From 328640aed0ff43680e0d58a045ec3eca83f04571 Mon Sep 17 00:00:00 2001
From: zeertzjq <zeertzjq@outlook.com>
Date: Wed, 14 Jan 2026 09:37:53 +0800
Subject: [PATCH] vim-patch:9.1.1323: b:undo_ftplugin not executed when
 re-using buffer

Problem:  b:undo_ftplugin not executed when re-using buffer
          (archy3)
Solution: explicitly execute b:undo_ftplugin in buflist_new() when
          re-using the current buffer

fixes: vim/vim#17113
closes: vim/vim#17133

https://github.com/vim/vim/commit/baa8c90cc0d214e036a3a7980d5cf95cae88a68d

Cherry-pick test_filetype.vim changes from patch 9.1.1325.

Co-authored-by: Christian Brabandt <cb@256bit.org>
---
 src/nvim/buffer.c                  | 13 +++++++++++++
 src/nvim/window.c                  |  4 ++--
 test/old/testdir/test_filetype.vim | 28 ++++++++++++++++++++++++++++
 3 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index 96ebfe6293..85cb824389 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -129,6 +129,18 @@ typedef enum {
   kBffInitChangedtick = 2,
 } BufFreeFlags;
 
+static void trigger_undo_ftplugin(buf_T *buf, win_T *win)
+{
+  window_layout_lock();
+  buf->b_locked++;
+  win->w_locked = true;
+  // b:undo_ftplugin may be set, undo it
+  do_cmdline_cmd("if exists('b:undo_ftplugin') | exe b:undo_ftplugin | endif");
+  buf->b_locked--;
+  win->w_locked = false;
+  window_layout_unlock();
+}
+
 /// Calculate the percentage that `part` is of the `whole`.
 int calc_percentage(int64_t part, int64_t whole)
 {
@@ -1945,6 +1957,7 @@ buf_T *buflist_new(char *ffname_arg, char *sfname_arg, linenr_T lnum, int flags)
     assert(curbuf != NULL);
     buf = curbuf;
     set_bufref(&bufref, buf);
+    trigger_undo_ftplugin(buf, curwin);
     // It's like this buffer is deleted.  Watch out for autocommands that
     // change curbuf!  If that happens, allocate a new buffer anyway.
     buf_freeall(buf, BFA_WIPE | BFA_DEL);
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 1514c26558..4fcf1ad85a 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -117,13 +117,13 @@ static int close_disallowed = 0;
 /// Used for autocommands that temporarily use another window and need to
 /// make sure the previously selected window is still there.
 /// Must be matched with exactly one call to window_layout_unlock()!
-static void window_layout_lock(void)
+void window_layout_lock(void)
 {
   split_disallowed++;
   close_disallowed++;
 }
 
-static void window_layout_unlock(void)
+void window_layout_unlock(void)
 {
   split_disallowed--;
   close_disallowed--;
diff --git a/test/old/testdir/test_filetype.vim b/test/old/testdir/test_filetype.vim
index 4cd6b764ca..022091b06a 100644
--- a/test/old/testdir/test_filetype.vim
+++ b/test/old/testdir/test_filetype.vim
@@ -1196,6 +1196,34 @@ func Test_filetype_indent_off()
   close
 endfunc
 
+func Test_undo_ftplugin_on_buffer_reuse()
+  filetype on
+
+  new
+  let b:undo_ftplugin = ":let g:var='exists'"
+  let g:bufnr = bufnr('%')
+  " no changes done to the buffer, so the buffer will be re-used
+  e $VIMRUNTIME/defaults.vim
+  call assert_equal(g:bufnr, bufnr('%'))
+  call assert_equal('exists', get(g:, 'var', 'fail'))
+  unlet! g:bufnr g:var
+
+  " try to wipe the buffer
+  enew
+  bw defaults.vim
+  let b:undo_ftplugin = ':bw'
+  call assert_fails(':e $VIMRUNTIME/defaults.vim', 'E937:')
+
+  " try to split the window
+  enew
+  bw defaults.vim
+  let b:undo_ftplugin = ':sp $VIMRUNTIME/defaults.vim'
+  call assert_fails(':e $VIMRUNTIME/defaults.vim', 'E242:')
+
+  bwipe!
+  filetype off
+endfunc
+
 """""""""""""""""""""""""""""""""""""""""""""""""
 " Tests for specific extensions and filetypes.
 " Keep sorted.