How can I show the syntax-colored current line wrapped on top of current line in nowrap buffer?

Question

I know this can look a lot like a duplicate of Vim wrap only current line, but none of the answers give a real solution to the original problem, as all of them state that per-line soft wrapping is not possible, proposing alternatives like hard wrapping the line and undoing the change afterwards, or showing the wrapped line in an ad-hoc buffer.

Furthermore, 6 years have passed, Vim has improved, and I wonder if new solutions exist.

A strategy that looks promising to me is to have an autocmd CursorHold showing a popup populated by the current line only, with horizontal scrolling taking into account stuff I use all the time, like number, relativenumber, list lcs.

Leaving aside for now the autocmd/augroup part of the strategy, what I've come up with is the following,

function! Foo()
  call popup_create(getline('.'),
    \ #{ line: 'cursor', col: screencol() - virtcol('.') + 1, moved: [line('.'), 0, col('$')], })
endfunction

which does a pretty good job in terms of formatting,

Except that the line is not syntax-colored (the 3 would have to be the same color as the other digits a few lines above).

I've found out that the first argument to popup_create can be a buffer number, which means that popup will be populated with that buffer. This seems useful, but I haven't been able to use it to show only the current line. I can start from the current line, via firstline option, but don't know how to set a final line, as lastline doesn't do the job¹ (probably I don't understand what it is for). Here's the second attempt I'm referring to:

function! Foo()
  call popup_setoptions(
    \ popup_create(
    \   bufnr('.'),
    \   #{ line: 'cursor', col: screencol() - virtcol('.') + 1, moved: [line('.'), 0, col('$')], }
    \ ),
    \ #{ wrap: 1, firstline: line('.')})
endfunction

and here's the result

Well, there's also the problem of the background of the popup, but that's likely easier to fix.

---

(¹) In the sense that I get the same output as the one in second screenshot, whther or not I put lastline: line('.') + 1 or lastline: line('.') in in popup_setoptions's second argument.

Answer 1

So the trick was to use maxheight and compute it based on how much text is on the current line and how much space is in the terminal width.

With the following solution

function! Foo()
  let available_screen = &columns - max([0, screencol() - virtcol('.')])
  call popup_setoptions(
    \ popup_create(
    \   bufnr(),
    \   #{ line: 'cursor', col: screencol() - virtcol('.') + 1, moved: [line('.'), 0, col('$')], }
    \ ),
    \ #{
    \    wrap: 1,
    \    firstline: line('.'),
    \    maxheight: float2nr(ceil(len(getline(line('.')))*1.0/available_screen)),
    \    scrollbar: 0
    \  })
endfunction

nnoremap <silent> <Leader>sb :call Foo()<cr>

(and temporarily changing moved to [0, 0, 0] to make popups permanent), I obtained this result: For reference, this is without the popups:

Clearly, when hitting the left border of the terminal, the popup has to stop moving left and so it is not aligned with the underlying text, but I think this can't be avoided (unless manually wrapping words back on previous lines).