Ubuntu 16.04: screen unlock fails if my phone is plugged in?

[Paul Smith]

I've been having this weird problem where _sometimes_ when I try to
unlock my screen I enter my password and hit ENTER and it just hangs
forever.  Other times it works fine.  Ubuntu 16.04.2.

I had figured out a workaround that avoided having to kill my session:
if I use C-A-Fn to get to another (graphical) login screen and log in
as another user (I had a test user account on the system), then use the
"Switch User" option to switch back to my first session, it unlocks my
screen.

However yesterday I found a smoking gun: it turns out this only happens
if my phone (Nexus 5 / Android) is plugged into my computer's USB port
(done to charge the phone).  If I unplug the phone then I can log in.

Does this ring any bells with anyone or does anyone have any thoughts
about where to look for logs, config settings, etc. that might be
involved?  Google doesn't help me much.

Now that I know what it is it's not such a big deal to work around it,
but it's still annoying.

Cheers!