When you attach a new display to your Mac, be it a primary or secondary display, the system should recognize the new addition and configure the system to use it. When it does this, the displays may all blink to a blue output for a second or two, and then output your desktop environment to the display, usually in extended desktop mode, though it may use mirrored mode.
This behavior is the system determining the make, model, and capabilities of the display so it can properly use it. There are times when this may not work, though. In such instances, … Read more