SYNC extension (X11)
Counters and alarms inside the X server, so clients can wait on each other without polling.
The X protocol is asynchronous: a client fires off requests with no idea when they actually happen. The X Synchronization Extension, shipped with X11R6 in 1994, adds counters that the server or a client can increment and alarms that trigger when a counter crosses a value. It was meant for multimedia — keeping audio and video in step — but found its most important job elsewhere.
Through EWMH's _NET_WM_SYNC_REQUEST, a window manager can ask an application to report back once it has redrawn after a resize, instead of piling the next resize onto a half-painted frame. Without it, the contents lag visibly behind the border. A later revision added fences, which compositing window managers and GL drivers use to keep tearing at bay.