Just an unqualified thought?
It makes me think of an aspect ratio setting being displayed in the screen.
I had a quick look in the downloaded manual and it does talk of different (selectable?) aspect ratios when using Live View.
PS using my Olympus/OM cameras that are natively 4:3 ratio I set it to 3:2 ratio and see such thin lines in the viewfinder & screen but as I shoot raw I get the whole 4:3 sensor image. However, if I was shooting jpeg I would only get the cropped 3:2 image file.
NB I do the above for compositional and print reasons.