Result from CocInfo

## versions

vim version: VIM - Vi IMproved 9.1 9011908
node version: v23.9.0
coc.nvim version: 0.0.82-9d00dc1 2025-12-15 17:22:19 +0800
coc.nvim directory: /home/lrne/.vim/plugged/coc.nvim
term: dumb
platform: linux

## Log of coc.nvim

2026-01-11T16:18:20.362 INFO (pid:27112) [plugin] - coc.nvim initialized with node: v23.9.0 after 443
2026-01-11T16:18:25.287 INFO (pid:27112) [attach] - receive notification: showInfo []

Describe the bug

Open a single line file, insert and undo a new line before the first line, and the file becomes empty

Reproduce the bug

We will close your issue when you don't provide minimal vimrc and we can't
reproduce it

• Create file mini.vim with：

  set nocompatible
  set runtimepath^=~/.vim/plugged/coc.nvim
  filetype plugin indent on
  syntax on

• Run echo a b c > foo.txt

• Start (neo)vim with command: vim -Nu mini.vim foo.txt

• Operate vim.

  1. On line 1, input O
  2. Insert foo
  3. Click <esc> u

  Expect: a b c

  Actual: empty file