There has been the need to clarify some rumors that have been circulated on social media:
Yes, #neovim uses an different undofile format than #vim. In an ideal world we would not need this, but the way neovim implements buffer properties is completely different than vim, and this state is part of what is undo-able.
However, Neovim does NOT delete or corrupt Vim undofiles (nor has vim any problem with neovim undofiles), for two simple reasons:
1. By default, neovim have used a different path for state files than vim since a decade. Thus the two editors will just safely ignore each others state files, just like you used any pair of editors at the same time, like vim and emacs.
2. even if you forcibly override both neovim and vim to look for state files in the same directory, there is no risk for corruption. Both editors share a versioning logic for undofiles where incompatible versions are directly detected and left unread and unwritten. These are never overwritten without asking the user for confirmation.
