To reproduce: turn scrivening mode on, select folder in binder that has documents inside, press down arrow key to navigate to first of those documents.
Now you’re unable to move with the arrow keys anymore.
If you wait like 10 secs, does it let you again? This has been reported before, and it may be a document loading issue.
The problem is indeed the Scrivenings editor stealing the focus. This has been fixed and will be available in the next release.