Screen Dimming on ` key press

When playing a mod (e.g. ROT or DOM), the ` key (above tab) causes the screen’s brightness to toggle between normal and dark. This behavior does not occur during the standard main campaign. I don’t see a key binding related to this. Any idea what is going on?

UPDATE: Only does this on Single player games. Is there a way to disable it? I can of course play single player as multiplayer, but it would be nice to know if it can be turned off.

It was dimming in Custom Game because it’s a console command window that’s available there.