I just want to share with you a few images of this happening to my X100f viewfinder.
To be honest I have no clue what is it.
I bought the camera at Calumet UK but I think I should contact Fujifilm UK right?
That's the ERF (electronic rangefinder) window/patch. If you are in OVF mode, try pushing the lever on the front of the camera toward the lens itself (the opposite direction compared to when you switch between EVF and OVF mode). It should disappear. If not, it is completely broken.
It does look broken still though. Because it should be a perfect square, and yours looks like it's a bit crooked?
My "T" got stuck like that once. I just banged the camera bottom on my (wooden) table a couple times and it went back down. (I really didn't want to wait for repairs) That was a couple years ago, it hasn't gotten stuck since. If you got yours recently, i'd just go for an exchange though.