Introduce new "popup" match display

Using Neovim's floating window, or Vim 8.x popup.

autoload/matchup/matchparen.vim

@@ -9,6 +9,8 @@ scriptencoding utf-8
 let s:save_cpo = &cpo
 set cpo&vim
 
+let s:winid = 0
+
 function! matchup#matchparen#init_module() " {{{1
   if !g:matchup_matchparen_enabled | return | endif
 
@@ -508,6 +510,8 @@ function! s:do_offscreen(current, method) " {{{1
     call s:do_offscreen_statusline(l:offscreen, 0)
   elseif a:method ==# 'status_manual'
     call s:do_offscreen_statusline(l:offscreen, 1)
+  elseif a:method ==# 'popup'
+    call s:do_offscreen_popup(l:offscreen)
   endif
 endfunction
 
@@ -536,7 +540,84 @@ function! s:do_offscreen_statusline(offscreen, manual) " {{{1
 endfunction
 
 " }}}1
+function! s:do_offscreen_popup(offscreen) " {{{1
+  let l:original_filetype = &filetype
 
+  if exists('*nvim_open_win')
+    " neovim floating window
+    call s:close_floating_win()
+
+    " Set default width and height for now.
+    let buf = nvim_create_buf(v:false, v:false)
+    let s:winid = nvim_open_win(buf, v:false, {
+          \ 'relative': 'cursor',
+          \ 'row': 1,
+          \ 'col': 0,
+          \ 'width': 42,
+          \ 'height': &previewheight,
+          \ 'style': 'minimal'
+          \ })
+
+    call nvim_buf_set_var(buf, 'cursorword', 0)
+    call nvim_buf_set_option(buf, 'filetype',  l:original_filetype)
+    call nvim_buf_set_option(buf, 'buftype',   'nofile')
+    call nvim_buf_set_option(buf, 'bufhidden', 'delete')
+    call nvim_buf_set_option(buf, 'swapfile',  v:false)
+
+    " assumes cursor is in original window
+    autocmd matchup_matchparen CursorMoved <buffer> ++once
+      \ call s:close_floating_win()
+
+  elseif exists('*popup_create')
+    " vim8 popup
+    let s:winid = popup_create('', {
+          \ 'line': 'cursor+1',
+          \ 'col': 'cursor',
+          \ 'moved': 'any',
+          \ })
+
+    call setbufvar(winbufnr(s:winid), 'cursorword', 0)
+    call setbufvar(winbufnr(s:winid), '&filetype', l:original_filetype)
+  endif
+
+  call s:populate_floating_win(a:offscreen)
+endfunction
+
+" }}}1
+function! s:populate_floating_win(offscreen) " {{{1
+  let l:adjust = matchup#quirks#status_adjust(a:offscreen)
+  let l:lnum = a:offscreen.lnum + l:adjust
+  let l:line = getline(l:lnum)
+
+  let l:body = split(l:line, '\n')
+  let body_length = len(l:body)
+  let height = min([body_length, &previewheight])
+
+  if exists('*nvim_open_win')
+    " neovim floating win
+    let width = max(map(copy(l:body), 'strdisplaywidth(v:val)'))
+    call nvim_win_set_width(s:winid, width)
+    call nvim_win_set_height(s:winid, height)
+
+    call nvim_buf_set_lines(winbufnr(s:winid), 0, -1, v:false, [])
+    call nvim_buf_set_lines(winbufnr(s:winid), 0, -1, v:false, l:body)
+    call nvim_win_set_cursor(s:winid, [1,0])
+
+  elseif exists('*popup_create')
+    " vim8 popup
+    call popup_settext(s:winid, l:body)
+  endif
+endfunction
+
+" }}}1
+function! s:close_floating_win() " {{{1
+  if win_id2win(s:winid) > 0
+    execute win_id2win(s:winid) . 'wincmd c'
+  endif
+  let s:winid = 0
+endfunction
+
+" }}}1
 function! MatchupStatusOffscreen() " {{{1
   return substitute(get(w:, 'matchup_statusline', ''),
         \ '%<\|%#\w*#', '', 'g')

doc/matchup.txt

@@ -612,7 +612,7 @@ Module matchparen~
     Sets the method to use to show off-screen matches.
     Possible values are:
 
-    `'status' (default): Replace the |status-line| for off-screen matches.
+    `'status'` (default): Replace the |status-line| for off-screen matches.
 
     If a match is off of the screen, the line belonging to that match will be
     displayed syntax-highlighted in the status line along with the line number
@@ -623,6 +623,8 @@ Module matchparen~
     `'status_manual'`: Compute the status-line but do not display it (future
     extension).
 
+    `'popup'`: Use neovim floating window or vim8 popup to show match.
+
   scrolloff~
     When enabled, off-screen matches will not be shown in the statusline while
     the cursor is at the screen edge (respects the value of 'scrolloff').