New versions of neovim will not read undofiles written by nvim before this merge (there will be an error message about incopmatible version). Nvim 0.4 (or an master up to bda12927be ) can be used to recover older undofiles, so if you worried about unsaved changes lurking around in undofiles it would make sense to keep such version around somewhere to recover them.
This is a necessary change to keep tree-sitter and plugins dependent on byte-level buffer change events fully working with undo states from a undofile. If there is a clear demand we might implement reading of the old format. Such recovered buffers will not be be fully functional with plugins relying on buffer updates or tree-sitter, however.
On empty buffers, when editing the first line, the line is buffered, causing offset to be < 0. While the buffer is not actually empty, the buffered line has not been flushed (and should not be) yet, so the call is valid but an edge case.
This caused segfaults when calling win_findbuf in an `on_detach`
callback, when the callback was triggered when closing the last window
containing the buffer.
Sloppy code inherited from Vim caused user scripts to be able
to observe the cursor line in an invalid intermediary state,
due to Neovim change callbacks being unbuffered unlike Vim listeners.
Manifested in Vimscript executed from the callback possibly erroring
when `:call`:ing any function,
due to the implicit range `curwin->w_cursor.lnum,curwin->w_cursor.lnum`
failing validation.
Fixed by deferring the call to `changed_lines()` until after
`curwin->w_cursor.lnum` gets its correct value.
