Windows can have the same problem when you run out of space, but it will at least give you a helpful UI to clean everything up.
Its not clear to me either why it appeared as if there was free space, but it might be the btrfs
/df
incompatibility the other poster raised.
Good luck, and reach out if there are further issues :)
Ha, I had basically the same bug (functionally) in GNOME a few months back. Moving the mouse off the screen scrolled it around.
The top left was anchored at least so it was easy to reset.