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.
If I recall from the original thread, it came from someone doing it because they realized they could, because it was implemented with flexibility, and not because anyone ever specifically thought it would be a good idea.
Basically mapping what the OS thinks of as the screen, where the display renders that virtual screen, how a touchpad maps to it, how a touchscreen on the display maps to them both can be complicated.
Everything basically figures itself out now, but they still use the tools that happily accept a transformation matrix to changes things around.
My hope is to see a rotating monitor setup with a display that holds still.