SDL2 still doesn’t support HighDPI on Windows.
As a workaround you can call the WinAPI functions to tell Windows that your applications is “DPI-Aware” (which just means “Yes, I really want a window of the size I asked for with the actual native resolution, and I’ll scale things in my UI up myself if necessary”).
SDL_GetDesktopDisplayMode resolution reported in Windows 10 when using app scaling - #4 by Eric_Wasylishen has example code for this (loading the corresponding function dynamically so the code still works on older windows versions that don’t support them). Of course that’s in C, but I assume that it’s possible to do the same in Ada. Call that code before creating a window (I usually call it quite early in main(), before calling SDL_Init())