It already is disabled for other backends, but this didn't happen with the software renderer. Attempting to change it while running causes the change to visually happen (including switching to the normal render settings UI instead of the barren one for the software renderer), but doesn't actually change the backend itself (it'll still use the software renderer at the next launch).
WindowModal allows alt+tabing to the render window, but prohibits interaction
with parent windows (controller settings window and the main dolphin window).
Pokechu22