popup_list() cluttered even when no popup is showing

[Aster89]

Explain the issue

Most issues are related to bugs or problems. In these cases, you should
include a minimal working example and a minimal vimrc file (see below), as
well as:

1. What vim version are you using? If using neovim, Are you using
   tree-sitter or classic matching? terminal Vim, v9.2.0600
2. Steps to reproduce: echo popup_list() after an off-screen popup has been shown and dismissed
3. Expected behavior: []
4. Observed behavior: [the id of the popup]

Minimal working example

1. I've stripped my vimrc (see below),
2. then opened a C++ file,
3. set the viewport such that the either but not both of a matching {/} is off screen,
4. checked that echo popup_list() prints []
5. moved the cursor to the other of the matching pair, thus trigger the off-screen popup,
6. checked that echo popup_list() prints [the id]
7. moved the cursor away from any matchable entity, e.g. to an empty line, so that no popup is showing anymore,
8. verified that, unexpectedly, echo popup_list() still prints the same thing as at step 7. It should print [] instead.

Minimal vimrc file

Please provide a minimal vimrc file that reproduces the issue. The
following should often suffice:

if empty(glob('~/.vim/autoload/plug.vim'))
  silent !curl -fLo ~/.vim/autoload/plug.vim --create-dirs
    \ https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
  au VimEnter * PlugInstall --sync | source $MYVIMRC
endif
call plug#begin('~/.vim/plugged')
Plug 'junegunn/vim-plug'
Plug 'andymass/vim-matchup'
call plug#end()
let g:matchup_matchparen_offscreen = {'method': 'popup', 'highlight': 'OffscreenPopup', 'fullwidth': 1, 'syntax_hl': 1}
let g:matchup_matchparen_stopline = 1500
let g:matchup_delim_stopline = 9000

[Aster89]

After step 8, doing call popup_clear() correctly dismisses the popup, but then repeating steps 3 to 5 triggers this error:

E158: Invalid buffer name: -1

which I think it's revealing that the intention is indeed to reuse the ID obtained with the first call.

By the look of https://github.com/andymass/vim-matchup/blob/master/autoload/matchup/matchparen.vim, it feels like the approach of reusing an assigned ID is pretty pervasive, so I guess doing what I ask is fairly complex.

Maybe I should conclude that I should write code that is agnostic to what popup_list() returns...