fix: Do NOT count popup windows on auto duplication (#474)

autoload/fern/internal/viewer/auto_duplication.vim

@@ -11,7 +11,7 @@ function! fern#internal#viewer#auto_duplication#init() abort
 endfunction
 
 function! s:duplicate() abort
-  if len(win_findbuf(bufnr('%'))) < 2
+  if s:count_non_popup_windows('%') < 2
     return
   endif
   " Only one window is allowed to display one fern buffer.
@@ -21,3 +21,22 @@ function! s:duplicate() abort
   let bufname = fern#fri#format(fri)
   execute printf('silent! keepalt edit %s', fnameescape(bufname))
 endfunction
+
+function! s:count_non_popup_windows(expr) abort
+  let winids = win_findbuf(bufnr(a:expr))
+  return len(filter(winids, {_, v -> !s:is_popup_window(v)}))
+endfunction
+
+if exists('*win_gettype')
+  function! s:is_popup_window(winid) abort
+    return win_gettype(a:winid) ==# 'popup'
+  endfunction
+elseif exists('*nvim_win_get_config')
+  function! s:is_popup_window(winid) abort
+    return nvim_win_get_config(a:winid).relative !=# ''
+  endfunction
+else
+  function! s:is_popup_window(winid) abort
+    return getbufvar(winbufnr(a:winid), '&buftype') ==# 'popup'
+  endfunction
+endif