(Pyls, Jedi language server, Pyright) + diagnosticls not showing preview window

[sjeon7]

• nvim --version: NVIM v0.5.0-844-g8c4648421
• nvim-lsp version(commit hash): 4f19567
• :checkhealth result

health#lspconfig#check
========================================================================
## Checking language server protocol configuration
  - INFO: diagnosticls: configuration checked.
  - INFO: pyright: configuration checked.

health#completion_nvim#check
========================================================================
## general
  - OK: neovim version is supported

## completion source
  - OK: all completion source are valid

## snippet source
  - INFO: You haven't setup any snippet source.

• What language server(If the problem is related to a specific language server):
  Python language servers (Pyls, Jedi language server, Pyright) and diagnosticls

• Operating system/version:
  Ubuntu 18.04 LTS

How to reproduce the problem from neovim startup

1. With minimal init.vim below
2. All the language server installed
3. Any python linter installed (flake8 is used but it can be pylint or any other linter)
4. Open python file and preview window is not showing

all plug#begin() 
    Plug 'neovim/nvim-lspconfig'
    Plug 'nvim-lua/completion-nvim'
call plug#end()

function! s:check_back_space() abort
    let col = col('.') - 1
    return !col || getline('.')[col - 1]  =~ '\s'
endfunction

inoremap <silent><expr> <TAB>
    \ pumvisible() ? "\<C-n>" :
    \ <SID>check_back_space() ? "\<TAB>" :
    \ completion#trigger_completion()
inoremap <expr> <S-TAB> pumvisible() ? "\<C-p>" : "\<S-TAB>"

" Set completeopt to have a better completion experience
set completeopt=menuone,noinsert,noselect

lua<<EOF
local lspconfig = require('lspconfig')

local on_attach = function(_, bufnr)
  vim.api.nvim_buf_set_option(bufnr, 'omnifunc', 'v:lua.vim.lsp.omnifunc')
  require'completion'.on_attach()
end

lspconfig['pyright'].setup{
    on_attach = on_attach,
}
lspconfig.diagnosticls.setup {
  filetypes = { "python" },
  init_options = {
    filetypes = {
      python = {"flake8"},
    },
    linters = {
      flake8 = {
        debounce = 100,
        sourceName = "flake8",
        command = "flake8",
        args = {
          "--format",
          "%(row)d:%(col)d:%(code)s:%(code)s: %(text)s",
          "%file",
        },
        formatPattern = {
          "^(\\d+):(\\d+):(\\w+):(\\w).+: (.*)$",
          {
              line = 1,
              column = 2,
              message = {"[", 3, "] ", 5},
              security = 4
          }
        },
        securities = {
          E = "error",
          W = "warning",
          F = "info",
          B = "hint",
        },
      },
    },
  }
}
EOF

Actual behaviour

Without diagnosticls, preview window pops up well.

After setting diagnosticls, preview window is not showing.

Other functionalities such as showing diagnostic, code completion etc. work but preview window is not showing.
I have tested with Pyls, Jedi LS, Pyright with diagnosticls, and all of them failed to show preview window. I also tested Nvim-completion and Deoplete which both completion manager have identical issue.

Expected behaviour

Language server should hover preview window even with diagnosticls.

[sjeon7]

nvim-lua/completion-nvim#279
This PR fixes this. I think this issue can be closed after the PR is merged.