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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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Linux phones aren’t supported because it’s an Xorg feature. Usually Linux phones use Wayland for the better (touch) experience. If someone wanted to they could implement it on a Wayland compositor, but given that no other OS I know of supports diagonal mode, I wouldn’t hold my breath.