https://www.reddit.com/r/feedthebeast/comments/1botv2x/all_guis_are_shifted_to_the_right_of_my_screen/

Experienced it myself playing with overloadedarmorbar1.20.1-1

Seems to have fixed itself after removing and re-equiping the elytra.
However, even "fixed" when you put the elytra on the inventory menu still temporarily jumps before readjusting itself

It seems to only apply when you have less than a full armor bar. If you have 20 armor then the gui stays even with an elytra on

I cannot currently reproduce this with only this mod and JEI. There is something else involved.

hmm, I'll dig through the mod list to see what works