From e615db06046312e74886fa1ef98feb5a9db2a7c3 Mon Sep 17 00:00:00 2001
From: Bram Moolenaar <Bram@vim.org>
Date: Thu, 20 Jan 2022 21:00:54 +0000
Subject: patch 8.2.4166: undo synced when switching buffer in another window

Problem:    Undo synced when switching buffer in another window.
Solution:   Do not sync undo when not needed. (closes #9575)
---
 src/buffer.c                |  6 +++++-
 src/testdir/test_timers.vim | 23 +++++++++++++++++++++++
 src/version.c               |  2 ++
 3 files changed, 30 insertions(+), 1 deletion(-)

(limited to 'src')

diff --git a/src/buffer.c b/src/buffer.c
index 18aa2a09e7..d8d0f3cb5f 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -1744,7 +1744,11 @@ set_curbuf(buf_T *buf, int action)
 	{
 	    win_T  *previouswin = curwin;
 
-	    if (prevbuf == curbuf)
+	    // Do not sync when in Insert mode and the buffer is open in
+	    // another window, might be a timer doing something in another
+	    // window.
+	    if (prevbuf == curbuf
+			 && ((State & INSERT) == 0 || curbuf->b_nwindows <= 1))
 		u_sync(FALSE);
 	    close_buffer(prevbuf == curwin->w_buffer ? curwin : NULL, prevbuf,
 		    unload ? action : (action == DOBUF_GOTO
diff --git a/src/testdir/test_timers.vim b/src/testdir/test_timers.vim
index 790b7b0d05..84fe05e557 100644
--- a/src/testdir/test_timers.vim
+++ b/src/testdir/test_timers.vim
@@ -484,5 +484,28 @@ func Test_timer_outputting_message()
   call delete('XTest_timermessage')
 endfunc
 
+func Test_timer_using_win_execute_undo_sync()
+  let bufnr1 = bufnr()
+  new
+  let g:bufnr2 = bufnr()
+  let g:winid = win_getid()
+  exe "buffer " .. bufnr1
+  wincmd w
+  call setline(1, ['test'])
+  autocmd InsertEnter * call timer_start(100, { -> win_execute(g:winid, 'buffer ' .. g:bufnr2) })
+  call timer_start(200, { -> feedkeys("\<CR>bbbb\<Esc>") })
+  call feedkeys("Oaaaa", 'x!t')
+  " will hang here until the second timer fires
+  call assert_equal(['aaaa', 'bbbb', 'test'], getline(1, '$'))
+  undo
+  call assert_equal(['test'], getline(1, '$'))
+
+  bwipe!
+  bwipe!
+  unlet g:winid
+  unlet g:bufnr2
+  au! InsertEnter
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/version.c b/src/version.c
index 350b7e10b0..eaeb0a1c37 100644
--- a/src/version.c
+++ b/src/version.c
@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4166,
 /**/
     4165,
 /**/
-- 
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