Emacs 25.3.1 Suddenly Starts Fullscreen?

[Brendan Leber]

Sometime in the past day or two my emacs installation has begun to start
and take up the entire screen.  I've checked and I don't have an
.emacs.desktop or .Xdefaults file which according to my searching might
be the cause.  Nor is there a process runngng as a daemon.  I've
restarted my shell and rebooted my system to no avail.

It also happens if I start with `emacs -Q`.

I'm running a version I've compiled from the release tar.gz file.  It
reports the version as "GNU Emacs 25.3.1".  My OS is Ubuntu 16.04.3
LTS.  

Can any of you fine folks help point me to what might be causing this?

Thanks!
B

-- 
C++ tries to guard against Murphy, not Machiavelli.  — Damian Conway