The account provider search treated its whole input as a host, so a full user
ID was probed as https://@alice:example.org, which never resolves, and the
permissive URL fallback then offered that invalid address as a provider.
Recognise a complete user ID and resolve the homeserver from its server name,
preserving a port when there is one. Input that starts with @ but is not yet a
complete user ID resolves nothing, so no candidate is produced while the user
is still typing. Detection reuses MatrixPatterns.isUserId and UserId.domainName
rather than a new pattern.
hayaksi1
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