XFIXES
The X extension that swept up the odds and ends the core protocol forgot, and that modern compositors can't live without.
Keith Packard wrote XFIXES around 2003, alongside Damage and Composite, and the name is refreshingly honest: a box of patches. The most important one is regions, sets of rectangles stored in the server, which let a compositor work out what needs repainting without shipping coordinates back and forth.
The rest is just as practical. A client can be told when a selection changes owner, which made clipboard managers possible without polling. It can fetch the cursor image, so VNC and screen recorders draw it correctly, and it can hide the cursor. Version 5 added pointer barriers, invisible walls between monitors. None of it is glamorous, and nearly every X desktop from 2005 onwards assumes it is there.