I have several notes on window systems:
Cached SOA desktop is about a REST architecture applied to a windowing system, with each 16×16 tile being a separate resource, using a cache invalidation protocol for redraws.
Speculative plans for BubbleOS talks briefly about Wercam, a secure windowing system.
What does a futuristic OS look like? talks about what I think a “cool” or “futuristic” OS should look like, or rather what “outdated” OSes look like.
A minimal window system talks about how to make a windowing system use a minimal amount of code, considering different design alternatives.
Pixel stream talks about a protocol for windowing systems.
Real time windowing asks about how to make windowing systems guaranteed responsive.