Xorg does support having multiple monitors with different DPI, however applications need to use the RandR extension to obtain that per-monitor information as an

Xorg does support having multiple monitors with different DPI, however applications need to use the RandR extension to obtain that per-monitor information as an older API that provided per-screen information (which doesn't make sense for Xinerama anyway) is hardcoded for backwards compatibility reasons (an attempt was made a couple of Xorg releases ago to remove the hardcoded value and present a more appropriate value but it broke a bunch of programs so it was reversed).