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Twitter post by Daniel Feldman (@d_feldman): Linux is the only major operating system to support diagonal mode (credit [Twitter] @xssfox). Image shows an untrawide monitor rotated about 45 degrees, with a horizontal IDE window taking up a bottom triangle. A web browser and settings menu above it are organized creating a window shape almost like a stepped pyramid.
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Probably would fall into scope of a compositer in Wayland, rather than the protocol. I suspect it originated with old CRT displays. Sometimes they can appear scan diagonally.
Even without that usecase, I think it's great to have around in order to support novel displays and display-like devices.