Fern may wrongly overwrites a non-fern buffer.

[mityu]

Problem

Fern wrongly overwrites a new, non-fern buffer in certain situation.

Reproduce steps

• Prepare this repro.vim

set nocompatible

" Please modify this path.
set runtimepath^=~/.cache/vim/pack/minpac/opt/vim-fern/

autocmd VimEnter * ++once call timer_start(10, {-> s:repro()})

function s:repro() abort
  Fern /
  split
  enew
endfunction

• Run Vim with vim -u repro.vim
• The current buffer (bufnr must be 2) has a file tree display although it's a anonymous, non-fern buffer.

Environment

• Vim 9.1.842
• Neovim v0.11.0-dev-1108+g5a86360400

The cause of this bug

1. At the split command at line 2 in s:repro function, fern's auto duplication feature will create new fern buffer and trigger asynchronous viewer draws. At this time, the draws are not done yet and the new buffer is empty.

+----------------------------------------+
|  bufnr:2                               |
|  bufname:fern://...                    |
+----------------------------------------+
|  bufnr:1                               |
|  bufname:fern://...                    |
+----------------------------------------+

2. enew command at line 3 in s:repro function opens a new empty buffer. However, since enew reuse the current buffer if it's not modified, the current buffer will be a non-fern buffer with keeping its bufnr same to one that the previously created fern buffer had.

+----------------------------------------+
|  bufnr:2                               |
|  bufname:(anonymous)                   |
+----------------------------------------+
|  bufnr:1                               |
|  bufname:fern://...                    |
+----------------------------------------+

3. Since the bufnr is same, fern.vim's drawer will draw its contents to current buffer though it's no longer a fern buffer.

+----------------------------------------+
|  bufnr:2                               |
|  bufname:(anonymous, wrongly written)  |
+----------------------------------------+
|  bufnr:1                               |
|  bufname:fern://...                    |
+----------------------------------------+

(EDIT: Fix buffer positions in figures)

[mityu]

Sample screenshot. Ran the reproduce steps on Vim.

[lambdalisue]

I'll check it after Vimconf. Let me know if I forget about this issue.

[mityu]

OK 👍.

Please don't mind, I don't think this is a critical issue; this rarely happens and we can restore the previous buffer contents with undo ;).

[mityu]

@lambdalisue ping

[lambdalisue]

I could reproduce the issue but it seems a bit complicated...

[lambdalisue]

@mityu Could you try #516?

[mityu]

Seems to be working really well 👍
Thank you.