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.
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?