This vital service composites individual windows to form what will be displayed, and routes clicks/taps to the correct app.
WindowServer
How to tell apart unexpected quits, WindowServer crashes, kernel panics, and more – and what to do about them.
What is wrong when, out of the blue, the display freezes for ten seconds, then you see a black screen and are asked to log back into your account? Malware perhaps?
How to understand Console’s log entries for a normal shutdown, and a normal startup.
The Eclectic Light Company 