How to pre-render a terminal buffer before displaying it in a popup

Question

I am currently trying to write my first plugin. So this is my first attempt at vimscript. I want to display the output of a shell command in a popup window.

This can easily achieved with:

command Cat call CatCurFile()

function! CatCurFile()
    let buf = term_start(['cat', @%], #{hidden: 1})
    let winid = popup_create(buf, #{minwidth: 120, minheight: 70})
endfunction

The popup window is built up from top to bottom, which is not pretty. I would like the have the whole terminal buffer pre-rendered somehow, so that it instantly displays everything.

cat is just an example here. The actual application is a bit slower unfortunately.

I have tried

command Cat call CatCurFile()

function! CatCurFile()
    let buf = term_start(['cat', @%], #{hidden: 1})
    let winid = popup_create(buf, #{hidden: 1, minwidth: 120, minheight: 70})
    popup_show(winid)
endfunction

but the popup is not showing. Instead an error saying menu not found is displayed. Is the id not passed right?