Sign placed in signcolumn disappears after confirming selection

[ValenTheRed]

FAQ

• [X] I have checked the FAQ and it didn't resolve my problem.

Announcement

• [X] I have checked Breaking change announcement.

Minimal reproducible full config

if has('vim_starting')
  set encoding=utf-8
endif
scriptencoding utf-8

if &compatible
  set nocompatible
endif

let s:plug_dir = expand('/tmp/plugged/vim-plug')
if !filereadable(s:plug_dir .. '/plug.vim')
  execute printf('!curl -fLo %s/autoload/plug.vim --create-dirs https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim', s:plug_dir)
end

execute 'set runtimepath+=' . s:plug_dir
call plug#begin(s:plug_dir)
Plug 'hrsh7th/nvim-cmp'
Plug 'hrsh7th/cmp-buffer'
Plug 'hrsh7th/cmp-nvim-lsp'
Plug 'hrsh7th/vim-vsnip'
Plug 'neovim/nvim-lspconfig'
call plug#end()
PlugInstall | quit

" Setup global configuration. More on configuration below.
lua << EOF
local cmp = require "cmp"
cmp.setup {
  snippet = {
    expand = function(args)
      vim.fn["vsnip#anonymous"](args.body)
    end,
  },

  mapping = {
    ['<C-n>'] = function()
      local cmp = require('cmp')
      if cmp.visible() then
        cmp.select_next_item({ behavior = cmp.SelectBehavior.Insert })
      else
        cmp.complete()
      end
    end,
    ['<C-p>'] = cmp.mapping.select_prev_item({ behavior = cmp.SelectBehavior.Insert }),
    ['<CR>'] = cmp.mapping.confirm({ select = false }),
  },

  sources = cmp.config.sources({
    { name = "nvim_lsp" },
    { name = "buffer" },
  }),
}

local capabilities = require('cmp_nvim_lsp').default_capabilities()

require'lspconfig'.cssls.setup {
  capabilities = capabilities,
}

-- the custom vim.ui.input
vim.fn.sign_define("UIInputPromptPrefix", { text = "I", texthl = "Type" })

vim.ui.input = function()
  local buf = vim.api.nvim_create_buf(false, true)
  vim.api.nvim_buf_set_option(buf, "bufhidden", "wipe")

  -- normal windows
  --vim.cmd.vsplit()
  --local win = vim.api.nvim_win_get_number(0)
  --vim.cmd.buffer(buf)

  -- floating windows
  local win = vim.api.nvim_open_win(buf, true, {
    relative = "cursor", row = -3, col = 0, width = 32,
    height = 1, zindex = 1, border = "rounded", title = "Input", noautocmd = true,
  })

  vim.fn.sign_place(0, "UIInput", "UIInputPromptPrefix", buf, { lnum = 1 })
end
EOF

Description

So, I came across this bug while using my own vim.ui.input handler. As evident in this asciicast, selecting with <C-n> or <C-p> and confirming with <CR> causes the sign placed with vim.fn.sign_place to disappear. This happens in both floating and normal windows.

Curiously, if I :set signcolumn=yes after opening the window but before confirming the selection, the sign persists. But setting it afterwards is no good.

Steps to reproduce

1. Run the config with nvim -u
2. :lua vim.ui.input()
3. Enter random text into the floating buffer for cmp to pick up. Select one of the options and hit <CR>.

Expected behavior

The sign in signcolumn should persist.

Actual behavior

The sign in signcolumn disappears.

Additional context

I feel that I'm doing something wrong considering signs by gitsigns.nvim don't disappear, but I don't know what...