[arch-general] How to remove secondary display shown in Gnome 3 settings

[Francis Gerund]

Hello.

While trying to fix what I thought was an application program window
placement problem, it turns out that the problem was that Gnome 3 seems to
be detecting a display which does not seem to exist.

Clicking the Gnome "Settings" icon, then the "Displays" icon seems to show
both a primary and a secondary display.
But there is only one monitor and one graphics card!

The primary display is correct:

1366 x 768 resolution, 16 x 9 aspect ratio.

But even thought there is no second monitor or graphics card, "Displays"
shows a secondary display as:

848 x 480 resolution, 16 x 9 aspect ratio.

So, how can I delete the non-existent secondary display.  (In the good old
days, "Displays" included a button to detect displays.  Not now).

1)  Wild guess: 848 x 480 seems like some sort of phone or tablet display,
but 16 x 9 wouldn't seem to be a phone display.

2)  A non-existent display - I worry: have I been hacked?

This is on an unmodified, standard, name-brand computer running up-to-date
Arch 64-bit.