Run this in on_attach to show diagnostics in a floating window

setup_show_diagnostics.lua

local setup_show_diagnostics = function(buffer)
  local opts = {
    border = 'rounded',
    close_events = {
      'BufLeave',
      'CursorMoved',
      'DiagnosticChanged',
      'FocusLost',
      'InsertEnter',
    },
    focussable = false,
  }
  local augroup_name = 'lsp-show-diagnostics'
  vim.api.nvim_create_augroup(augroup_name, { clear = true })
  vim.api.nvim_create_autocmd('CursorHold', {
    buffer = buffer,
    callback = function()
      vim.diagnostic.open_float(nil, opts)
    end,
    group = augroup_name,
  })
  vim.api.nvim_create_autocmd('DiagnosticChanged', {
    buffer = buffer,
    callback = function(args)
      vim.diagnostic.open_float(nil, opts)
    end,
    group = augroup_name,
  })
end

Fix for bad background:

vim.api.nvim_set_hl(0, 'Pmenu', { ctermbg = 0 })

Set delay for CursorHold:

set updatetime=500