MOUSE_SENSITIVITY and units_per_pixel (i.e. screen resolution, relative to 640x480) are honoured
As far as I can tell, behaviour is the same as before the SDL2 update.
With altinput on:
mouse moves 1:1 with the Windows mouse (excusing the fact that its coordinates get out of sync when you leave the window, as it has always done). [MOUSE_SENSITIVITY and units_per_pixel ignored]
It seems to work as expected: With altinput off:
With altinput on: