Registration: shows subdomain instead of main domain with delegation

[krille-chan]

Description

The registration form shows the subdomain instead of the main domain for a Matrix - synapse server setup with delegation
The issue is repro for both the Android app and the https://fluffychat.im/web/

To Reproduce

• Setup a synapse server with delegation on a subdomain, for example matrix.mydomain.org
• Make sure https://mydomain.org/.well-known/matrix/server is pointing to matrix.mydomain.org
• As Homeserver enter "matrix.mydomain.org" and click the "Register" button
• The Username field shows @ .... :matrix.mydomain.org

Current results: the Username field shows the subdomain the synapse server is running at.
Expected results: the Username fields shows the server_name for the homerserver.yaml or is able to detect that a .well-known file exist for the subdomain and that the main domain should be used for the Username.

I confirm that the registration works, including the 3pid email registration and the use of a token on my own matrix - synapse server.
Although the registration dialog shows @username:matrix.mydomain.org the user is correctly created with @username:mydomain.org
So at the end this is rather a cosmetic issue, that is confusing though.

[github-actions]

This issue is stale because it has been open for 120 days with no activity.

[github-actions]

This issue was closed because it has been inactive for 14 days since being marked as stale.