Mysteriously, a rectangle has appeared in the middle of the screen. How to get rid of it? I have a iMac (27-inch, Late 2009) running OS X Yosemite v 10.10.2.  Mysteriously, a rectangle has appeared in the middle of the screen.

Proportionately, it looks like a golden section.
It floats on top of everything else.



Close up:



Almost like a ghost of something, but I don't remember opening anything that size.

Any ideas about this?

Could someone/something have hacked my machine?

How do I get rid of it?
A floating black rectangle appeared on my Mac screen after upgrade to High Sierra operating system. It is due to an Accessibility option and can be removed as follows:

System Preferences > Accessibility > Zoom > Options > Show preview rectangle when zoomed out

I do not know how it appeared, but I am glad that it is gone.