With the gnus included in recent debian packaged i386 emacs 25.2, in the
Group buffer, an M-g to get count of messages of a foreign imap group at
imap.mail.yahoo.com hangs.  If C-g to stop then a second M-g succeeds.

Sticking in some protocol trace showed

    out >> 1 CAPABILITY
    out >> 1 LOGIN "username" "password"

    in  << 1 OK CAPABILITY completed
    out >> 2 SELECT "mailboxname"
    
    in  << 1 OK LOGIN completed
    hang ...

I suspect nnimap.el interprets "1 OK CAPABILITY" response as being login
completion, and issues the SELECT.  Could that be right?

I don't know if a server is supposed to allow pipelined SELECT before
login completes.  It seems recent imap.mail.yahoo.com doesn't like it.
I see nnimap-login uses nnimap-command so I take it the intention in
gnus is not to attempt such a thing anyway, but wait for login success.

I get some joy from the diff below setting nnimap-sequence so it starts
"2 LOGIN".  The first M-g then succeeds for me as hoped.

I think the reason a second M-g on the group succeeds is that
nnimap-sequence is left incremented after the first attempt, so it goes
"3 LOGIN" or whatever bigger number.

I have this on a rather lagged connection, if that might make a
difference.  I see the login taking a noticeable second or so, but don't
know if that's by the server or the connection.

-------------------
Gnus v5.13
GNU Emacs 25.2.1 (i686-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2017-09-12, modified by Debian