Horizontal scrollbar in email archives

I see an odd horizontal scrollbar when viewing the email archives in
Firefox 84.0.2.  Specifically it happens when I am on a 'message id'
link:

    https://www.postgresql.org/message-id/20210108155040.GC18394%40momjian.us

Screenshot attached, with orange scrollbar.  I think it is related to
the new 'thread' drop-down list.  If I choose "Whole thread", it
disappears, and so does the thread drop-down list.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee