[Bug]: Adding a public share to your Nextcloud does not work if no user is specified and the user is not logged in

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

In Nextcloud 28 and later right now this does not work because the federated file sharing plugin for the Files app is not compatible with the changes introduced in Nextcloud 28. But, even if it did, it is likely that the problem described below still happens :-)

In the public share page, if outgoing federated shares are enabled, the top right menu includes an Add to your Nextcloud item. When clicked an input field is shown, where it is expected that user@yourNextcloud.org is set. That sends a request to the sharer server to create a federated share with the remote server and user specified in the input.

However, in the dawn of time, only the remote server was expected to be set in the field. This legacy behaviour is still supported. In that case the page redirects to the Files app in the remote server with some special query fields, so once the Files app is loaded the sharer server is asked to create a federated share with the remote server and the now known user.

The problem is that, if the user was not already logged in when sending the Add to your Nextcloud form, before the Files app is opened the user will get redirected to index.php/login?redirect_url=/index.php/apps/files#remote={HOST}&token={SHARE_TOKEN}&owner..., and after logging in the part after the # will be lost and the user will be simply redirected to /index.php/apps/files without causing the addition of the public share. This is reproducible at least on Nextcloud 22.0.0; I did not check further back to find when the regression was introduced, though.

Steps to reproduce

• Setup Nextcloud server A
• Setup Nextcloud server B
• In server A, create a public share
• Ensure that no user is logged in server B in the current browser window
• Open the link to the public share
• In the top right menu, click on "Add to your Nextcloud"
• Enter the address of server B (only the address, without user@)
• In server B, log in as a user
• Open the Files app

There is no dialog to add the remote share to your account

Expected behavior

There is a dialog to add the remote share to your account

Installation method

None

Nextcloud Server version

master

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

Start with apps/files_sharing/js/public.js and apps/federatedfilesharing/js/external.js, then trace the login redirect used before the Files app loads. Reproduce the server A/server B flow with the user logged out on server B; done means the remote-share dialog appears after login when only the server address was entered.