X11 supports mixed DPI with the RANDR extension. Applications can use the per-monitor information provided by RANDR to get the DPI information for each output,

X11 supports mixed DPI with the RANDR extension. Applications can use the per-monitor information provided by RANDR to get the DPI information for each output, and use this to calculate font and widget sizes. Everything looks nice as long as you don't have a window spanning multiple screens. The problem is that while Qt has supported this functionality since 5.9, GTK seemingly has no interest in implementing it and most other toolkits or raw Xlib programs don't support it either.