Steps to reproduce:
1) Open a long .org file
2) Tap on the "view as plain text" button (top bar, second from the right)
3) The file is now shown as text, and the keyboard should have popped up
4) Tap the "back" button
The lower third of the screen should now be covered by a white rectangle, covering a part of the opened file.
This was tested on an iPhone 6S, running iOS 12.1.1 and Beorg 2.8.0 (99).
Thank you for reporting this, I'll get fixed in the next release.