How to load method definition in a popup?

Question

This is a followup to this question, where I would like to load the method definition in a pop-up rather than a split screen (or the whole screen for that matter).

Attempt 1: using nui.vim

I’m using nui.nvim for this, their pop-up method seems perfect for this purpose.

I have this in my lsp server key binding:

  for _, lsp in ipairs(servers) do
      lspconfig[lsp].setup {
          ..
          on_attach = function(client, bufnr)
            // jumps to method definition
            vim.keymap.set('n', 'gd', vim.lsp.buf.definition, bufopts) 
            // opens method definition in a split screen
            vim.keymap.set('n', 'gv', function() 
                    vim.cmd([[
                        vsplit
                        ]])
                        vim.lsp.buf.definition()
            end, bufopts)
 
            // loads ‘hello world’ in a pop-up window
            vim.keymap.set('n','gn', function()
              popup:mount()
              vim.api.nvim_buf_set_lines(popup.bufnr, 0, 1, false, { "Hello World" })
            end, bufopts)

            // loads an empty pop-up window
            vim.keymap.set('n','gx', function()
              popup:mount()
              vim.api.nvim_buf_set_lines(popup.bufnr, 0, 1, false, { vim.lsp.buf.definition() })
            end, bufopts)
          end
      }
  end

result: didn’t work.

Attemp 2: using on_list and vim.lsp.util.open_floating_preview

I was going over the docs from lsp-neovim, and I found these two interesting:

vim.lsp.util.open_floating_preview()
open_floating_preview({contents}, {syntax}, {opts})
    Shows contents in a floating window.

and

on_list receives a table with:
items table[], structured like setqflist-what
title string, title for the list.
context table|nil. ctx from lsp-handler

I tried this:

  local function on_list(options)
    vim.lsp.util.open_floating_preview(options)
  end


  for _, lsp in ipairs(servers) do
      lspconfig[lsp].setup {
          capabilities = capabilities,
          on_attach = on_attach,
          flags = lsp_flags,
          on_attach = function(client, bufnr)
            vim.keymap.set('n', 'gd', vim.lsp.buf.definition{on_list=on_list}, bufopts)

          end
      }
  end

result: didn’t work