MIT-SCREEN-SAVER

The core protocol has a primitive screen saver: after xset s minutes of inactivity the server blanks the screen or draws a pattern, with no client involved. Jim Fulton and Keith Packard wrote the extension for X11R6 in 1994 so that a program could stand in instead. The client registers a window with XScreenSaverSetAttributes, the server maps it over everything else when the timer runs out and takes it down at the first keypress, and ScreenSaverNotify reports what happened. The extension shows up in xdpyinfo and is reached through the libXss library.

The window part saw little use; XScreenSaver long preferred to measure idleness itself. The call that took on a life of its own was XScreenSaverQueryInfo, which returns milliseconds since the last input. That is how xautolock knows when to lock, where xprintidle gets its number and how chat clients set themselves to "away". That any client at all could learn exactly when the user left the keyboard was not something anyone worried about in 1994, and something Wayland later refused outright, which is why idle detection there goes through the compositor with a protocol of its own.