Weird full screen

[Skertelles]

So I not sure if I can post this here but I getting this weird thing . When ever I go full screen on the omega adventure mod on full screen , the main menu is mostly on the right  of my monitor , and I can’t select create new world because I can’t see the button and when I am in the world I created when I enter full screen I can’t see my hot bar. When I play pixlemon generations it’s normal on full screen . So how do I fix this ? 
 

simpler summary. In game resolution seems off when in full screen .

 

I not sure why but I can’t post the picture I have on imugur.

86E97B2B-7AB5-4A85-8709-1CE8B10DCC79.webp