I might be mistaken. Maybe it did not start fullscreen but just too big?
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My screen size code right now is pretty simple.
.: In CREATE Event :.
viewWidth = 1920 /4;
viewHeight = 1080/4;
windowScale = 4;
var horSize = viewWidth * windowScale;
var verSize = viewHeight * windowScale;
window_set_size(horSize,verSize);
//then i set the surface size to the same values,
surface_resize(application_surface,horSize,verSize);
And then lastly i center the window to the center of your screen.
So there is no fullscreen mode when you start the game!
How small is your screen?
Mayeb i can add a "windowed size " property in that case where you have the options x1, x2, x3, x4 etc
My active screen's resolution is 1440 X 900. You code seem to default to 1920 X 1080. What I do for my games is I check the user's screen dimensions with "display_get_width" and adapt the screen accordingly (simplified code below) :
var dw = display_get_width();
if(dw >= 1920){ w= 1920; h= 1080;}
else if(dw >= 1280){ w= 1280; h= 720;}
else if(dw >= 640){ w= 640; h= 360;}
surface_resize(application_surface,w,h);
display_set_gui_size(1920, 1080);
window_set_size(w,h);