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_windows('%') < 2
     return
   endif
   " Only one window is allowed to display one fern buffer.
@@ -21,3 +21,9 @@ function! s:duplicate() abort
   let bufname = fern#fri#format(fri)
   execute printf('silent! keepalt edit %s', fnameescape(bufname))
 endfunction
+
+function! s:count_windows(expr) abort
+  let winids = win_findbuf(bufnr(a:expr))
+  let wintypes = map(winids, {_, v -> win_gettype(v) })
+  return len(filter(wintypes, {_, v -> v !=# 'popup'}))
+endfunction