the Post and Replies section)

Expected behavior

The username should be passed to the redirect URL and not just the last part of the original URL

System

Browser and version: Firefox
Add-on version: 2.0
Mastodon: v4

Possible solution

As best I can tell, the issue is this line:

mastodon-simplified-federation/src/content_script/mastodonInject.js

Line 12 in 9197e7f

const username = window.location.pathname.split("/").slice(-1)[0];

is just taking the last part of the URL after the last /.
This works fine for https://mastodon.social/@username
But with a URL like https://mastodon.social/@username/with_replies, it incorrectly uses the "with_replies" part as the username.

Perhaps something like

const username = window.location.pathname.split("/").find(string => string.startsWith('@'))

would be more flexible?

Also, just wanted to say Thank You for this extension, it's very convenient.

Thanks a lot for that detailed bug report/writeup! Without having tested/reproduced the issue that seems very plausible and easy to fix, so feel free to create a PR and test it.

So it would be great if you could try out the development version and submit a patch.