After executing :Diffoff, foldmethod stays at diff. Can be reproduced as following:

1. Open two files in vertical splits, have foldmethod set to marker
2. Switch to visual mode, select lines in both
3. :Diffthis
4. Execute :Diffoff in both splits
5. :set foldmethod?

On the side-note, I would be handy to have single command that exits the diff mode and restores the window layout back to what it was.

Tested on nvim v0.10.2