The problem is similar to this: https://forums.spiralknights.com/en/node/113413

It occurs not on my pc but on someone's I do play with, so I don't have all the specs, but what I remember is: (will update later)

-Asus Laptop
-Win10
-7th generation Core i7
-12 Ram
-2VRam/ Nvidia GeForce 900 series m/mx
-1920 x 1080 recommended resolution

Latest Java is installed and copied over into the game files replacing the old java_vm folder, as it is in my own pc.

We have tried going into windowed and stretching it to the size - games recognizes it as a "2400-ish x something" resolution, - BUT the different buttons in the in-game menus seem off to where you can really click them.

Changing the screen-resolution in windows doesn't work.

ALSO: Yesterday when we tried to fix it, we connected over Team-Viewer as the game was still starting from the patch-window and it just magically started into normal working fullscreen mode. Today we had no luck though.

TL;DR - It refuses to cover the full screen at any resolution, except for one time. When stretched it messes up the "hitbox" of the menu buttons.