OK, so I have this weird bug.
I have set up my monitors on top of each other
As you can see the screens aren't totally aligned which is the way I want it.
Now, here's the bug: I can't fullscreen windows on the top monitor. If I fullscreen the window it will only fill the left or right part of the screen (matching the distance from the right edge of the laptop to the right edge of the big screen) It's like the bottom screen is blocking the middle part of the screen.
This behaviour is not present if I align the screens vertically. If I drag the corners of the window I can use that part of the screen, it's only when I fullscreen that the problem occurs.
Not sure if this info helps but the only program that I'm able to fullscreen properly is my virtual windows desktop (qemu/virt-manager).