Weird desktop image on screen

[MR ZenWiz]

My partner has a Dell Inspiron 3785 with a 1600x900 17" screen.  The
main monitor is an HDMI LG 32"  full HD screen.  I've set the HD
monitor to be the primary display, and it works just fine - most of
the time.  She runs with the laptop lid closed, and when it is open,
only the upper left 1600x900 part of the 1920x1080 full screen is
visible on the laptop monitor while the full HD screen is visible on
the HD monitor.  This makes sense to me.

There is one oddity that appears from time to time, and the only fix
I've found so far is a reboot, which stinks.

Occasionally, the desktop background image will appear on the HD
monitor in the 1600x900 space on the top left corner of the HD
monitor, and also around the right and bottom of that image is the
remains of the full HD image of the same background.  Sort of like
this:

+-------------------------------+----+
|                                       |     |
|                                       |     |
|                                       |     |
|                                       |     |
+-------------------------------+     |
+------------------------------------+

I've tried playing with the display settings, nothing there seems to
help.  Of the four display settings for the usual
two-monitors-of-different-sizes issues, I haven't checked any - we
just use the default.  I don't recall if logging out fixed the
problem, but I think not.

Any idea why this might be happening?

Thanks.

Mark