Hi Dave
Yet again dosbox(-x).conf gives an answer for output options:-
# output: What video system to use for output (surface = software (SDL_Surface); openglnb = OpenGL nearest; openglpp = OpenGL perfect; ttf = TrueType font output).
# Possible values: default, surface, overlay, ttf, opengl, openglnb, openglhq, openglpp, ddraw, direct3d.
So far default, surface, ddraw and the opengl* variants all give the same result: dosbox-x does not run fullscreen; overlay results in dosbox-x crashing on startup.
Regards
Pete