Strange boxes on El Capitan

[apomin]

I use an old mac mini for music
Some day ago two boxes appeared on the screen, one gray and one dotted
They won't go away even after rebooting
I am a Windows guy, don't know much about Mac
What could it be?

 

[for this]

I don't know what it is. It doesn't look like Zoom's preview (System Preferences>Accessibility>Zoom>More Options). I would try shutting it down and turning it back on, like in this thread.

https://discussions.apple.com/thread/8322541

 

[apomin]

I tried safe restart and discovered that it was actually a monitor issue. The boxes were right there even on the startup screen. My Asus has some controls at the bottom and I must have touched one of those. In a way rebooting the mac helped, thank you