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TIL - Linux supports tilted monitors... apparently 22° is best
(www.tomshardware.com)
This is a most excellent place for technology news and articles.
Doesn't need a protocol, just compositor support. Unless you want windows to be aware of being rendered at an angle, that is.
Also I do wonder how broken that stuff is under X as the WM protocols that I remember definitely assumed axis-aligned monitor and windows.