I don’t think this is something we do. If you open a terminal in full screen mode (drop down terminal mode in Authentic theme), then the ambient light level drops significantly (in case you’re working in a completely dark room), and what I think is really happening, is that your monitor light sensor reacts and dims the screen’s brightness.
The solutions are few:
Don’t work in complete darkness and have some constant ambient light around your monitor’s ambient light sensor
Disable ambient light sensor in your monitor menu, and control brightness manually
Use stand-alone Terminal module (i.e. Webmin / Tools → Terminal)
It was a very strange behavior as if it was breathing in and out. But when I clicked on the title bar to minimize it than it would stop. My Monitor had this feature called Samsung Magic so I switched it standard with full 100% brightness and now it stopped dimming.
I bought a new 2k monitor a few months ago. I thought my eyes were just getting worse than they already were until I mentioned it on another forum and somebody told me about the same thing.
They really should give us old, half-blind already people the heads up on new features like this.